# Patent

## Title
Surface planarization of thin silicon films during and after processing by the sequential lateral solidification method

## Abstract

Systems and methods for reducing a surface roughness of a polycrystalline or single crystal thin film produced by the sequential lateral solidification process are disclosed. In one arrangement, the system includes an excimer laser (110) for generating a plurality of excimer laser pulses of a predetermined fluence, an energy density modulator (120) for controllably modulating the fluence of the excimer laser pulses such that the fluence is below that which is required to completely melt the thin film, a beam homoginizer (144) for homoginizing modulated laser pulses in a predetermined plane, a sample stage (170) for receiving homoginized laser pulses to effect melting of portions of the polycrystalline or single crystal thin film corresponding to the laser pulses, translating means for controllably translating a relative position of the sample stage (170) with respect to the laser pulses, and a computer (110) for coordinating the excimer pulse generation and fluence modulation with the relative positions of the sample stage (170) to thereby process the polycrystalline or single crystal thin film by sequential translation of the sample stage (170) relative to the laser pulses.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional of U.S. patent application Ser. No. 09/979,201, filed Feb. 4, 2002, now U.S. Pat. No. 6,830,993, issued Dec. 14, 2004, which is a national stage of International Application PