# Patent

## Title
Advanced excimer laser annealing for thin films

## Abstract

The present disclosure relates to a new generation of laser-crystallization approaches that can crystallize Si films for large displays at drastically increased effective crystallization rates. The particular scheme presented in this aspect of the disclosure is referred to as the advanced excimer-laser annealing (AELA) method, and it can be readily configured for manufacturing large OLED TVs using various available and proven technical components. As in ELA, it is mostly a partial-/near-complete-melting-regime-based crystallization approach that can, however, eventually achieve greater than one order of magnitude increase in the effective rate of crystallization than that of the conventional ELA technique utilizing the same laser source.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of and claims the benefit under 35 U.S.C. 120 to U.S. patent application Ser. No. 13/505,961, filed on Dec. 6, 2012, which is a 371 of PCT/US10/55106, filed on Nov. 2, 2010, which claims benefit under 35 U.S.C. 119(e) to U.S. Provisional application No. 61/294,288, filed on Jan. 12, 2010; U.S. Provisional application No. 61/291,663, filed on Dec. 31, 2009; U.S. Provisional application No. 61/291,488, filed on Dec. 31, 2009; U.S. Provisional application No. 61/286,643, filed on Dec. 15, 2009; U.S. Provisional application No. 61/264,082, filed on Nov. 24, 2009; U.S. Provisional application No. 61/257,657, filed on Nov. 3, 2009; and U.S. Provisional application No. 61/257,650, filed on Nov. 3, 2009, the contents of which are incorporated in its entirety by reference herein.
This application is also a continuation-in-part of U.S. patent application Ser. No. 12/776,756, filed on May 10, 2010, which claims benefit under 35 U.S.C. 119(e) to U.S. Provisional application No. 61/294,288, filed on Jan. 12, 2010; and U.S. Provisional application No. 61/291,663, filed on Dec. 31, 2009.
This application also claims benefit under 35 U.S.C. 119(e) to U.S. Provisional application No. 61/692,316, filed on Aug. 23, 2012; U.S. Provisional application No. 61/674,017, filed on Jul. 20, 2012; U.S. Provisional application No. 61/659,517, filed on Jun. 14, 2012; and U.S. Provisional application No. 61/646,480, filed on May 14, 2012, the contents of which are incorporated in its entirety by reference herein.


BACKGROUND
In the field of semiconductor processing, a number of techniques have been described to convert thin amorphous silicon films into polycrystalline films. One such technique is excimer laser annealing (“ELA”). ELA is a pulsed-laser crystallization process that can produce polycrystalline films having uniform crystal grains on substrates, such as, but not limited to, substrates that are intolerant to heat (e.g., glass and plastics). Examples of ELA systems and processes are described in commonly-owned U.S. Patent Publication Nos. 20090309104, entitled “Systems and Methods for Creating Crystallographic-Orientation Controlled Poly-Silicon Films,” filed Aug. 20, 2009; 20100065853, entitled “Process and System for Laser Crystallization Processing of Film Regions on a Substrate to Minimize Edge Areas, and Structure of Such Film Regions,” filed Sep. 9, 2009, and 20070010104, entitled “Processes and Systems for Laser Crystallization Processing of Film Regions on a Substrate Utilizing a Line-Type Beam, and Structures of Such Film Regions,” filed Mar. 9, 2006.
Conventional ELA tools use a single line beam that is continuously scanned at a low velocity over the surface of a sample with large overlap between pulses (e.g. 95%) to establish a large number of pulses per unit area in a single scan. Thus, in ELA, a region of the film is irradiated by an excimer laser to partially melt the film, which subsequently crystallizes. Repetitive partial melting of the film can lead to formation of small-grained polycrystalline films; however, the method often suffers from microstructural non-uniformities, which can be caused by pulse to pulse energy fluctuations and/or non-uniform beam intensity profiles. A large number of pulses is not only required to induce the cumulative effects that lead to more uniform grain size, but also to mitigate the effects of the short axis beam edges. In the beam edge segments of the beam, the energy gradually reduces to zero. Depending on the location in the film, location-dependent variation in the initial pulse energy sequence can occur. Such variation is not easily removed by the subsequent ELA process and artifacts in pixel brightness (i.e., mura) may result. FIG. 1A illustrates a random microstructure that may be obtained with ELA. The Si film is irradiated multiple times to create the random polycrystalline film with a uniform grain size. FIG. 1B depicts a conventional ELA single-scan, showing the cross section of the line beam 101 on its short axis as the beam 101 scans a film 104. The beam 101 is advanced in the direction of the arrow 102 and a region 103 of the film 104 can be irradiated with multiple laser pulses as the beam 101 moves across the film 104.
Further, crystallization methods and tools that can be used for obtaining a uniform grain structure (“UGS”) at very high throughput have been reported. For example, such a system is disclosed in United States Application Publication No. 20070010104 entitled “Processes and Systems for Laser Crystallization Processing of Film Regions on a Substrate Utilizing a Line-Type Beam, and Structures of Such Film Regions.” UGS is a single pulse irradiation process that can involve complete-melt crystallization (“CMC”) and/or partial-melt crystallization (“PMC”) of the film being crystallized. An additional feature of the UGS process is the position-controlled firing of laser pulses so that partial or complete melting occurs only in those regions where columns/rows of pixel thin-film transistors (“TFTs”) reside. When the stepping distance between pulses exceeds the width of the line beam, unirradiated regions (e.g. amorphous as-deposited Si) of the film remain in between such columns. This selective-area crystallization (“SAC”) process can thus have very high throughput as the average number of pulses per unit area could be less than one.
However, none of the prior tools are especially well optimized for ELA for very large films, for example as used in televisions that have a low density of pixels. Conventional ELA is an inefficient process for such substrates, in that time and resources are wasted to crystallize the Si substrate between pixel locations. While UGS tools allow one to skip those areas, the material that is obtained is significantly more defective than the typical ELA material and also uniformity of the material may not be sufficient when typical radiation conditions are used.
SUMMARY
In one aspect, the present disclosure relates to a method for crystallizing a thin film. In some embodiments, the method can include providing a thin film having a plurality of device formation regions; generating a line beam having a uniform portion and two edge portions, while continuously translating the thin film semiconductor in a first direction, irradiating a first device formation region with the uniform portion of the line beam; and irradiating a second device formation region, distinct from the first device formation region, with the uniform portion of the line beam. In some embodiments, the first device formation region overlaps the second device formation region. In some embodiments, the method can include continually irradiating successive device formation regions until the each device formation region has been irradiated at least once. In some embodiments, the method can include, after each device formation region has been irradiated at least once, continuously translating the film in a second direction, opposite the first direction while irradiating successive device formation regions. In some embodiments, the method can include, after each device formation region has been irradiated at least once, translating the film back to a starting position and continuously translating the film in the first direction, irradiating successive device formation regions. In some embodiments, the method can include after each device formation region has been irradiated at least once, shifting the film in a direction perpendicular to the first direction. In some embodiments, the method can include, after each device formation region has been irradiated once, shifting the film in a direction parallel to the first direction.
In some embodiments, the method can include irradiating each device formation region at least twice. In some embodiments, the method can include irradiating each device formation region at least four times. In some embodiments, the method can include irradiating each device formation region at least six times.
In some embodiments, the device formation region can include a region where a pixel is to be formed. In some embodiments, the device formation region can include a region where a thin film transistor is to be formed. In some embodiments, the device formation region can include a region where capacitor is to be formed. In some embodiments, the method can include forming a capacitor in a capacitor region adjacent to the first device formation region. In some embodiments, the device formation region can include a region where a column of thin film transistors are to be formed. In some embodiments, the device formation region can include a region where a plurality of columns of devices are to be formed. In some embodiments, the device formation region can include a portion of a first column of devices and a portion of a second column of devices, wherein the portion of a first column of devices is proximal to the portion of a second column of devices. In some embodiments, the device formation region can include a region in a first column of pixels and a region in a second column of pixels.
In some embodiments, the line beam can have a fluence sufficient to partially melt the film. In some embodiments, the line beam can have a fluence sufficient to completely melt the film. In some embodiments, the line beam can have a fluence sufficient to near completely melt the film. In some embodiments, the line beam can have a fluence below the partial melting threshold, but sufficient to modify the structure of the thin film. In some embodiments, the film can be a semiconductor film.
In one aspect, the present disclosure relates to a system for crystallizing a thin film. In some embodiments, the system can include optics for generating a line beam having a uniform portion and two edge portions; and a computer controlled stage, wherein the computer controlled stage holds a thin film having a plurality of device formation regions and is programmed to continuously translate the stage in a first direction in order for the beam to irradiate a first device formation region with the uniform portion of the line beam; and irradiate a second device formation region, distinct from the first device formation region, with the uniform portion of the line beam.
In another aspect, the present disclosure relates to a thin film including a plurality of regions for formation of devices, wherein each region includes a first crystallized portion and a second crystallized portion, wherein both the first and second crystallized portions include uniform, isotropic crystal grains and the first crystallized portion has larger crystal grains than the second crystallized portion. In some embodiment, the plurality of regions can include at least a first column of regions for formation of devices and a second column of regions for formation of devices. In some embodiments, a first crystallized portion of the first column can be located proximal to a first crystallized portion of the second column. In some embodiments, a second crystallized portion of the first column can be located proximal to a second crystallized portion of the second column.
Another aspect of the present disclosure relates to a method for crystallizing a thin film, the method including providing a thin film having a plurality of device formation regions arranged into a plurality of columns; generating a line beam having a uniform portion and two edge portions, while continuously translating the thin film semiconductor in a first direction, irradiating a first column of device formation regions with the uniform portion of the line beam with a first number of beam pulses; irradiating a second column of device formation region with the uniform portion of the line beam with a second number of beam pulses; and irradiating a third column of device formation regions with the uniform portion of the line beam with a second number of beam pulses, wherein, the first column is proximate to the second column and the third column is spaced apart from the second column. In some embodiments, the first number of beam pulses can be greater than the second number of beam pulses. In some embodiments, the second number of beam pulses can be greater than the first number of beam pulses.



BRIEF DESCRIPTION OF THE DRAWINGS
The following description will be more readily understood with references to the following drawings in which:
FIG. 1A illustrates a random microstructure that may be obtained with ELA;
FIG. 1B depicts a conventional ELA single-scan;
FIGS. 2A-2C depict exemplary energy profiles of laser pulses according to an embodiment of the present disclosure;
FIG. 2D depicts a single shot irradiated plasma enhanced chemical vapor deposition (PECVD) amorphous silicon film;
FIG. 3A depicts a non-periodic pulse ELA system, according to an embodiment of the present disclosure;
FIG. 3B depicts a sample used in the non-periodic pulse ELA system, according to an embodiment of the present disclosure;
FIG. 4 illustrates an exemplary profile of the beam pulse, according to an embodiment of the present disclosure:
FIG. 5A depicts a non-periodic pulse ELA process, according to an embodiment of the present disclosure;
FIG. 5B is an exploded view of region 590 in FIG. 5A according to an embodiment of the present disclosure;
FIG. 6 depicts a non-periodic pulse ELA process, according to an embodiment of the present disclosure;
FIG. 7 depicts a first non-periodic pulse scan as described in FIG. 5A, and also includes a second scan in the reverse direction of the film, according to an embodiment of the present disclosure;
FIG. 8A depicts the crystallographic structure of a film after one irradiation, according to an embodiment of the present disclosure;
FIG. 8B is an illustration of the crystal structure in FIG. 9A, according to an embodiment of the present disclosure;
FIG. 8C depicts an atomic force microscope (“AFM”) scan of the surface of a film after one irradiation at a higher energy density but still in the PMC regime, according to an embodiment of the present disclosure;
FIG. 8D is an illustration of the crystal structure in FIG. 8C according to an embodiment of the present disclosure;
FIG. 8E shows a circular region that was formed upon lateral crystallization from an unmelted seed, according to an embodiment of the present disclosure;
FIG. 9 depicts an interface response function of a thin film, according to an embodiment of the present disclosure;
FIG. 10A depicts FTR and BTR for 150 nm a-Si on a glass substrate with a 300 nm oxide layer in vacuum, according to an embodiment of the present disclosure;
FIG. 10B is similar to 10A except 10B depicts results in vacuum, according to an embodiment of the present disclosure;
FIG. 11A depicts a graph of time in nanoseconds (x-axis) versus normalized reflectance values (y-axis) for a 200 nm a-Si film with a 300 nm oxide surface layer in air at 1.32 CMT and in vacuum at 1.4 CMT, according to an embodiment of the present disclosure;
FIG. 11B is an image of the microstructure obtained in the air environment.
FIG. 11C is an image of the microstructure obtain in the vacuum environment.
FIG. 12 is a schematic of a thin film have a plurality of pixel columns, according to embodiments of the present disclosure.
FIG. 13 is an illustration of a grain structure of an excimer laser annealing (ELA) processed film.
FIG. 14A is an illustration of scan of a top-hat-shaped beam over a film having a plurality of pixels, arranged in columns, according to embodiments of the present disclosure.
FIG. 14B is a schematic illustrating the translation of a beam across a thin film, according to embodiments of the present disclosure.
FIG. 14C is an illustration of scan of a top-hat-shaped beam over a film having a plurality of pixels, arranged in columns, for differential crystallization, according to embodiments of the present disclosure.
FIG. 15 is an illustration of a grain structure of an advanced excimer laser annealing (AELA) processed film, according to embodiments of the present disclosure.
FIG. 16 is an illustration of scan of a top-hat-shaped beam over a film having a plurality of pixels, arranged in columns, according to embodiments of the present disclosure.
FIG. 17A is an illustration of scan of a top-hat-shaped beam over a film having a plurality of pixels, arranged in columns, for differential crystallization, according to embodiments of the present disclosure.
FIG. 17B is an illustration of scan of a top-hat-shaped beam over a film having a plurality of pixels, arranged in columns, for differential crystallization, according to embodiments of the present disclosure.
FIG. 18A is an illustration of scan of a top-hat-shaped beam over a film having a plurality of pixels, arranged in columns, for differential crystallization, according to embodiments of the present disclosure.
FIG. 18B is an illustration of scan of a top-hat-shaped beam over a film having a plurality of pixels, arranged in columns, for differential crystallization, according to embodiments of the present disclosure.
FIG. 19 is an illustration of a grain structure of an advanced excimer laser annealing (AELA) processed film using differential crystallization, according to embodiments of the present disclosure.
FIG. 20 is an illustration of scan of a top-hat-shaped beam over a film having a plurality of pixels, arranged in columns, for differential crystallization, according to embodiments of the present disclosure.
FIG. 21 depicts an exemplary AELA system, according to embodiments of the present disclosure.
FIG. 22 is an illustration of scan of a top-hat-shaped beam over a film having a plurality of pixels, arranged in columns, according to embodiments of the present disclosure.



DESCRIPTION
The present disclosure relates to systems and methods for using non periodic pulse laser techniques in combination with partial melt crystallization and complete melt crystallization techniques to form uniform polycrystalline films. In some embodiments, non periodic pulse ELA is used to produce fine grained uniform crystalline films from an amorphous as-deposited Si films devoid of pre-existing crystallites, for example films obtained by low pressure chemical vapor deposition (LPCVD), plasma-enhanced chemical vapor deposition (PECVD), sputtering, or e-beam evaporation. In some embodiments, a flood irradiation method can be used to either produce a fine grained uniform crystalline film or to produce a precursor film for a non-periodic pulse irradiation method. The flood irradiation method can be a two shot partial melt process wherein an amorphous silicon film devoid of any pre-existing crystallites (e.g., a PECVD film) is transformed in two steps into a fine grained uniform crystalline film with grains having average lateral dimensions exceeding the film thickness, i.e., small columnar grains. The flood irradiation method can also be an extended duration single shot partial melt process wherein an amorphous silicon film devoid of any pre-existing crystallites (e.g., a PECVD film) is transformed into a fine grained uniform crystalline film with grains having average lateral dimensions less than the film thickness. The flood irradiation method also can be a complete melt process wherein an amorphous silicon film of any kind having oxide interfaces at both the top and the bottom of the film is transformed into a low-defect small equiaxed grain Si film.
A non-periodic pulse ELA method and tool is described using position controlled sequential triggering of lasers. The system can be implemented using multiple lasers to create distinct non-periodic laser pulses in the crystallization process, e.g., distinct in that each laser pulse results in a separate partial melting and solidification cycle and non-periodic in that the intervals between pulses are not the same. Multiple lasers are used in a coordinated pulse sequence to irradiate and crystallize selected areas of a film in a single scan or in multiple scans. Multiple scans may be desirable to reach a larger number of melting and solidification cycles in regions of interest in order to benefit from the cumulative effects of multiple irradiations observed in ELA that lead to more uniform polycrystalline films with, for example, a tighter grain-size distribution.
Non-Periodic Pulsing
Exemplary sequences of laser pulses are depicted in FIGS. 2A-2C. The y axis represents energy density and the x axis represents time. FIG. 2A depicts a periodic pulse rate of a laser that can be used for a conventional ELA process. The periodic laser repetition rate results in a laser pulse pattern that is evenly spaced in the time domain. FIG. 2B represents an example of non-periodic pulsing disclosed herein where a second pulse 105 is fired in close time relation to the first pulse 106. Then, a third pulse 107 is fired at time interval different than the interval between the first pulse 106 and the second pulse 105. FIG. 2C illustrates an embodiment for which both the pulse rate and the laser power (energy density) of the laser pulses are different. Thus, the irradiated film experiences a non-periodic pulse rate and variable irradiation energy. Because of the relatively short time between the first pulse 106 and the second pulse 105, the regions irradiated by the first pulse 106 and the second pulse 105 experience an increased overlap.
The time delay between the first pulse 106 and the second pulse 105 can be less than half of the time interval between the first pulse 106 and the third pulse 107. In some embodiments, the time interval between first pulse 106 and the second pulse 105 is less than one tenth or less than one twentieth or less than one hundredth the time interval between the first pulse 106 and the third pulse 107. The time delay between the first pulse 106 and the second pulse 105 can be about three microseconds to about one millisecond, about five microseconds to about 500 microseconds, and about 10 microseconds to about 100 microseconds.
Thus, FIGS. 2B and 2C present a non-periodic pulse pattern that employs two closely spaced or a “train” of two laser pulses; however, a greater number of closely spaced pulses, e.g., 3-5 or more, corresponding to three to five or more lasers or laser cavities may be employed. In such embodiments, where a higher number of closely spaced pulses from different lasers, e.g., laser beams from either two different laser energy sources or two different laser carriers of the same laser energy source, are used, the targeted region is irradiated a correspondingly greater number of times. For example, n pulses from n laser sources may be closely spaced to form a train of n laser pulses and a single region will experience n irradiations in a single scan. The beam may have similar widths as in conventional ELA process.
The two consecutive pulses in a pulse train need not be at the same energy density. For example, if the film is still hot from the first pulse, the second pulse could be at a lower energy density than the first pulse. Likewise, a higher energy density may be used to compensate for the changes in optical properties upon the first pulse (amorphous silicon absorbing slightly better than crystalline silicon for UV light). Proper choices for the energy density of the second pulse may thus take in account both effects and possibly others as well so that the film will experience the same degree of melting. Here, the degree of melting is understood to be a measure of melting independent of the details of melting, which may vary significantly as a function of precursor phase (amorphous or crystalline), heterogeneity (e.g., uniformly defective or having defective cores surrounded by larger and cleaner grains), and surface morphology (smooth or rough, for example having periodicity similar to the wavelength of light). The same degree of melting is thus achieved when the extent of melting during the second pulse is equivalent to that of the first pulse, for example about 80% of the film. In a multiple scan process that aims to benefit from the cumulative effects that lead to more uniform polycrystalline films it is desired that most of the pulses result in the same degree of melting so that the process is most efficient.
Thus, as shown in FIG. 2C, the first laser pulse and the second laser pulse can have different energy densities. Specifically, FIG. 2C depicts the first laser pulse having a smaller energy density than the second pulse. However, in some embodiments, the second laser pulse has a smaller energy density than the first laser pulse. Further, in a multiple scan process, the offset between the energy densities of the first pulse and the second pulse may be different or absent in different scans. For example, the offset in energy density between the first and second pulses in a first scan may be selected to compensate for a change in optical properties while in a second scan, the offset may be selected to compensate for temperature. In some embodiments, even though the two pulses may have different energy densities, a second lower energy pulse may cause the same amount of melting in the film as a first higher energy pulse due to residual heat in the film from the first pulse.
In one embodiment, the present system creates non-periodic laser pulses by using coordinated triggering of pulses from a plurality of laser sources (as is also possible using a single laser source having multiple laser cavities, e.g., tubes) to produce a series of pulses closely spaced in the time domain. A plurality of laser sources may be incorporated into a single laser system. A laser system is a computer controlled system that uses computer controlled techniques to irradiate a substrate in a predetermined manner, e.g., the computer controls the firing of the lasers and the movement of the stage, and one or more laser cavities to produce one or more laser beams. Each laser beam corresponds to one laser source. Each laser beam can be produced from a stand alone laser, or a laser cavity which is part of a plurality of laser cavities contained within one laser system.
Tools having multiple laser cavities, e.g. tubes, have been disclosed previously to (1) increase the pulse energy by simultaneously triggering and subsequently combining multiple pulses and (2) increase the pulse duration by delayed triggering of various tubes and subsequently combining them, as discussed in U.S. Pat. No. 7,364,952, entitled “Systems and Methods for Processing Thin Films,” issued Apr. 29, 2008. In other words, pulses are combined to provide a modified single melting and solidification cycle. Non-periodic pulse ELA is different in that it uses the pulses of various lasers in separate melting/solidification cycles. However, the pulses are close enough in the time domain that they show significant overlap while the stage is traveling at high speed.
Further, the non-periodic pulse ELA method and tool also can be used to perform selective-area crystallization of a film in order to crystallize only those areas of the film that will be formed into electronics. The non-periodic pulse ELA method and tool provides selective-area crystallization resulting in crystal growth in a first region of the film, followed by a break determined by the repetition rate of the lasers and then substantial overlap in the second pulses of the two or more lasers resulting in crystal growth in a second region of the film. The timing between laser pulses gives rise to non periodic laser pulse sequences and substantial overlap in irradiated regions, which is discussed in detail below. Such methods and systems can be used for ELA processes at high throughput.
In selective-area crystallization, the film is crystallized at locations where electronic devices are made (in a subsequent process not discussed here). However, not all electronic devices need equally uniform or even equally conductive material. For example, small TFTs may be much more demanding in terms of crystalline uniformity than large TFTs or even large capacitors. Also, TFTs that are used for current driving may require better uniformity than TFTs used for switching. Thus, of a total area of a particular region to be crystallized, only a fraction may need to be crystallized with a high number of laser pulses to obtain a region of high crystalline uniformity and conductivity, while the remainder may be processed with less pulses or even a single pulse. Selective area crystallization non periodic pulse ELA provides a framework for only scanning selected areas of a film, thereby reducing processing time.
Non-Periodic Pulse ELA
A non periodic ELA system includes one or more of the following features: multiple lasers or laser tubes, and means for delayed triggering of subsequent pulses to have pulses in short succession. The system also can include position controlled triggering of the pulses so that the laser beam pulses irradiate a specific position on the substrate. The timing of the two pulses spaced closely in time should be such that the irradiated portion of the film is allowed to solidify in between pulses, while the position control ensures that the irradiated region is properly located on the substrate, for example to create a column of pixel TFTs or circuits. It is further desirable for the laser beam pulse to have a top-hat beam profile with a beam width that is sufficient to have the sequence of pulses overlap a selected region.
The number of laser sources may be chosen based on various considerations such as throughput, laser power, panel size, display size, system design, and tool maintenance. A larger number of lasers will generally result in higher crystallization rates, but will also necessitate a larger number of optical elements, which may result in more complicated and costly system design. Also, a larger number of lasers may result in increased downtime of the tool because of more frequent need for service, for example tube replacement. Exemplary values for the number of lasers may be two to four or more lasers each having a power of around 600 W or more to process glass panels that may be larger than two m2 and possibly as large as five or 7.5 m2 to make displays having diameters as large as 30, 40, or 50 inches or more.
Non periodic pulse ELA tools can offer the following benefits over conventional ELA and/or UGS tools:
1. Efficient power delivery to preselected regions: by virtue of position control, regions in-between pixel TFTs/circuits are not needlessly crystallized. This leads to higher effective crystallization rates.
2. Elimination of beam-edge related artifacts: beam edges do not impinge on pixel TFT/circuit regions so that crystallized regions therein all experience the exact same sequence of pulses.
3. Optimizing of pulse sequence: regions are irradiated by a sequence of pulses from multiple laser sources and during a multitude of scans and as such the sequence can be optimized (e.g., pulse energy, pulse duration, pulse pre-heating).
4. Mitigating beam non-uniformities on the long axis by implementing a perpendicular displacement between scans. (Beam non-uniformities on the short axis also may be mitigated by an effective parallel displacement within or in between scans, i.e., by shifting the lateral placement of the beam with respect to the regions of interest).
Multiple scans are typically required in non periodic pulse ELA to obtain satisfactory material uniformity. The SAC operation of non periodic pulse ELA typically results in higher through put rates than conventional ELA. In addition, the pulse number with non periodic pulse ELA required to obtain an acceptably uniform crystal structure may be less than that required with conventional ELA. In conventional ELA, the beam edges overlap the area of interest resulting in variation in the crystal structures of the irradiated region along the direction of the scan. The variation in crystal structure was for example discussed in Im and Kim, Phase transformation mechanisms involved in excimer laser crystallization of amorphous silicon films, Appl. Phys. Lett. 63, (14), Oct. 4, 1993, wherein the variation of grain size as a function of energy density in partially melted low pressure chemical vapor deposition (“LPCVD”) films was discussed; LPCVD amorphous Si films are believed to contain small crystallites that trigger crystallization leading to films having a grain size that increases with energy density. In plasma enhanced chemical vapor deposition (“PECVD”) films, melting and solidification processes are further complicated by the absence of such crystallites. Thus, the crystallization is preceded by formation of crystals through a nucleation process. When the nucleation density is low, this may result in disc-shaped crystal structures as is for example visible at the very edge of a single-shot, i.e., one laser pulse, irradiated PECVD amorphous Si film shown in FIG. 2D. FIG. 2D shows an edge region 120 of a single shot PECVD amorphous Si film. This edge region 120 has both an amorphous Si portion 122 and a crystalline Si portion 124. However, the transition region 126 between amorphous Si and crystalline Si is not a sharp edge, but a heterogeneous region containing a mixture of crystalline and amorphous material. The non-uniformity of the film after the first irradiation thus is impacted by the existence of grain size variation and or disc-shaped crystal structures. Such non uniformities may not easily be removed in subsequent radiations. In conventional ELA, even after as many as 10 pulses or more, the effect of the energy density gradient of the first pulse beam edge may still be visible. A large pulse number is therefore needed to erase the history of the first pulse beam edge.
As disclosed herein, SAC using non-periodic pulse ELA may require fewer pulses to achieve an equally uniformly crystallized film. As discussed in greater detail below, the energy profile across the short axis of a line beam contains leading and trailing edges of gradually changing energy density and a central flat region of relatively constant energy. The term line beam, as used herein, refers to a beam having a width substantially smaller than the length of the beam, i.e., the beam has a large aspect ratio. In conventional ELA, the beam edges are a significant source of material non-uniformity. In non periodic pulse ELA, the beam edges are positioned outside the region of interest so that the region of interest is irradiated with a top-hat portion of a first pulse. Further, the energy density of the beam can be optimized to create the most uniform starting material for the cumulative process so as to reduce the number of pulses required to reach a desired level of material uniformity.
System for Performing Non-Periodic Pulse ELA
FIG. 3A depicts a non-periodic pulse ELA system. The system includes a plurality of laser pulse sources 110, 110′ operating for instance at 308 nm (XeCl) or 248 nm or 351 nm. A series of mirrors 206, 208, 212 direct the laser beam to a sample stage 180, which is capable of scanning in the y− direction. The beam is shaped into a line beam having a length of for example about 360 mm, or about 470 mm, or about 720 mm, or any length that is suitable for processing a glass panel in one, two, or more scans. The system may also include a slit 140 that may be used to control the spatial profile of the laser beam and energy density meter 216 to read the reflection of slit 140. Optional shutter 228 can be used to block the beam when no sample is present or no irradiation is desired. Sample 170 may be positioned on stage 180 for processing. Further, homogenizers may be used to provide a more uniform top hat beam profile. An attenuator may be used. The beam energy is controlled by controlling the laser directly. The stage 180 can be a linear translation stage, and can have the ability to do sideways translations. Optionally, the system may include a pulse extender 213 and a mirror 214 to create extended duration pulses.
The sample translation stage 180 is preferably controlled by a computing arrangement to effectuate translations of the sample 170 in the planar y direction, as well as optionally in the x and the z directions. In this manner, the computing arrangement controls the relative position of the sample 170 with respect to the irradiation beam pulse. The repetition and the energy density of the irradiation beam pulse also are controlled by the computing arrangement. It should be understood by those skilled in the art that instead of the beam source 110, 110′ (e.g., the pulsed excimer laser), the irradiation beam pulse can be generated by another known source of short energy pulses suitable for at least partially melting (and possibly fully melting throughout their entire thickness) selected areas of the semiconductor (e.g., silicon) thin film of the sample 170 in the manner described herein below. Such known sources can be a pulsed solid state laser, a chopped continuous wave laser, a pulsed electron beam and a pulsed ion beam, etc. Typically, the radiation beam pulses generated by the beam sources 110, 110′ provide a beam intensity at sample level in the range of 400 mJ/cm2 to 1 J/cm2 or 1.5 or more, a pulse duration (FWHM) in the range of 10 to 300 nsec, and a pulse repetition rate in the range of 10 Hz to 300 Hz to 600 Hz or 1.2 kHz or more.
The exemplary system of FIG. 3A may be used to carry out the processing of the semiconductor thin film of the sample 170 in the manner described below in further detail. A mask/slit can be used by the exemplary system of the present disclosure to define the profile of the resulting masked beam pulse, and to reduce the non-uniformity of the adjacent portions and edge regions of the portions of the semiconductor thin film when these portions are irradiated by such masked beam pulse and then crystallized.
For example, a line beam for the non periodic pulse ELA process can have a width of about 100 or less to 300 microns to about 400 to 600 or more microns. The fluences of the ELA beams are selected to not induce complete melting of the film. Therefore, the ELA beam should have a fluence lower by about 5% to 30% or more of the fluence value that induces complete melting in the given film. The fluence value that induces complete melting is dependent upon the thickness of the film and the duration of the pulse. Further, the ELA beams could have relatively low repetition rates of about 300 Hz to about 600 Hz. The disclosed high power lasers provide sufficient energy per pulse to provide adequate energy density across the length of an irradiated region so that the pulse may melt a film within that region.
The ELA line beam may be created from a relatively low frequency laser source, such as used in certain systems available from JSW (The Japanese Steel Works, Ltd., located at Gate City Ohsaki-West Tower, 11-1. Osaki 1-chome, Shinagawa-ku, Tokyo. Japan). High frequency lasers, such as available from TCZ, are not well suited for the non periodic pulse ELA process as the required scan velocity, which is dictated by pulse repetition rate and the pitch of the TFTs or circuits, becomes very high.
As illustrated in FIG. 3B, a semiconductor thin film 175 of the sample 170 can be directly situated on, e.g., a glass substrate 172, and may be provided on one or more intermediate layers 177 there between. The semiconductor thin film 175 can have a thickness between 100 Å and 10,000 Å (1 micron) so long as at least certain necessary areas thereof can be at least partially or completely melted throughout their thickness.
According to an exemplary embodiment of the present disclosure, the semiconductor thin film 175 can be composed of silicon, (e.g., an amorphous silicon thin film) germanium, silicon germanium (SiGe), etc. all of which preferably have low levels of impurities. It is also possible to utilize other elements or semiconductor materials for the semiconductor thin film 175. The intermediary layer 177, which is situated immediately underneath the semiconductor thin film 175, can be composed of silicon oxide (SiO2), silicon nitride (Si3N4), and/or mixtures of oxide, nitride or other materials.
An exemplary profile of the beam pulse 200 is illustrated in FIG. 4, which also can be shaped by the optics of the system illustrated in FIG. 3A and/or produced by a mask. In this exemplary embodiment, the energy density of the beam pulse 200 has a profile 220 with an energy density that is below the complete melting threshold, i.e., the energy density of the beam pulse at which the film completely melts. In particular, this profile 220 includes a top portion 205, a leading edge portion 210 and a trailing edge portion 215. The top portion 205 of this embodiment extends for a width C, within which the energy density is approximately constant. The width C may be between 100 microns to 1 mm. The leading edge portion 210 can extend for a distance D1 (e.g., between 50 microns and 100 microns), and the trailing edge portion 215 may extend for a distance D2 (e.g., also between 50 μm and 100 μm). The leading edge potion 210 has a section with a length of D1P, which extends from to the point when the energy density is approximately constant to a lower point of the crystallization threshold, i.e., the energy density of the beam pulse at which the film crystallizes. Similarly, the trailing edge potion 215 has a section with a length of D2P which extends from the point of the crystallization threshold, to a higher point of when the energy density is approximately constant. The top portion 205 is commonly referred to as the “top hat” portion of the beam.
The system also can include multiple projection lenses to enable simultaneous scanning of multiple sections of a thin film. A system for allowing simultaneous scanning of multiple sections of a thin film is disclosed in U.S. Pat. No. 7,364,952, entitled “System and Method for Processing Thin Films.” While the method and system have been described using a dual laser source, additional lasers may be used as well.
The non-periodic laser pulse pattern is preferably obtained by the off-set firing of a plurality of lasers of the same repetition rate. As discussed above, the lasers can be controlled by a computer system to produce the pulse energy profiles depicted in FIGS. 2B-2C. As discussed above, while in the disclosed embodiments, two laser tubes are depicted, more than two laser tubes may be used for non-periodic pulse ELA. For example, three, four, five or more laser tubes, each emitting separate laser pulses may be used to provide up to three, four, five or more irradiations onto each portion of the film during each scan.
The film 170 can be an amorphous or polycrystalline semiconductor film, for example a silicon film. The film can be a continuous film or a discontinuous film. For example, if the film is a discontinuous film, it can be a lithographically patterned film or a selectively deposited film. If the film is a selectively deposited film, it can be via a chemical vapor deposition, sputtered, or a solution processed thin film, for example ink-jet printing of silicon based inks.
Non-Periodic Pulse ELA Method
FIG. 5A depicts a non-periodic pulse ELA process. FIG. 5A shows an exemplary illustration of a film that has been irradiated by two sets of two laser pulses, in which the first two laser pulses occur close together in time, followed by a delay (during which the substrate continues to move in the −y direction as indicated by arrow 980), and the second two laser pulses also occur close together in time. The process includes at least four irradiation steps, with two irradiation steps (steps 1 and 3) corresponding to pulses from a primary laser and two irradiation steps (steps 2 and 4) corresponding to pulses from a secondary laser.
FIG. 5A illustrates sequential translations of the thin film 175 of the sample 170 with respect to the pulses of the line-type beam 164 shaped by the optics of the system of FIG. 3A and/or patterned by a mask. FIG. 5B is an exploded view of region 590 in FIG. 5A. In this exemplary illustration of the irradiation of the semiconductor thin film 175 provided on the sample 170, the sample 170 is translated in a negative y− direction (arrow 980) with respect to the direction of the line-type beam 164. When the sample 170 is translated in this manner to a position such that the line-type beam 164 points at a first row 510 of the thin film 175, the beam source 110 is actuated by the computing arrangement so that a first line-type beam pulse 410 from a primary laser source 110 irradiates and at least partially melts, one or more portions 511-519 at the first row 510 of the semiconductor thin film 175. The profile and length of the first line-type pulse 410 shown in FIG. 5 substantially corresponds to the profile and length of the pulse 200 illustrated in FIG. 4. It is preferable for the width C of the top hat portion 205 of the first pulse 410 to be wide enough to irradiate and partially melt the entire cross-sections of the portions 511-519 in region 910. These portions can be designated to place certain structures (e.g., TFTs) therein so that they can be used to define the pixels. The re-solidified portions which are partially melted would likely possess small grain regions, but include relatively uniform material therein. The melted portions 511-519 re-solidify and crystallize so that they have uniform crystal grain growth therein.
Second, a second line beam pulse 410′ from a secondary laser source 110′ irradiates the thin film 175 to induce partial melting of the thin film 175. The top hat portion of the second line beam pulse 410′ irradiates a second region 920 of the thin film 175 to partially melt the entire cross sections of portions 511-519. As shown in FIG. 5, region 910 and region 920 have significant overlap and form a first crystallized region 960. In the disclosed non periodic pulse ELA process, the overlap between the first region and the second region can be greater than 70%, greater than 85%, greater than 90%, greater than 95% or greater than 99%.
After the first row 510 is irradiated and partially melted using the line-type pulses 410 and 410′ as described above, the sample 170 is translated in the negative y direction (via a control of the computing arrangement so that the beam 164 impinges on a second row 520 of the semiconductor thin film 175 provided on the sample 170. As for the first row 510 and upon reaching the second row 520, the primary laser source 110 is actuated by a computing arrangement to generate a third line-type pulse 420 from the primary laser which irradiates and either at least partially or fully melts one or more sections 521-529 in region 940 of the second row 520 in substantially the same manner as described above with respect to the irradiation of the first row 510. Then, a fourth line beam pulse 420′ from the secondary laser source 110′ irradiates the thin film 175 to induce partial melting of the thin film 175 including sections 521-529. The top hat portion of the fourth line beam pulse 420 irradiates a fourth region 950 of the thin film 175. As shown in FIG. 5, third region 940 and fourth region 950 have significant overlap to form a second crystallized region 970. In the disclosed non periodic pulse ELA process, the overlap between the first region and the second region can be greater than 70%, greater than 85%, greater than 90%, greater than 95% or greater than 99%.
This translation of the sample 170 (so that the impingement of the line-type beam 164 moves from the first row 510 to the second row 520 of the semiconductor thin film 175) is executed for a distance D. The distance D can be also referred to a pixel row periodicity or pixel pitch because the translation of the sample 170 via the distance D is performed for other rows of the sample 170.
The translation of the sample 170 with respect to the impingement thereof by the beam 164 can be performed continuously (e.g., without stopping). The computing arrangement can control the lasers 110, 110′ to generate the corresponding pulses 410, 410′, 420, 420′ based on a predefined frequency. In this manner, it is possible to define the velocity V of the continuous translation of the sample 170 with respect to the impingement of the semiconductor thin film 175 by the line-type pulses 410′, 410, 420′, 420 so that the respective rows 510, 520 of the thin film 175 are accurately irradiated by the pulses. For example, this velocity V of the translation of the sample 170 can be defined as follows: V=D×flaser where flaser is the frequency of each of the lasers. Thus, if the distance D is 200 μm and the flaser is 300 Hz, the velocity V can be approximately 6 cm/sec, which can be a constant velocity.
While the sample 170 does not have to be continuously translated with respect to the impingement thereof by the beam 164, the actuation of a primary laser source 110 and secondary laser source 110′ can be controlled based on a positional signal provided by the translation stage 180. This signal may indicate the position of the sample 170 relative to the position of the impingement thereof by the line-type beam 164. Based on the data associated with such signal, the computing arrangement can direct the actuation of the laser sources 110, 110′ and the translation to the sample 170 to achieve an effective irradiation of specific portions (e.g., rows) of the semiconductor thin film 170. Thus, the location controlled irradiation of at least portions of the semiconductor thin film 175 can be achieved using a line-type beam 164.
All four irradiations partially melt the region and the molten region then quickly solidifies to form a crystallized region. The area of the thin film 175 where the first region 910 and the second region 920 overlap forms the first crystallized region 960. The area of the thin film 175 where the third region 940 and the fourth region 950 overlap forms the second crystallized region 970.
The film velocity and the repetition rate (frequency) of the first and second laser pulses determine the location of subsequent crystallized regions on the film. In one or more embodiments, the first and second crystallized regions 960 and 970 also can overlap, in which case, as the film is scanned in the y direction, the entire film surface can be crystallized.
As shown in FIG. 5A, the first and second crystallized regions 960 and 970 do not overlap. Thus, the non-periodic pulse sequence can be used to selectively crystallize only certain regions of interest, for example, the pixel TFTs or circuits 511-519 and TFTs or circuits 521-529 in an active-matrix device such as a display or a sensor array. In this SAC embodiment, there is no overlap between the first and second crystallized regions 960 and 970. Because of the lack of overlap, the stages on which the sample is held can move at higher velocity to increase the spacing between the first and second crystallized regions 960 and 970 to match the periodicity of the matrix type electronics. Such increase in stage velocity can result in a significant increase in the overall processing throughput. For example, in the pixel array of a display, the density of electronics is rather low, for example, having a pixel pitch of several hundreds of μm or more, e.g., more than 1 mm or more, a significant increase in throughput can be achieved by only crystallizing those regions. Accordingly, the stage can be moved at faster speeds for a given laser pulse rate to accomplish full crystallization of the selected areas on the film. Exemplary values for throughput for an SAC non-periodic pulse ELA system are referenced in the Examples section of this application. Thus, non-periodic pulse SAC's throughput improvement enables more competitive throughputs for large panels, e.g. Gen8 panels ( ̃2.20×2.50 m2), such as required for large television manufacturing.
FIG. 6 depicts a similar scan to the scan shown in FIG. 5A, except that the first and third line beam pulses 1000, 1010 have a lower energy density than the second and fourth line beam pulses 1020 and 1030. This figure corresponds to the energy densities depicted in FIG. 7C. The energy densities can be range from about 20% to about 70% of the complete melting threshold. Generally, in non periodic pulse ELA, the first melting and solidification cycle can be optimized for providing the most uniform crystal structure so as to benefit the cumulative process in ELA resulting in sufficient uniformity material with low defect density. For example, the first pulse may be at an energy density higher than the complete melting threshold. Such higher energy density could for instance easily be achieved by simultaneously firing the first two pulses to result in only a single melting and solidification cycle (i.e., not distinct ones). Likewise, the first two pulses may be triggered with small delay so as to form combined pulse having longer pulse duration which may benefit the uniformity of the partially melted material even further, in particular when the starting material is a PECVD deposited a-Si film.
FIG. 7 depicts a first non-periodic pulse scan as described in FIG. 5A, and also includes a second scan in the reverse direction of the film 1100. In the first scan of FIG. 7, five regions 1110, 1112, 1114, 1116, and 1118 are irradiated as the scan proceeds in a first direction 1120. As discussed with respect to FIG. 5A each of the five regions 1110, 1112, 1114, 1116, and 1118 correspond to a region irradiated by a first line beam pulse 1122 and an area irradiated by a second line beam pulse 1124. Each irradiation results in partial melting and subsequent crystallization of the irradiated region. The overlapping region formed by the region irradiated by a first line beam pulse 1122 and the region irradiated by the second line beam pulse 1124 corresponds the first region 1110. After all five regions of the film have been irradiated in a first scan, the film is shifted in the positive x direction and a second scan occurs in the opposite direction as the first scan, in the direction of arrow 1130. A multiple scan conventional ELA technique is disclosed in WO 2010/056990 entitled “Systems and Methods for Crystallization of Thin Films.” In some embodiments, the film is not shifted in the x direction before the scan or the film can be shifted in the negative x− direction in between the first and second scans. The second scan, as shown in FIG. 7, results in irradiated regions 1132, 1134 and 1136, etc. This multiple pass scan can provide a higher quality crystallographic film. The film can be scanned one, two, three, four, five or more times.
Thus, the non-periodic pulse ELA system can be capable of executing multiple scans to reach a desired number of pulses, e.g., a four laser tube system can be used in a five scan process to reach a total of 20 pulses per unit area of the film. The technique allows for accurate control of the pulse energy sequence for each segment of the film. For example, in non-periodic pulse ELA, the first pulse in each train of pulses during the first scan may be at a lower fluence than subsequent scans. In some embodiments, the last pulses to impinge the surface may be at a lower energy density so as to induce surface melting in order to reduce the surface roughness of ELA processed films. Further, each segment of the pixel TFTs or circuits or any part thereof may have the exact same pulse energy density sequence as the irradiation thereof with beam edges can be entirely avoided. The avoidance of beam edges to impinge the areas of interest means that the cumulative process may more rapidly converge to a material with desirable uniformity and as such, the total pulse number for such material may be reduced compared to the conventional ELA process. Thus, the benefits of the method are twofold: reduced average number of pulses as a result of selective-area crystallization and reduced number of pulses in areas of interest because of reduced initial non-uniformity of the material after the first pulse as a result of avoiding irradiation with the edges of the beam.
Compared to previously discussed ELA methods, the beam width in non-periodic pulse selective-area crystallization ELA may often be less; it need only be as wide as the width of the regions to be crystallized. Hence, surplus energy is available that can be used to increase the beam length. A longer beam length can be realized using larger dimension projection lenses. Also, the beam can be split into separate optical paths so as to simultaneously crystallize multiple regions in the film during scanning of the beam pulses. Increasing the length of the processed region upon a scan can reduce the total number of scans required to fully crystallize the film.
Additionally, selective-area crystallization non periodic pulse ELA can be used to precisely align the top hat portion of the beam such that the region of interest is not irradiated by the trailing edges of the beam. Ideally, the first irradiation of the area of interest should be with the top hat portion of the beam or at least be a portion of the line beam that is all of a similar energy density above the crystallization threshold of the film. In this way, by selectively irradiating the film such that beam edges do not irradiate the region of interest on the film, the number of scans required to create the required microstructure and uniformity within the film can be reduced.
In some embodiments, optics can be used to split the beams into two or more line beams which are each directed to another column of pixel TFTs or pixel circuits (or at least, locations where later pixel TFTs or circuits are to be fabricated). In this fashion, using a beam split into two line beams double the number of pulses per unit area can be achieved so that even fewer scans are necessary to reach complete crystallization. The multitude of parallel line beams may be used to impinge on adjacent columns of pixel TFTs/circuits or may be used to impinge on non-adjacent columns. The multitude of line beams may be generated using known ways of splitting beams and directing them on separate optical tracks. Split beams also may be rejoined to travel jointly through part of the optical path, for example through the projection lenses or even immediately after splitting. The split beams may travel parallel to each other and/or under an angle slightly offset with respect to each other. Splitting the beams while maintaining beam length would result in beams that have approximately 1/mth the width, where m is the number of line beams.
Particular parameters of the non periodic pulse ELA method depend on the beam width, which can in turn depend upon the width of the region to be crystallized. For example, the size of the active-matrix device may suggest certain pixel dimension. The pixel dimensions can give rise to new pixel layouts that take advantage of the non periodic ELA processing capabilities. For example, a 55 inch display having a 660 μm pixel pitch can require crystallized regions as wide as 300 μm. Further shrinkage of pixel dimensions (for example for ultra-high definition displays) and optimization of the design towards a layout more suitable with non periodic ELA crystallization schemes, can reduce the dimension of this region to for example below 150 μm. Optimization may further include having different layouts for pixels in two adjacent columns: TFT/circuits in adjacent columns may be placed closer to each other so that they can be overlapped within a single irradiation, after which the distance traveled to next region to be irradiated may be even larger.
Aside from pixel TFTs, TFTs also may be desirable in the periphery of the display, for example to make column and row drivers. Row drivers may need to have higher performance in order to process video signals. In some embodiments, SAC provides a sufficient area of crystallized material to integrate the desired drivers in the periphery of the display. In other embodiments, the non periodic pulse ELA can be followed by separate crystallization steps to more fully crystallize the periphery of the display. This could be done using the same laser and optical path by performing conventional scanned ELA in those regions. Alternatively, this could be done using a solid state laser shaped in a narrow line beam to perform sequential lateral solidification (“SLS”) or ELA. Alternatively, a 2D projection radiation tool to perform for example 2-shot SLS (i.e., two laser pulses per unit area as shown in U.S. patent application Ser. No. 12/063,814 entitled “Systems and Methods for Uniform Sequential Lateral Solidification of Thin Films Using High Frequency Lasers,” filed Oct. 31, 2008) or dot SLS (i.e., SLS using a mask having a dot pattern as shown in U.S. Pat. No. 7,645,337 entitled “Systems and Methods for Creating Crystallographic-Orientation Controlled Poly-Silicon Films,” issued Jan. 12, 2010). Such could be integrated into the same tool to benefit from the precision stages. As used herein, an x-shot process refers to irradiating each targeted area of the film x times.
As described above, selective-area crystallization involves crystallizing only the regions of interest in for example a matrix-type electronic device or circuit. Thus, the locations of crystallized regions need to be aligned with respect to the locations of the nodes in the matrix-type electronic device or circuit. Accordingly, in order to implement SAC, sample alignment techniques should be implemented. The step of sample alignment may be achieved according to various techniques. In one technique, sample alignment may be established using a crystallization system that further has the ability to position the sample in such a manner that the sample position can be reproduced in further processing steps for making electronic devices. One common way is when the panel is provided with fiducials or alignment marks that are detected prior to crystallization and to which the crystallization process is aligned. Such methods of sample alignment are commonly used in lithographic procedures to make thin-film transistors where sub-micron accuracy is in overlaying various features of such devices. Sample alignment in SAC need not be as accurate as in lithography. For example, the crystallized region can be larger than the region of interest by several microns or ten or more micron on each side.
In another technique, sample alignment is established by detecting the location of crystallized regions prior to fabricating the electronic devices. The location may be achieved through detecting regions wherein electronics are to be placed. The regions can be detected because the change from amorphous to crystalline may be microscopically visible as a result to a change in optical properties.
A system for sample alignment can include an automated system for detecting fiducials and aligning the sample to a known position with respect to that fiducial. For example, the system can include a computing arrangement for controlling movement and responding to an optical detector that can detect the fiducials on the film. The optical detector can be, for example, a CCD camera.
Uniform Partial Melt Crystallization of PECVD Amorphous Si Films
As discussed above, partial melt crystallization techniques are those in which one or more irradiations are used to crystallize a silicon film wherein at least the last pulse does not induce complete melting of the film. In some embodiments, a partial melt flood irradiation method can be used to either produce a fine grained uniform crystalline film or to produce a precursor film for a non-periodic pulse irradiation method. The partial melt flood irradiation method can be a two shot partial melt process wherein an amorphous silicon film devoid of any pre-existing crystallites (e.g., a PECVD film) is transformed in two steps into a fine grained uniform crystalline film with grains having average lateral dimensions exceeding the film thickness. The partial melt flood irradiation method also can be an extended duration single shot partial melt process wherein an amorphous silicon film devoid of any pre-existing crystallites (e.g., a PECVD film) is transformed into a fine grained uniform crystalline film with grains having average lateral dimensions less than the film thickness.
The work of Professor James Im has shown that super lateral growth (“SLG”) may occur in single shot irradiation processes at an energy density close to the complete melting threshold in a way that “near-complete melting” occurs (Im et al, APL 63, 1993, p 1969) resulting in lateral growth of grains having low intragrain defect density. Such material may be used to create TFTs with mobilities upwards from 100 cm2/Vs. However, the TFT uniformity of this material is poor, as the grain size is very sensitive to (1) pulse energy density, (2) heterogeneities in the precursor film, and (3) if a fully amorphous film is used, the stochastic nature of the crystal nucleation process. Multiple irradiations in this SLG regime, however, can result in more uniformly sized grains. This is made possible by the formation of periodic surface roughness in the film commensurate with the wavelength of the irradiating light, resulting in a self-stabilizing process. This approach has been commercialized as ELA, most commonly using a line beam. As discussed above, the ELA process is a cumulative process wherein an initially non-uniform polycrystalline film converges to a more uniform state due to multiple radiations in the near-complete melting regime. However, the ELA process can be more efficient if the initial polycrystalline state is uniform.
As discussed above, a more uniform polycrystalline film can be obtained using a UGS system or a non periodic pulse ELA system wherein the regions of interest are not irradiated with the edges of the beam. However, even regions initially irradiated with the top hat portion of the beam can suffer from non uniformity as a result of heterogeneities in the precursor film, and, in the case of a fully amorphous film, the stochastic nature of the crystal nucleation process. The present disclosure relates to methods and systems for performing partial melt crystallization to create a uniform initial crystallized polycrystalline film that can be beneficial for increasing the efficiency of the ELA processes described above (both conventional and non-periodic pulse). In other embodiments, the obtained PMC material with enhanced uniformity may itself be used for creating thin film electronic devices without further ELA processing. This may be beneficial in situations where lower performance thin-film devices (e.g. less than 100 cm2/Vs or as low as 10 cm2/Vs) are sufficient but uniformity of the film is still critical.
Partial melt crystallization (i.e., crystallization at energy densities below the near-complete melting threshold) was previously described for amorphous Si films deposited using LPCVD in Im and Kim, Phase transformation mechanisms involved in excimer laser crystallization of amorphous silicon films, Appl. Phys. Lett. 63, (14). Oct. 4, 1993. This study indicated that the LPCVD Si films are not fully amorphous and that small crystallites exist in the films that seed the crystallization. Because of the high density of crystallites, the lateral spacing between crystallites is extremely small and crystal growth occurs predominantly in a direction perpendicular to the plane of the film. The very small size of the grains makes this material attractive for making uniform TFTs. Such a single-shot crystallization of LPCVD films is one of what is referred to as UGS methods that are performed with flood irradiation tools that are further capable of stage-synchronized radiation of laser pulses (see U.S. Patent Application Publication No. 2006-0030164 A1, entitled “Process and system for laser crystallization processing of film regions on a substrate to minimize edge areas, and a structure of such film regions,” using a two dimensional projection system and U.S. Patent Application Publication No. 2007-0010104 A1, entitled “Processes and systems for laser crystallization processing of film regions on a substrate utilizing a line-type beam, and structures of such film regions,” using a line-beam ELA system). Potentially, this can be a method for making LTPS devices with a very high throughput. Such devices are currently considered for UD-LCD TV products (e.g. approximately 2000×4000 pixels, 480 Hz and 80′′) for which amorphous silicon is concluded to be of insufficient performance level (approximately 1 cm2/Vs for n-channel a-Si TFTs compared to up to 30 or even 50 cm2/Vs for n-channel UGS TFTs).
A PMC microstructure having very small columnar grains is by no means universally achieved in this partial melt energy density regime. Studies have shown that partial melt crystallization as is presently understood cannot be reproducibly used in manufacturing of small-grain uniform LTPS TFTs. Mariucci et al. (Thin Solid Films 427 (2003) 91-95) for example shows that very heterogeneous and partially very defective materials may be obtained (defective cores surrounded by larger and cleaner grains through lateral growth).
FIG. 8A depicts an AFM scan of the surface of a film after one irradiation at the low end of the PMC regime. It shows disc shaped structures surrounded by large protrusions indicative of lateral growth and correspondingly lateral mass flow as a result of the expansion of Si upon solidification. FIG. 8B is an illustration of the crystal structure in FIG. 8A. The crystal structure in 8B has a defective core 800. This structure is the result of a low density of nucleation events that seed lateral crystallization and result in disc shaped structures. The initial growth conditions are far from equilibrium. As such the crystals are highly defective. As growth fronts move in on each other, sufficient heat is released and leads to significant reheating of the film. The reheating can result in lower defect density lateral growth.
FIG. 8C depicts an AFM scan of the surface of a film after one irradiation at a higher energy density but still in the PMC regime. FIG. 8D is an illustration of the crystal structure in FIG. 8C. Here, the further heat introduced from higher energy density radiation results in re-melting of the defective core regions that were formed in the initial stages of phase transformation. The melt threshold of the defective core region is lower than that of the low defect density outer ring and as such will melt preferentially. Re-growth at these energy densities will be seeded from the outer ring and proceed inwards. This seeding produces a small protrusion at the center as a result of the expansion of Si upon solidification. These protrusions are visible in the AFM scan in FIG. 8C. The re-melting of defective core regions may result in films that are more uniform. FIG. 8D is an illustration of the crystal structure obtained at energy densities sufficient for near complete melting of the film. Near complete melting refers to a melting scheme in which the entire film is melted except for small seed regions that remain solid. Upon solidification of the film, crystal grains grow from these unmelted grains. FIG. 8E shows a circular region that was formed upon lateral crystallization from an unmelted seed.
The secondary melting of defective core regions may be influenced by the temporal profile of the laser pulse. For example, excimer lasers available from Coherent, Inc. (Santa Clara, Calif.) tend to have a temporal profile that shows peaks in intensity. The first peak may lead to the initial explosive crystallization of the film, while the second peak may result in the selective re-melting of defective core regions formed during the initial stages. The temporal profile of the laser is known to be variable over time, especially with aging of the laser gas. Ultimately, over time, a third intensity peak may appear. Thus, while the material after core re-melting may be more uniform, it is not easily reproducible over many pulses from a laser tool. Other lasers may have only a single intensity peak and the details of re-melting within the same pulse will likely be different.
One way to improve reproducibility of this microstructure, is to irradiate the film twice. The first pulse can be optimized for obtaining the defective core material, while the second pulse can be optimized for re-melting and thus cleaning the core regions. This may be done using two scans or a step and irradiate procedure wherein two pulses are irradiated at each location before the stage steps to the next location.
The present disclosure relates to a system for providing such a two part irradiation partial melt crystallization process in a more efficient manner, namely, in a single scan. A non periodic pulse ELA system can be used to generate a first laser pulse of the two part process to obtain an intermediate microstructure having large grains but poor uniformity across the film, while a second pulse is used to clean the intermediate microstructure to create a final uniform film. The present method thus teaches the delayed triggering of the second pulse (and possibly the fluence control of first or second pulse) to achieve an optimized energy density window for the re-melted core regions. The delayed triggering has been suggested before, but then to mimic pulse duration extension, and without optical losses by mirrors. Because the pulses are close and may overlap, this means that the film is not completely cooled or possibly not even completely solidified upon arrival of the second pulse, resulting in a more efficient use of energy density. Further, the energy densities of the first and second pulses can be the same or can be different. However, because the film may not be completely cooled prior to arrival of the second pulse, the film may experience a different degree of melting from the second pulse as compared to the first pulse.
The starting films are typically around 40 nm to 100 nm thick or even up to 200 nm thick Si films on SiO2-coated glass, quartz, or oxidized Si wafers. Thinner films are generally preferred as it reduces deposition time and it lowers the energy density needed for reaching a desired level of melting. The pulses can have pulse durations around 30 ns FWHM or more for example up to 300 ns FWHM or more. Generally, shorter pulses are more efficient in melting the Si films as less heat is lost to the underlying substrate and a higher throughput may be established. Films can be irradiated over the entire partial-melting energy-density range.
In another embodiment the disc shaped regions are avoided altogether while using microcrystallite devoid films (as obtained using PECVD). The disc shaped regions can be avoided by increasing the nucleation density. A higher nucleation density can lead to more vertical crystallization processes resulting in less lateral growth and less lateral mass flow. Higher nucleation density can be achieved by shifting to longer pulse durations because with long pulse durations the amorphous Si melting front moves more slowly. As is made visible in the interface response function (“IRF”) shown in FIG. 9, (describing the velocity of the solid-liquid interface with respect to its temperature) this means the temperature thereof is more supercooled with respect to the crystalline Si melting temperature Txm. The IRF in FIG. 9 shows temperature on the x axis and velocity of the crystal front on the y axis. The solidification region is the positive y region of the graph and the melting region is the negative y region of the graph. The dotted line corresponds to amorphous silicon while the solid line corresponds to crystalline silicon.
Thus, for a long pulse 900 having slow melting characteristics, nucleation starts rapidly and at deep supercooling conditions as indicated by point 905 on the amorphous Si IRF curve. From classical nucleation theory it is known that deep supercooling results in higher nucleation rates. Thus, a large number of nuclei are formed within in a short time and before the film starts to reheat as a result of the release of heat of fusion as those nuclei start to grow (a phenomenon referred to as recalescence). This high density of nucleation substantially eliminates lateral growth in the region because the nucleation growth will occur in the vertical direction. Substantial lateral growth can create a less homogeneous structure and an uneven film surface. Thus, by using long duration pulses, which impart less energy per unit time on the film, a film can be obtained similar to those obtained with (some) LPCVD films, wherein high densities of microcrystallites pre-exist.
With a short pulse 910, on the other hand, the melt front moves rapidly and is less supercooled. The condition schematically corresponds to 915 on the IRF. While supercooling is less than with long-pulse irradiated films, it is still sufficient for nucleation to occur, albeit at a lower rate. Hence, fewer nuclei are formed in the short time interval before significant recalescence occurs resulting in further heating of the film to temperatures where further nucleation is halted. Because of the lower density of nucleation, these types of films will experience more lateral growth and will result in heterogeneous crystal growth.
The regular excimer laser pulse can be short enough to enable the short pulse scenario, while, using the 8× pulse extender (to create an approximately 300 ns FWHM pulse), pulses can be created to be long enough to move into the long pulse scenario. Alternatively, the elongated pulse may be created using multiple laser tubes each fired in short sequence to induce a single melting and solidification cycle.
Therefore, a homogeneous crystalline film can be obtained through a single pulse partial melt process by using long pulses with slow melting characteristics. This film can be used as a precursor film for a conventional or non-periodic pulse ELA process.
Complete Melt Crystallization
In another aspect, irradiation in the complete melting regime is used to generate a fine grained uniform crystalline film or to produce an initially crystallized polycrystalline film that will benefit the subsequent cumulative ELA process. Complete melt crystallization (CMC) is a technique in which single shot irradiation is used to completely melt Si films and then the film crystallizes through nucleation (see U.S. Ser. No. 10/525,288, entitled, “Process and system for laser crystallization processing of film regions on a substrate to provide substantial uniformity, and a structure of such film regions”). CMC is one of what is referred to as UGS methods that are performed with flood irradiation tools that are further capable of stage-synchronized radiation of laser pulses (see U.S. Ser. No. 10/525,297, entitled “Process and system for laser crystallization processing of film regions on a substrate to minimize edge areas, and a structure of such film regions,” using a 2D projection system and U.S. Ser. No. 11/373,772, entitled “Processes and systems for laser crystallization processing of film regions on a substrate utilizing a line-type beam, and structures of such film regions,” using a line-beam ELA system).
The presently disclosed CMC method focuses on causing heterogeneous nucleation in a thin film to form low-defect small equiaxed grain Si films. The system uses high energy density pulses, for example, greater than 1.3 to 1.4 times the complete melting threshold of the film. The processing is performed in ambient air or any oxygen containing atmosphere. The process may be performed using films having an oxide surface layer or cap layer of less than about 50 nm in thickness. The system uses relatively long pulse durations, approximately 80 ns to about 500 ns (for example, 200 ns or 400 ns) in combination with a relatively thin Si film (in the range of 100 nm to 300 nm) on SiO2 glass, quartz wafers. By selecting the parameters of the process to induce a certain desired heterogeneous nucleation scenario, instead of a homogenous nucleation scenario taught in the prior art, nucleation can be achieved at both the interface between the film and the oxide surface layer and the film and the substrate. As a result of the above parameters, low-defect density crystals can be formed.
The disclosed CMC method can be used for making low-performance LTPS devices with a very high throughput. Such devices are currently considered for UD-LCD TV products (e.g. approximately 2000×4000 pixels, 480 Hz, 80 inches) for which amorphous silicon is concluded to be of insufficient performance level (approximately, 1 cm2/Vs for n-channel a-Si TFTs compared to up to 30 or even 50 cm2/Vs for n-channel UGS TFTs).
Complete melting is known to result in a variety of nucleation-induced microstructures depending on the radiation conditions and sample configuration; a description of the process can be found in S. Hazair, et al, “Nucleation-Initiated Solidification of Thin Si Films,” Mater. Res. Soc. Symp. Proc. Vol. 979 (2007). Many of these microstructures are characterized by a large degree of heterogeneity (variable grain sizes, highly defective regions), which will result in poor device uniformity. For example, the topic of the Hazair paper is the formation of flower-like grains (flg-Si) in which a defective core region is surrounded by a ring of low-defect-density “petal”-shaped grains.
One microstructure in particular, however, appears to be an exception to this and was first described in S. R. Stiffler, M. O. Thompson, and P. S. Peercy, Phys. Rev. Lett. 60, 2519 (1988). This microstructure consists of uniformly small grains distributed throughout the thickness of the film and with a very low intragrain defect density. Such a microstructure is expected to result in good device uniformity and possibly a reasonable device performance level. This is true even for bottom-gate TFTs because, unlike many other ways of preparing small-grain Si (including deposition techniques), the crystals at/near the bottom have low defect density and larger size. However, questions remained about the mechanisms behind formation of this microstructure and hence on the required conditions to obtain this reproducibly.
The small equiaxed grain Si (seg-Si) was described by Stiffler to be the result of homogeneous nucleation, i.e., nucleation of solids throughout the bulk of the liquid as opposed to only at interfaces. Stiffler based his conclusions on a combination of transient reflectance (“TR”) data and transient conductance (“TC”) data that showed a simultaneous drop in front side reflectance and the conductance of the film. This was argued to indicate nucleation throughout the bulk of the film. For twenty years this has been the accepted model to explain the presence of grains within the bulk of the film (i.e., not bordering the surface or the bottom interface). Recently, based on TR studies, it has been discovered that Stiffler's model is inaccurate.
The present TR studies instead present a model where it was postulated that seg-Si is the result of heterogeneous nucleation (i.e., at an interface) followed by volumetric recalescense, re-melting and re-solidification of defective core structures. Thus, the initial stages of this scenario are equivalent to that leading to flg-Si, with the difference being that the defective core regions re-melt and re-solidify in low-defect-density grains to form seg-Si.
For Stiffler's data, the microstructure characterization was based on top view planar SEM TEM, and AFM images. However, this was insufficient to explain all the features in the TR data. Specifically, Stiffler's model was unable to explain the drop in the back side TR (“BTR”) occurring before the drop in the front side TR (“FTR”) which could be observed from the experiments done in the vacuum atmosphere and with the removal of native surface SiO2 layer before laser irradiation.
Presently, based on bottom view planar as well as cross sectional TEM microstructure characterization, it has been determined that such a TR drop results in the microstructure with smaller grains near the bottom area which looks like growing upward and become larger at the top of the film. On the other hand, the almost simultaneous drops of both BTR and FTR is a necessary (but not sufficient) condition for formation of the seg-Si microstructure such as it was first observed by Stiffler (and further expected to be most optimal for making uniform TFTs).
Commonly, heterogeneous nucleation is understood to take place only at the bottom interface of the film. The drop in front side TR corresponds to the onset of nucleation at the top interface (i.e., at/near the surface) of the film. Then, a simultaneous onset of nucleation on both sides of the film (as evidenced by the simultaneous drop in TR signal at both front and back side TR) results in roughly double the amount of latent heat being released back into the film and thus a much more effective/extensive re-melting/re-solidification of defective core regions. Nucleation at/near the surface requires the presence of an interface. Such an interface could for instance be with a (native) oxide. Such oxide film may be present prior to irradiation or may be formed during radiation when oxygen is present. Depending on the atmosphere it is possible that other surface reactions occur that can lead to formation of appropriate interfaces for nucleation. Furthermore, it was discovered that without such top layer (e.g., by removal of native oxide) and without the ability of formation of such top layer during radiation (e.g., by radiating under vacuum), indeed no surface nucleation occurred and no seg-Si as observed by Stiffler was formed. Finally, in some samples irradiated at relatively low energy density, a simultaneous TR signal drop was observed, yet no Stiffler seg-Si was observed. It is presently believed that this may be the result of complete re-melting of the solids formed via nucleation at the top interface. In addition, film thinner than 100 nm may also see a simultaneous TR drop, however the amount of latent heat in the volume of the film appears insufficient to result in much effective/extensive re-melting/re-solidification of defective core regions.
FIGS. 10A and 10B depict the results of recent TR studies. FIG. 10A depicts FTR and BTR for 150 nm a-Si on a glass substrate without surface oxide layer in vacuum. The bottom line in the graph 1400 is the irradiation experienced by the film. The lines above are reflectance values for different values of CMT. The x axis of FIG. 10A is time in nanoseconds, the y axis is a normalized value of reflectance. FIG. 10B is similar to 10A except 10B depicts results in air. FIG. 10B shows that BTR signal drops (series of signals located bottom in the graph above laser signal) before the drops in FTR under the energy density of 1.38 CMT in which FTR signal seems to start dropping simultaneously to the BTR. Thus, even in the no vacuum scenario, higher energy is required to obtain seg-Si microstructure. As shown in FIGS. 10A and 10B, because the reflectance difference between solid and liquid is so large, one can distinguish the onset of transformation from solid to liquid and vice versa from TR data. Heterogeneous nucleation can be inferred considering both FTR and BTR data and the resultant microstructure (shown in FIG. 11B). FIG. 11A depicts a graph of time in nanoseconds (x-axis) versus normalized reflectance values (y-axis) for a 200 nm a-Si film in air at 1.32 CMT 1500 and in vacuum at 1.4 CMT 1510. FIG. 11B is an image of the microstructure obtained in the air environment. FIG. 11C is an image of the microstructure obtain in the vacuum environment. As can be seen in the two figures, FIG. 11B shows larger crystals throughout the thickness 1520 of the film. FIG. 11C shows good crystal quality near the surface of the film, but poor, small crystals near the interface with the substrate 1540. Thus, it can be seen that true 3D seg-Si is obtained in air where the reaction at surface can happen to form oxide layer for heterogeneous nucleation at surface as well as at bottom interface, not in vacuum where heterogeneous nucleation can occur only at the bottom interface.
The present method is of special interest to fabrication of bottom-gate TFTs because unlike many other ways of preparing small-grain Si (including deposition techniques), the crystals at/near the bottom have low defect density and larger size. Thus, typical bottom-gate LTPS TFTs suffer from low mobility and perhaps also high leakage current. Fabrication of bottom gate TFTs requires formation of a patterned metal film (the gate) underneath the Si film and separated therefrom by an insulating layer (the gate dielectric). During laser irradiation, such a metal film will act as a heat sink and will result in a shift of the local complete melting threshold (CMT) energy density. It is found that the condition for reaching seg-Si formation stays the same provided this local shift in CMT is taken in account. For example, for a 100 nm thick metal separated from the silicon film by a 100 nm thick oxide film, the shift in complete melt threshold could typically be 15% to 20% higher. One condition for seg-Si formation is thus to irradiate at an energy density greater than 1.3 to 1.4 times the local CMT. Care must be taken that the energy density is not too high to cause damaging of the surrounding film that does not have heat sinks through agglomeration or ablation. For example, for a film of 100 nm thickness on top of 100 nm thick oxide on top of a 100 nm metallic gate, the film can be irradiated at 1.4 times the local complete melt threshold, or between approximately 1.61 and 1.68 times the complete melt threshold of the surrounding film, which is below the damaging threshold of the film.
The experimental conditions that Stiffler used are somewhat distinct from the conditions of the present process. Stiffler used a shorter laser pulse (30 ns vs. the disclosed approximately 80 ns) and also used more thermally conductive substrates: either SOI (Si film on thin 250 nm SiO2 on Si substrate) or Si on sapphire. Generally, homogeneous nucleation requires very rapid quenching. The conditions of the presently disclosed process including glass substrates and longer pulses result in less rapid quenching, and thus a decreased likelihood of homogeneous nucleation and an increased likelihood of heterogeneous nucleation. The oxide thickness that Stiffler used is not enough to avoid rapid cooling. Therefore, the glass substrate gives much slower cooling than Stiffler's configuration. Thus, the present method implements useful, practical conditions in which Stiffler material may be obtained by virtue of the correct understanding of what happens.
The samples created according to embodiments of the present disclosure included 100 to 300 nm Si films on SiO2-coated glass, quartz, (or also oxidized Si wafers). An excimer-laser-based system (308 nm) was used to irradiate the films at various pulse durations (30 ̃250 n sec FWHM) and energy densities. In situ analysis was performed using front-side and backside transient reflectance measurements. Characterization of the irradiated materials was conducted using TEM. See also, Yikang “Vacuum Experiment Update: Microstructure analysis” (Sep. 2, 2009).
EXAMPLES
For large-diameter TVs, the pixel pitch may be 660 μm. With a 600 Hz laser, the scan velocity may thus be  ̃40 cm/s. Such a condition could be achieved using a 0.8 J pulse shaped into a 100 μm×75 cm beam for a  ̃640 mJ/cm2 pulse assuming a 60% optical efficiency. Then, using a 4-tube laser, five overlapped scans are needed to reach complete crystallization. For a 2.2×2.5 m2 panel, the crystallization time is then three parallel scans×(250 cm/40 cm/s)×5 overlapped scans=93.75 s. Taking a five second acceleration/deceleration time, a 10 second time between parallel scans, and a 60 second loading and unloading time. The total process time is then  ̃95+5×5+2×10+60=200 s. More conservatively, a five minute process time can be assumed. Then that equals 60/5×24×30= ̃8.5 k panels/month.
A conventional 20 shot, i.e., 20 laser pulses per unit area of film, ELA process would require simultaneous triggering of the four laser tubes to get a 400 μm×75 cm beam. For 20 shots, the scan velocity would thus be 1.2 cm/s and the crystallization time would be 3×(250/1.2)=625 s. Total process time, ignoring acceleration/deceleration times, is then 625+2×10+60=705 s. More conservatively, a 12.5 minute process time can be assumed, and the throughput is then  ̃3.4 k panels/month.
While there have been shown and described examples of the present invention, it will be readily apparent to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention. By way of example, it is appreciated that advancing a thin film in a selected direction can be accomplished by holding the laser beam stationary and moving the film relative to the laser source as well as the embodiment wherein the film is stationary and the beam is moving.
Advanced Excimer Laser Annealing for Thin Films
One aspect of the disclosure relates to a new generation of laser-crystallization approaches that can crystallize Si films for large displays at increased effective crystallization rates. The particular scheme presented in this aspect of the disclosure is referred to as the advanced excimer-laser-annealing (AELA) method, and it can be readily configured for manufacturing large organic light-emitting diode (OLED) TVs (e.g., 55 inch TVs). As in ELA, AELA is mostly a partial-/near-complete-melting-regime-based crystallization approach that can, however, eventually achieve greater than one order of magnitude increase in the effective rate of crystallization than that of the conventional ELA technique utilizing the same laser source. AELA is a selective area crystallization method where regions of interest, e.g., pixel columns, are irradiated in a single shot per scan with a uniform top hat portion of a laser beam, i.e. avoiding the beam-edge area, over multiple scans. In some embodiments, multiple pixels columns are irradiated in one shot per scan. In other embodiments, only a portion of the pixel, e.g., a thin film transistor (TFT) or a capacitor, is irradiated by each shot in a scan.
The described AELA method can deliver substantial gains in the effective rate of crystallization compared to the rates that can be expected from the conventional ELA method using the same laser source. AELA can accomplish this primarily as a consequence of (1) a potentially substantial reduction in the number of pulses needed to uniformly crystallize the film, which is achieved mostly by avoiding the beam-edge area, and additionally by executing a shot-to-shot optimization of the individual pulses, (2) a potential reduction in the area of crystallization, as only a fraction of a pixel area may require it, and (3) an efficient delivery of the laser power via constant- and high-velocity-based continuous scans using the location-controlled mode of firing the laser pulses.
OLED TVs represent a substantial category of new products for the flat-panel display industry. (D. K. Flattery, C. R. Fincher, D. L. LeCloux, M. B. O'Regan, and J. S. Richard, Information Display, 27 (2011) p. 8). This opportunity, as it happens, also calls for reexamining and evaluating various TFT technologies. The long-established and dominant a-Si:H TFTs are viewed as being not entirely suitable for the application due to the issues stemming from low field-effect mobility values and device stability, and, as a result, a more OLED-suitable backplane technology is being sought for the application. Blackplane technology refers to the technology used to make the transistors and capacitors for the flat panel display.
The excimer-laser crystallization-based low-temperature polycrystalline-Si (LTPS) TFT technology, particularly in the form of manufacturing-dominant ELA method, can be identified as an obvious candidate for the task. (T. Sameshima and S. Usui, MRS Symp. Proc. 71 (1986) p. 435; J. S. Im, H. J. Kim, and M. O. Thompson, Appl. Phys. Lett. 63 (1993) p. 1969; S. D. Brotherton, Semicond. Sci. Technol. 10 (1995) p. 721; S. Uchikoga and N. Ibaraki, Thin Solid Films, 383 (2001) p. 19). In actuality, the conventional ELA method has been repeatedly criticized and dismissed over the years as being ill suited for OLED displays in general, and large OLED TVs in particular. To this end, the metal-oxide TFTs and solid-phase-based crystallization techniques have been (and continue to be) actively pursued as alternative and promising “non-laser” backplane technology candidates. (J. K. Jeong, Semicon. Sci. Technol. 26 (2011) p. 034008; C. Spinella, S. Lombardo, F. Priolo, J. Appl. Phys., 84 (1998) p. 5383; J. H. Choi, D. Y. Kim, S. J. Park, B. K. Choo, and J. Jang, Thin Solid Films. 427 (2003) p. 289).
Many of the technical issues and problems that have been previously pointed out for ELA for OLED displays can be identified as being largely eliminated. That this is so can be concluded partially from the demonstrated success of the ELA technology in high-yield volume manufacturing of high-resolution OLED screens for mobile phones and tablets. The remaining issue for ELA technology pertains to the effective rate of crystallization (which is much lower than desired at least for the large OLED TV application). This is the rate at which a unit area of the substrate panel is processed, and it eventually dictates the process throughput, the monthly capacity of a system, and the investment and operation costs associated with using the method—all critical factors that figure significantly in deciding the manufacturing competitiveness of a technique. A substantial increase in the “per-power” rate of crystallization will go a long way in terms of making the approach that much more compelling.
The AELA methods and systems of the present disclosure increase significantly the effective crystallization rate by (1) recognizing and taking advantage of the specific technical requirements that are associated with the OLED TV application, (2) identifying and eliminating a ELA problem (i.e., the “edge” problem) that prevents the ELA method from becoming increasingly and highly efficient, and (3) configuring and implementing various and proven technical elements that permit efficient delivery of the laser power in a manner that satisfies the above considerations.
OLED TVs and ELA
One general point to keep in mind in developing a backplane technology optimized for large OLED TVs is that, in general, the effective-channel mobilities of TFTs can be much lower than those required for making high-resolution mobile displays (often with integrated driver and other circuits). (J. S. Im and R. S. Sposili, MRS Bull. 21, No. 3 (1996) p. 39). Lower mobility devices are acceptable for OLED TVs because the primary purpose of these TFTs is to satisfy the current-related requirements needed to uniformly drive the OLED pixel elements.
This tolerance for reduced-mobility TFTs should be viewed as an opportunity to increase the effective crystallization rate of the ELA method. (F. Simon, J. Brune, and L Herbst, Appl. Surf. Sci. 252 (2006) p. 4402). One of the main reasons for a relatively low crystallization/power efficiency of the conventional ELA approach (where approximately 20 shots are dispensed to process an area over the entire substrate) is that these 20-or-so shots are needed in order to increase both the size and quality of the resulting grains, to enable fabrication of high-mobility TFTs required for the mobile LTPS-based displays. (F. Simon, J. Brune, and L Herbst. Appl. Surf. Sci. 252 (2006) p. 4402). Because this requirement can be relaxed for OLED TVs, it is possible to substantially reduce the number of laser shots needed for making TFTs that function at a lower, but still sufficient, level of performance.
An additional opportunity for laser crystallization of large OLED TVs can be found in the geometrical details associated with the regions that require the crystallization treatment (i.e., the areas where polycrystalline Si must be created). For example, FIG. 12 depicts a thin film 1200, with a plurality of pixel columns 1202, 1204, 1206 and 1208. Each pixel column contains a plurality of pixels 1210, for example, three. Each pixel, shown in this exemplary film, has three portions: a TFT region 1212, a capacitor region 1214 and a LED portion 1214. TFT region 1212 is a region of film 1200 where TFTs will later be formed. As discussed in more detail below, TFT region 1214 has the most stringent crystallographic quality requirements. Capacitor region 1214 is where capacitor will later be formed. As discussed in more detail below, capacitor region 1214 has less stringent crystallographic quality requirements than TFT region 1212. LED portion 1214 is the region of the pixel where the LED will be formed and need not be crystallized at all. While FIG. 12 shows three regions in pixel 1210 having three regions, some pixel arrangements have only two regions: a TFT region and a capacitor region. In these embodiments, the LED is formed on top of the TFT and capacitor regions. The methods described below apply equally to either configuration. Film 1200 can be a thin film, e.g., semiconductor films, such as silicon. Film 1200 is generally deposited onto a substrate, e.g. glass. While FIG. 12 shows TFT region 1212 adjacent to capacitor region 1214, TFT region 1212 need not abut capacitor region 1214, but can be slightly spaced apart. Further, while FIG. 12 has been described in terms of pixels, TFTs and capacitors, any device can be formed in the crystallized device formation regions of the film.
Referring to FIG. 12, one can note that only a fraction the film and then only a fraction of pixel 1210 area, whose dimensions are relatively substantial to start with, may require the treatment. (This would still be the case even at the next-generation ultra high density (UHD) resolution, regardless of the OLED devices having a bottom- or top-emission structure, and regardless of the displays using a red, green, blue (RGB)- or white-OLED-based configuration). As such, one can save much crystallization time and/or laser power by crystallizing only those regions that require crystallization, provided that such a feat can be physically accomplished in a way that efficiently utilizes the laser power (i.e., does not require an operation at substantially reduced frequencies, or throws away/wastes much of the power in the process, both of which would defeat the original purpose of increasing the rate and efficiency of crystallization in the first place). Secondly, it is furthermore the case that only a part of the total area that must be crystallized within a pixel will actually end up being used in fabricating TFTs and, therefore, needs to be composed of a TFT-suitable material; a lower-quality polycrystalline material may suffice for the other regions (within which capacitors will be fabricated), as it could meet the requirements. In such a case, one can entertain the possibility of crystallizing these capacitor regions differentially and even more power-efficiently using fewer shots.
Unfortunately, as discussed below, the aforementioned opportunities associated with large OLED TVs are not those that can be technically exploited and seized by ELA within the framework of the conventional ELA method. The kind of equipment- and process-related progress and optimization that have previously served the method well (and which incidentally are always desirable and beneficial), will not endow the method with a possibility of fully leveraging the large-OLED-display application-specific opportunities.
Crystallization at the Leading Edge of the ELA Beam
The crystallization-related issues manifested at the leading edge of the slowly scanned beam correspond to an intrinsic aspect of the conventional ELA process that ultimately limits its efficiency. As the beam is translated, a first shot-crystallized region is created at the leading edge within which (1) an abrupt border exists between an amorphous and crystallized phases (which will crystallize differently during the subsequent irradiation taking place at higher energy densities), (2) an area with ultra-small-grained microstructure is obtained (as there must be an area inevitably crystallized, predominantly via melt-mediated explosive crystallization, at the lowest possible energy density), and (3) a crystallized region with non-uniform microstructure and surface morphology inevitably exists (since the region is being irradiated with a range of energy densities), as the beam edge profile, by definition, is not uniform. (J. S. Im, H. J. Kim, and M. O. Thompson, Appl. Phys. Lett. 63 (1993) p. 1969; Q. Hu, C. S. Lee, T. Li, Y. Deng, U. J. Chung, A. B. Limanov, A. M. Chitu, M. O. Thompson, and J. S. Im, MRS Symp. Proc. 1321 (2011) p. 197). FIG. 13A is an illustration of a thin film processed via ELA. As illustrated, ELA films have periodic non-uniformities. Regions 1300 represent highly regular portions of the film, having regular grains 1302 with uniform grain sizes, e.g., 308 nm. However, ELA films have periodic non-uniformities 1304 between the highly regular regions 1300.
As a result of the aforementioned factors, the possibility of simply reducing the ELA process shot number to correspondingly increase its effective crystallization rate (by increasing the step distance between the shots) cannot be fully taken advantage of. Reducing the shot number will, for a given beam profile, inevitably lead to a physically larger presence of increasingly non-uniform regions that will be periodically manifested; while it will be a trivial exercise to obtain a material with a smaller average grain size (for generating TFTs still with a sufficient average mobility value) by simply reducing the shot number, it is the accompanying microstructural non-uniformity problem manifested more severely (as a result of the fundamentals associated with melt-mediated crystallization dynamics) at the leading edge that will ultimately limit the degree to which the effort can succeed. (J. S. Im, H. J. Kim, and M. O. Thompson. Appl. Phys. Lett. 63 (1993) p. 1969; Q. Hu, C. S. Lee, T. Li, Y. Deng, U. J. Chung, A. B. Limanov, A. M. Chitu, M. O. Thompson, and J. S. Im, MRS Symp. Proc. 1321 (2011) p. 197).
One way to recognize the extent to which the edge problem corresponds to an intrinsic and unavoidable feature of the conventional ELA method (and is actually not identical, therefore, to the often mentioned ELA issues that arise largely from various non-uniformities introduced by lasers and beams with poor characteristics), is to imagine the process being executed using a “perfect” ELA system. Consider a system consisting of (1) a laser with zero pulse-to-pulse energy fluctuation, and (2) optical components that create the incident beam with a perfectly uniform and precisely shaped spatial profile; such an imaginary system with a set of ideal specifications cannot eliminate the edge issues. (Note that a partial remedy of the situation by decreasing the slope of the edge profile is energetically not free, and does not eliminate the phase boundary and explosive crystallization factors mentioned above.) Since the conventional ELA method is configured and developed to “blanket” crystallize essentially the entire display area, any possibility of implementing selective and reduced-area crystallization at the pixel/sub-pixel level is simply not an option that can be considered by the conventional ELA method.
Advanced ELA Method: Technical Elements
As discussed above, the conventional ELA method crystallizes a-Si films by translating a long and narrow top hat shaped beam, i.e., a beam with a high aspect ratio, perpendicular to the length at a low velocity over the films relative to a film. The beam is slowly scanned such that it overlaps approximately 95 percent of the uniform top-hat portion of the width of the beam (delivering  ̃20 shots in the process) to fully crystallize the entire area of the substrate scanned by the beam. (Demanding applications that require a high degree of microstructural uniformity (e.g., OLED displays can often require even higher-shot-numbers.) With the AELA method, the following technical fundamentals that differentiate the method are incorporated and utilized in order to energy- and time-efficiently relegate the edge area of the beam into a non-critical part of the display while enabling multiple-pulse irradiation of the pre-selected regions within the pixels.
Location-Controlled Irradiation
One of the foundational technical elements of the AELA method corresponds to the location-controlled crystallization of the films. FIG. 14A is an illustration of scan of a top-hat-shaped beam 1400 over a film 1402 having a plurality of pixels 1404-1418, arranged in columns 1420-1424, each pixel having an area to be crystallized 1426-1436, according to embodiments of the present disclosure. The basic idea here, as shown in FIG. 14A, is to use an appropriately top-hat shaped beam with sufficiently narrow edges to process the critical areas of a column of pixels using the uniform part of the top-hat beam while the edge areas are positioned in the non-critical parts of the layout. The diagram illustrates how the uniform portion of a top-hat shaped beam 1400 (as discussed above) is used to “flood” irradiate a column of pixels 1422. Beam 1400 irradiates the areas to be crystallized 1426-1436 within the pixels 1404-1418 while moving at a constant high velocity. The next shot 1446 is triggered in a position controlled manner to coincide with, and irradiate, the next column 1424 to be crystallized. The position control also ensures that the edge regions of the top hat shaped beam 1400 are outside areas of interest 1426-1436, i.e., areas of interest 1426-1436 are only irradiated by the uniform portion of the top hat beam 1400. Further, each area of interest 1426-1436 is irradiated one time per scan, i.e., a one shot scan. As used herein, “shot” refers to an irradiation of a region with the film. A “scan” refers to processing the entire film through a sequence of successive “shots” of regions of interest of the film.
Top-hat shaped beam 1400 can be a line beam, generated using a laser. Beam 1400 can have a width of about 5 microns to about 100 microns, depending on the width of the area to be crystallized 1426-1436 and the pixel pitch (the spacing of pixels) of the film. In some embodiments, the width of the beam can be sized to be less than one pixel width (e.g., the width of a TFT or a capacitor); greater than one pixel width and less that than two pixels; or greater than the width of two pixels, depending on the desired irradiation scheme. The beam can be trimmed through the use of optics to minimize edge region and maximize the uniform top hat portion. As seen in FIG. 14C (as compared to FIG. 5B above), using beam trimming techniques known to those of ordinary skill in the art, beam 1450 can be trimmed such that uniform region 1452 is maximized and edge regions 1454, 1456 are minimized. Such trimming also can reduce edge regions that result in crystallization, by reducing the width of the edge region that is above the crystallization threshold 1458 of the film. Accordingly, only a small portion of the film melts and crystallizes in the non-uniform edge regions of the beam.
Area to be crystallized 1426-1436 can be the entire pixel, a TFT within the pixel, a capacitor within the pixel, both the pixel and the capacitor within the pixel, or a plurality of pixel columns (discussed below).
FIG. 14B is a schematic illustrating the translation of beam 1400 across thin film 1402, according to embodiments of the present disclosure. Because multiple pulses are needed to optimally crystallize areas of interest and each scan of the film only delivers a single shot to each area of interest, multiple back and forth scans are required to optimally crystallize areas of interest. Accordingly, as shown in FIG. 14B, a first scan 1405 is performed in one direction. One first scan 1405 is complete, the direction of movement of the film is reversed and a second scan 1460 is performed in the opposite direction. This method repeats until each area of interest is processed the requisite number of times, e.g., 2-10 times, depending on the required quality. For example, for TFTs, the area of interest for the TFT can be processed 4-10 times. For capacitors, the area of interest can be processed 2-4 times. In some embodiments, after completion of a scan, the film is returned to its starting position and the film is scanned again in the same direction as the first scan, instead of in the reverse direction.
Continuous Scan at a Constant High Velocity
Irradiating regions that may be physically apart from each other, in a location-controlled manner, can be most simply executed by precisely positioning the beam, irradiating a region, and then stepping into the next region to be irradiated, and repeating the cycle. This solution, however, leads to an enormous and unacceptable amount of time being wasted solely on accelerating and decelerating the beam/stage between the pulses. The second AELA element pertains to avoiding such a situation by irradiating the crystallization-requiring regions while moving the beam or sample continuously and constantly at a very high translation velocity (as dictated by the laser pulse frequency and the geometry of the adjacent regions to be irradiated), and using the stage-position-controlled laser-pulse firing capability to deliver the beam with the required level of spatial precision; any variation in the sample translation speed is avoided.
Multiple Shot Processing Via Multiple Scan
In order to generate polycrystalline Si films that will provide a satisfactory level of performance, flood irradiating a crystallized region with multiple shots is most likely necessary. (This should at least initially be the case; eventually and ultimately one can even envision the possibility of employing a single shot-based crystallization technique such as complete-melting crystallization (CMC) (Y. Deng, Q. Hu, U. J. Chung, A. M. Chitu, A. B. Limanov, and J. S. Im. MRS Symp. Proc. 1245 (2010) p. 257), or partial-melting crystallization (PMC) using a temporally modified/stretched pulse. (Q. Hu, C. S. Lee, T. Li, Y. Deng, U. J. Chung, A. B. Limanov, A. M. Chitu, M. O. Thompson, and J. S. Im. MRS Symp. Proc. 1321 (2011) p. 197)). The third element of the AELA method, at least for large OLED TVs, takes care of this multiple-shot requirement through multiple scanning of the regions (see FIG. 14B) while implementing the above technical elements, until the necessary number of shots is delivered to the pre-selected regions. One useful crystallization option, which can be implemented in order to improve the uniformity of the crystallized material processed using a spatially non-uniform beam, corresponds to intentionally introducing limited translation/repositioning of the beam in the line-beam parallel direction (while the beam is outside the display area between the scans), shown in FIG. 7, and in the perpendicular direction (during the irradiation of the pixel columns), so as to prevent a region from being repeatedly irradiated with a hot (or cold) spot within the beam.
AELA Films
FIG. 15A is an illustration of a grain structure of an advanced excimer laser annealing (AELA) processed film. AELA processed films 1500 have random, yet uniform microstructure of crystal grains 1502. AELA processed films have regular, uniform, equiaxed, isotropic grain patterns, as shown in both the illustration and the image. Equiaxed crystals refer to crystals that have axes of approximately the same length. Crystals of equal axial length have more plans on which to slip and thus have higher strength and ductility.
Multiple Shots Per Scan Via Sequential Firing
As discussed above, for those high-power lasers in which multiple tubes are used, one can administer multiple shots during each scan, potentially up to the number of tubes in the laser. The number of fired to the multi-shot-irradiated region during a scan can be as high as the number of laser tubes in the system, provided that each tube can produce a pulse with enough energy to generate a sufficiently long and wide beam. This can be done simply by sequentially and rapidly firing the tubes non-simultaneously as the beam is rapidly moved relative to the sample, a short and precisely controlled interval between the pulses (in the range of several tens of microseconds) means that a rather insignificant translation between these sequential shots takes place, thereby leading to multiple-shot processing of the irradiated area, even at very high AELA translation speeds. Any potential issues arising from a minute increase in the background temperature of the film/substrate can be addressed by properly adjusting the energy density of the subsequent pulse. When implemented optimally, this sequential-firing option can (1) permit efficient utilization of the prodigious amounts of power being generated from the multi-tube-based laser systems, (2) reduce the overall process time by reducing the required number of scans: (and thereby decreasing the total amount of “wasted” time associated with decelerating and accelerating the stage after the each scan), and (3) decrease the sample translation speed, making the precise-motion-control task that much more manageable. The ability to sequentially fire multiple laser tubes with a precisely controlled time interval between the pulses can also be useful in single-shot irradiation (during a scan) of separate and repeating regions that may not be equidistant from one another. As discussed below, sequential firing can be used to achieve differential crystallization.
Irradiation of Multiple Pixels
Depending on a specific combination of display- and AELA-system-related factors, it may be advantageous to process two or more columns of pixels per each shot. This is particularly, but not exclusively, the case when the pixels' dimensions are small compared to the width of the beam. Here, multiple pixel regions can be processed with each shot and the translation speeds can be controlled so that it either steps one-pixel distance per shot, or steps multiple pixel distance per each shot at proportionally higher velocities.
FIG. 16 depicts an illustration of scan of a top-hat-shaped beam 1601 over a film 1600 having a plurality of pixels 1608-1630, arranged in columns 1602-1606, where each irradiation of film 1600 covers more than one pixel column, according to embodiments of the present disclosure. For example, beam 1601 is appropriately sized to irradiate two pixel columns 1602, 1604 in one shot 1632. The second shot of the scan 1634 irradiates the next two pixel columns 1605, 1606. While FIG. 16 shows irradiation of two entire pixel columns, the location controlled beam 1601 can be sized and positioned and the film 1600 can be moved in such a manner as to irradiate portions of multiple pixel columns, instead of the entire pixel in each column, or can irradiate three, four, five or more pixel columns in one shot. It is also possible to consider power-efficiently irradiating multiple pixels simultaneously using multiple beams that are optimally shaped and positioned in accordance with the geometrical details of the areas to be crystallized.
Benefits of the AELA Method
AELA's technical elements described above can be exploited to positively and flexibly differentiate the method from the conventional ELA approach. Leveraging and optimizing these new capabilities and benefits, some of which are listed below, can ultimately lead to substantial increases in the overall crystallization rate compared to that of the conventional ELA method using an identical laser source.
Shot Number Reduction
One of the major benefits associated with the AELA method is that, with its location-controlled irradiation capability, the devices can completely avoid the problematic edge regions by simply relegating them to non-utilized or less device-critical areas of the display. This solution fully decouples the performance-uniformity correlation that prevents the conventional ELA method to take advantage of the “lower-mobility” opportunity. With AELA, it possible to obtain small (as well as large) grained and uniform polycrystalline films using significantly reduced shot numbers. In the limit of improving the laser- and beam-related characteristics, it can be possible to implement single-shot-based PMC (F. Simon, J. Brune, and L Herbst, Appl. Surf. Sci. 252 (2006) p. 4402) or two-shot AELA techniques.
Additionally, the energy-density sequence of the AELA process (i.e., energy density vs. shot number) is no longer limited and dictated by the spatial profile of the beam and translation step distance. Instead, it is now possible to consider a shot-to-shot adjustment of the irradiation parameters (e.g., energy density, pulse duration, etc). This capability bestows a previously inaccessible level of flexibility to optimize the process, in general, and to reduce the number of shots, in particular. Here, the first pulse can be identified as being the most critical shot. It is the shot that actually crystallizes the amorphous film, and it is where complex melting and solidification sequences involving all three condensed phases of Si are encountered; the details of the transitions determine the resulting polycrystalline microstructure and surface morphology that, in turn, affect the subsequent evolution of the film. It is particularly significant that it is now possible to first-shot irradiate the film at any incident energy density—including and especially at high values; this is in contrast to the ELA method in which a region at the leading edge is always inevitably first-shot crystallized at the lowest possible crystallization value, wherein melt-mediated explosive crystallization leads to the generation of an extremely defective fine-grained material. Additional and subsequent shots, which deal with partial melting of polycrystalline films also can be further adjusted to permit rapid generation of the films that will produce uniform TFTs with sufficient mobility, stability, and leakage current characteristics using a minimal number of pulses.
Differential Crystallization
With AELA, one can also consider the possibility of crystallizing different regions within a pixel area differently, and thereby further reduce the total amount of power needed to process a substrate (and increase the effective rate of crystallization in the process). For instance, as shown in FIG. 12, a typical pixel structure consists of a region occupied by a capacitor and a region, typically smaller, within which TFTs are placed. Because the microstructural quality of crystallized Si required for the capacitor is generally lower than that needed for making TFTs, if and when various technical elements are configured properly, one can benefit from substantially further reducing the number of shots for treating such an area, and benefit from commensurately increasing the effective rate of crystallization.
FIG. 17A depicts an illustration of scan of a top-hat-shaped beam 1701 over a film 1700 having a plurality of pixels 1710-1732, arranged in columns 1702-1708, where each irradiation of film 1700 covers a first portion of the pixel column 1734-1756, according to embodiments of the present disclosure. For example, beam 1701 is appropriately sized to irradiate only first portion 1734-1738 of pixel column 1702 in one shot 1760. The second shot of the scan 1762 irradiates the next first portions 1740-1744 of the second pixel column 1762. The third shot 1764 irradiates the next first portions 1746-1750 of the third pixel column 1706, and the fourth shot 1706 irradiates the next first portions 1752-1756 of the fourth column 1708 and so on, until the entirety of pixel columns are irradiated. First portions 1734-1756 can be irradiated in subsequent scans until the desired number of irradiations of first portions 1734-1756 occur.
FIG. 17B depicts an illustration of scan of a top-hat-shaped beam 1701 over a film 1700 having a plurality of pixels 1710-1732, arranged in columns 1702-1708, where each irradiation of film 1700 covers a second portion of the pixel column 1768-1790, according to embodiments of the present disclosure. For example, beam 1792 is appropriately sized to irradiate only second portion 1768-1772 of pixel column 1702 in one shot 1793. The second shot of the scan 1794 irradiates the next second portions 1774-1778 of the second pixel column 1704. The third shot 1795 irradiates the next second portions 1780-1784 of the third pixel column 1706, and the fourth shot 1796 irradiates the next portions 1786-1790 of the fourth column 1708 and so on, until the entirety of pixel columns are irradiated. Second portions 1768-1790 can be irradiated in subsequent scans until the desired number of irradiations of second portions 1768-1790 occur.
In one embodiment, the first portions 1734-1756 correspond to regions where TFTs will later be formed. In this case, first portions 1734-1756 are irradiated between about 4 times to about 10 times to create the desired crystallographic structure in first portions 1734-1756. Second portions can correspond to regions where capacitors will be formed. In this case, second portions 1768-1790 can be irradiated 1-4 times to achieve the desired crystallographic structure.
FIG. 18A depicts an illustration of scan of a top-hat-shaped beam 1801 over a film 1800 having a plurality of pixels 1810-1832, arranged in columns 1802-1808, where each irradiation of film 1800 covers a first portion of a first pixel column 1834-1838 and a first portion of a second pixel column 1840-1844, combining multiple pixel column and differential crystallization, according to embodiments of the present disclosure. For example, beam 1801 is appropriately sized to irradiate a first portion of a first pixel column 1884-1844 and a first portion of a second pixel column 1840-1844 in a first shot 1802 and a third pixel column 1846-1850 and a first portion of a fourth pixel column 1852-1856 in a second shot 1863. First portions 1834-1856 can be irradiated in subsequent scans until the desired number of irradiations of first portions 1834-1856 occur.
FIG. 18B depicts an illustration of scan of a top-hat-shaped beam 1876 over a film 1800 having a plurality of pixels 1810-1832, arranged in columns 1802-1808, where each irradiation of film 1800 covers a second portion of a first pixel column 1864-1868 and a first portion of a second pixel column 1870-1874, combining multiple pixel column and differential crystallization, according to embodiments of the present disclosure. For example, beam 1876 is appropriately sized to a second portion of a first pixel column 1864-1868 and a first portion of a second pixel column 1870-1874. Second portions 1864-1868 can be irradiated in subsequent scans until the desired number of irradiations of first portions 1864-1868 occur. Additionally, the remainders of second portions are crystallized through shots 1878 and 1880.
In one embodiment, the first portions can correspond to regions where TFTs will later be formed. In this case, first portions can be irradiated between about 4 times to about 10 times to create the desired crystallographic structure in first portions. Second portions can correspond to regions where capacitors will be formed. In this case, second portions can be irradiated 1-4 times to achieve the desired crystallographic structure.
FIG. 19A is an illustration of a grain structure of an advanced excimer laser annealing (AELA) processed film using differential crystallization, according to embodiments of the present disclosure. FIG. 19A depicts a film 1900 having a plurality of crystallized areas, for later device formation 1902, 1904. Each crystallized region 1902, 1904 includes two sub regions: a large grain region 1908, 1912 and a small grain region 1906, 1910. Both regions have uniform, regular, equiaxed, isotropic grain patterns. However, because large grain regions 1908, 1912 have been irradiate more times than small grain region 1906, 1910, large grain regions 1908, 1912 has larger grains than small grain regions 1906, 1910. Accordingly TFTs likely will formed in larger grain regions 1908, 1912 and capacitors will be formed in small grain regions 1906, 1910.
FIG. 20 is an illustration of scan of a top-hat-shaped beam over a film having a plurality of pixels, arranged in columns, for differential crystallization, according to embodiments of the present disclosure. In the method of FIG. 10, film 2000 includes device regions 2004-2014. Each device region corresponds to a region to be crystallized where a later device, such as a pixel, transistor, and/or capacitor will be formed. Each device region 2004-2014 includes two sub-regions 2016-2038. In a first pulse 2002, a beam irradiates the film 2000 in region 2040, which irradiates sub-regions 2020-2030. In a second pulse 2042, the beam irradiates film 2000 in sub-regions 2028-2038. Accordingly, after two pulses, some sub-regions are irradiated twice 2028, 2030, while other sub-regions are irradiated only once, 2020-2026 and 2032-2038. Accordingly, if one scans the entire film in this way, after one scan, the film would contain some regions that were irradiated once (small grain regions for capacitor formation) and some regions that were irradiated twice (large grain regions for TFT formation). Further, if two scans of the film were performed, some regions would be irradiated twice and some regions would be irradiated four times.
Differential crystallization can be achieved using one laser or with sequential firing techniques using multiple lasers/tubes. By controlling the translation speed of the film and the energy of the laser, sequential firing can be used to create distinct or overlapping regions, where each region has different crystalline characteristics. Accordingly, sequential firing can be used to create to separate, distinct regions with different crystalline properties or can be used to create multiple overlapping regions with different crystalline properties. For example, when using sequential firing and a first pulse from a first laser/tube overlaps a second pulse from a second laser/tube, the resultant film will have a first crystalline region corresponding to the first pulse, a second crystalline region corresponding to the second pulse and a third crystalline region corresponding to the overlap between the first and second pulses. The overlap between the first and second pulses/shots can range from minimal to almost complete overlap. Further, multiple scans can be used to achieve the required shots per area.
Reduced-Area Crystallization
An option and opportunity to crystallize a reduced area fraction of the film using AELA presents one with a route to increase the effective rate of crystallization. As alluded before, this is possible because (1) the pixels are spatially arrange in a repetitive periodic manner, (2) the pixel dimensions for large OLED TVs are no longer minuscule, (3) it may be feasible and sufficient to uniformly crystallize only a fraction of the pixel area, and (4) the laser power can be distributed in AELA only to those selected areas with a negligible amount of power loss while operating at high power levels.
As can be inferred from the benefits listed above, much can be gained, at least in the case of OLED TVs, by breaking away from the well-established and long-refined formula of the conventional ELA method. The AELA method relates to delivering the minimal number of uniformly shaped and precisely tailored beam pulses only to those locations that need to be uniformly crystallized. The main contribution of the AELA method over that of ELA can be distilled in terms of its superior productivity; it can deliver a higher level of productivity than can be achieved using ELA with an identical laser.
AELA Systems
One aspect of the AELA method has to do with the way in which it can be implemented, and various potential ways through which the approach and the displays technology can further evolve and co-evolve. Many of the technical elements and capabilities, which are needed for implementing AELA, have been already developed and tested in various LTPS manufacturing environments. (F. Simon, J. Brune, and L Herbst, Appl. Surf. Sci. 252 (2006) p. 4402; J. S. Im, A. B. Limanov, P. C. van der Wilt, U. J. Chung, A. M. Chitu, Information Display, 9 (2007) p. 14). Given this background, a first-generation AELA system can be configured and developed using the laser and optical components employed in the conventional ELA systems (with the top-hat shaped beams that are  ̃75 cm in length and several hundred microns in width), and the high-velocity motion-control technology that is being utilized in Gen 4 manufacturing sequential-lateral solidification (SLS) systems. The air-bearing linear-motor stages used in the SLS systems capable of precise high-velocity translation have already been developed for handling larger substrates for other applications, and are already available for Gen 8 substrates. An additional factor of significance is that the AELA system is fully backward compatible; it is straightforward to use an AELA system to execute ELA (either selectively within a substrate, or to process the entire substrate). This backward compatibility should greatly reduce the risks associated with adopting this new approach. These factors should essentially eliminate the prolonged initial development/evaluation period, and enable AELA to be readily accommodated as a new manufacturing method. Finally, it should be noted that AELA can be optimized—mostly by irradiating the films at higher energy densities—to also provide uniform large-grained polycrystalline films for making high-mobility TFTs in the range of  ̃100 to 200 cm2/Vsec; it should be possible to accomplish this, as a consequence of the reasons mentioned previously, at higher effective crystallization rates/reduced shot numbers compared to ELA. The availability of such high-performance TFTs via AELA means that for any possible product and manufacturing roadmap scenarios that would require or benefit from the availability of such high-mobility TFTs (e.g., high-brightness, high-resolution, and high-refresh-rate LCDs and OLED displays) can be fully addressed by AELA; as it stands, basically all other “non-laser” approaches, including those that utilize metal-oxide TFTs, direct deposition, or solid-phase-crystallization techniques, presumably cannot accommodate these high-mobility-TFT-requiring roadmap scenarios so readily. All in all, the various factors enumerated above should assist in essentially eliminating a prolonged initial development/evaluation period, and enable AELA to be readily implemented as a new manufacturing method.
For example, FIG. 21 depicts an exemplary AELA system. FIG. 21 shows substantially a similar system as used for non-periodic pulse ELA shown in FIG. 3A, except that only one excimer laser 110 is used. In some embodiments stage 180 is a high-velocity motion-controlled SLS stage.
In general, the film itself is not required to move during crystallization; the laser beam or a mask defining the laser beam shape could be scanned across the film instead to provide a relative motion of the irradiated region and the film. However, moving the film relative to the laser beam can provide improved uniformity of the laser beam during each subsequent irradiation event.
Once initiated by leveraging what is already available, the AELA method should be able to keep the momentum of progress by leveraging what can further be accomplished. This can involve optimizing various aspects of the AELA- and display-related particulars. Here, one can envision how (1) crystallized microstructure/TFT performance, (2) device, circuit, and pixel layout, and (3) laser- and beam-related details can be all optimized to sustain the continued increases in the productivity of the AELA method. When these factors are optimally designed to fully leverage the fundamental advantages and flexibility offered by AELA, it is possible to estimate using a set of realistic parameters that greater than one order of magnitude increase in the rate of effective crystallization can be achieved (when compared to that accomplished by using the conventional ELA method based on an identical laser source). As the effective crystallization rate increases, the crystallization system throughput increasingly becomes burdened by other factors such as the time it takes to load and unload the substrates. With AELA, the crystallization time may decrease sufficiently to warrant a multi-chamber/single-laser-configured system to remedy the situation.
Although AELA is quite flexible, and is certainly well matched for making large OLED TVs, the degree to which the method can be used for smaller displays at higher pixel densities remains unclear. AELA can be a viable method for manufacturing high-resolution tablet/notebook LCD/OLED screens. (It should at least be more effective than the conventional ELA method, which is presently considered by many as being the most volume-manufacturing-compatible option for producing these displays.) For these products, multi-pixel processing using a many-pixel-wide beam with sharply defined edges may be one way to at least partially implement, and commensurately partially benefit from, some elements of AELA. Alternatively, the situation may be well suited for implementing AELA using thin-beam excimer laser crystallization systems that employ high power lasers with high frequencies at reduced per-pulse energies. (It should be noted that both the thin-beam-based and projection-based excimer laser crystallization methods could also be configured to boost the throughput by performing reduced-area crystallization only of selected regions.) (D. S. Knowles, J. Y. Park, C. Im, P. Das, T. Hoffman, B. Burfeindt, H. Muenz, A. Herkommer, P. C. van der Wilt, A. B. Limanov, and J. S. Im, Proc. SID 36, (2005) p. 503; R. S. Sposili and J. S. Im, Appl. Phys. A. A67 (1998) p. 273; R. S. Sposili and J. S. Im, Appl. Phys. Lett. 69 (1996) p. 2864; J. S. Im, M. A. Crowder, R. S. Sposili, J. P. Leonard, H. J. Kim, J. H. Yoon, V. V. Gupta, H. J. Song, and H. S. Cho, Phys. Stat. Solidi A, 166, 603 (1998)).
Even though it may not be as influential and significant as the leading edge, the trailing edge of the beam can also be of concern, as it can additionally introduce a degree of microstructural and/or surface-morphology-related non-uniformities; with AELA, these issues are trivially and preemptively addressed. For the multiple-column-wide beams, the trailing edge can be managed by appropriately adjusting the widths of the beam and the edge.
Finally, the advantages and capabilities afforded by an AELA system need not be confined to crystallization. The essence of the system lies in its capability to efficiently deliver well-defined amounts of power at, and only at, those regions that need to be irradiated. As such, it may be possible to consider the use of the systems to other potential processes and applications (e.g., laser doping and activation, laser patterning and ablation, etc.).
Location Controlled ELA Techniques
For small screen applications, for example, tablets or phones, the pixel pitch is smaller, making selective area crystallization using AELA more difficult. Accordingly, for these applications, it is generally preferred to crystallize the entire film. However, with existing ELA technologies, the resultant film can have periodic non-uniformities, as shown in FIGS. 13A and 13B, which can result in poor quality displays. Accordingly, a new ELA method. ELA+ has been developed to eliminate the non-uniformity issues with ELA and reduce the number of shots to the film. ELA+ uses a sharply defined beam, as shown in FIG. 14C, to do location controlled ELA across the entire film. ELA+ is location controlled because while ELA results in uncontrolled non-uniformities, in ELA+, the non-uniformities can be strategically placed, such that later formed pixels will not be placed in the non-uniform regions of the film. This can be accomplished by using a beam sized greater than the width of two device formation regions, for example, two, three, four or up to the width of ten device formation regions, and carefully controlling the regions of overlap between a first irradiation of the beam and a second irradiation of the beam.
FIG. 22 is an illustration of scan of a top-hat-shaped beam over a film 2200 having a plurality of pixels, arranged in columns 2202-2214, according to embodiments of the present disclosure. The beam 2216 is sized such that in one shot 2222, three pixel columns 2202-2206 are irradiated. Then, the beam is advanced (either through movement of film 2200 or the beam itself) and in a second pulse 2218, the next three pixel columns 2208-222 are irradiated. A third shot, 2220, irradiates column 2214 and the next two columns, and so on until the entire film is irradiated. By sizing the beam to the pixel pitch of the film, regions containing pixels are uniformly irradiated and the non-uniform regions are located in between shots/pulses 2224, 2226.
The above described film can be any type of semiconductor or oxide film amenable to laser irradiation processing. For example, the film can be composed of silicon, germanium, oxides, or composite semi-conductors (e.g., gallium arsenide). Exemplary oxide films include titanium oxide and zinc oxide films, wherein the zinc oxide films can include gallium indium zinc oxide. In some embodiments, the irradiation of the oxide film can be modified to adjust the parameters of melting and solidification of the oxide film or to result in a solid state annealing process with no melting of the film.
While there have been shown and described examples of the present invention, it will be readily apparent to those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention. Those skilled in the art will recognize, or be able to ascertain, using no more than routine experimentation, numerous equivalents to the specific compositions and procedures described herein. Such equivalents are considered to be within the scope of this invention, and are covered by the following claims.

## Claims

1. A method for crystallizing a thin film, the method comprising:
providing a thin film having a plurality of device formation regions;
generating a line beam having a uniform portion and two edge portions, while continuously translating the thin film semiconductor in a first direction, irradiating a first device formation region with the uniform portion of the line beam having a fluence that is sufficient to partially melt the thin film and the first region re-solidifying and crystallizing to form a first crystallized region; and
irradiating a second device formation region, distinct from the first device formation region, with the uniform portion of the line beam having a fluence that is sufficient to partially melt the thin film and the second region re-solidifying and crystallizing to form a second crystallized region;
wherein the first and second crystallized portions comprise uniform, isotropic crystal grains and the first crystallized portion has larger crystal grains than the second crystallized portion.

2. The method of claim 1, wherein the first device formation region overlaps the second device formation region.

3. The method of claim 1, comprising continually irradiating successive device formation regions until the each device formation region has been irradiated at least once.

4. The method of claim 1, comprising, after each device formation region has been irradiated at least once, continuously translating the film in a second direction, opposite the first direction while irradiating successive device formation regions.

5. The method of claim 1, comprising, after each device formation region has been irradiated at least once, translating the film back to a starting position and continuously translating the film in the first direction, irradiating successive device formation regions.

6. The method of claim 1, comprising, after each device formation region has been irradiated at least once, shifting the film in a direction perpendicular to the first direction.

7. The method of claim 1, comprising, after each device formation region has been irradiated once, shifting the film in a direction parallel to the first direction.

8. The method of claim 1, comprising irradiating each device formation region at least twice.

9. The method of claim 1, comprising irradiating each device formation region at least four times.

10. The method of claim 1, comprising irradiating each device formation region at least six times.

11. The method of claim 1, wherein the device formation region comprises a region where a pixel is to be formed.

12. The method of claim 1, wherein the device formation region comprises a region where a thin film transistor is to be formed.

13. The method of claim 1, wherein the device formation region comprises a region where capacitor is to be formed.

14. The method of claim 1, comprising forming a capacitor in a capacitor region adjacent to the first device formation region.

15. The method of claim 1, wherein the device formation region comprises a region where a column of thin film transistors are to be formed.

16. The method of claim 1, wherein the device formation region comprises a region where a plurality of columns of devices are to be formed.

17. The method of claim 1, wherein the device formation region comprises a portion of a first column of devices and a portion of a second column of devices, wherein the portion of a first column of devices is proximal to the portion of a second column of devices.

18. The method of claim 1, wherein the device formation region comprises a region in a first column of pixels and a region in a second column of pixels.

19. The method of claim 1, wherein the line beam has a fluence sufficient to partially melt the film.

20. The method of claim 1, wherein the line beam has a fluence sufficient to completely melt the film.

21. The method of claim 1, wherein the line beam has a fluence sufficient to near completely melt the film.

22. The method of claim 1, wherein the line beam has a fluence below the partial melting threshold, but sufficient to modify the structure of the thin film.

23. The method of claim 1, wherein the film comprises a semiconductor film.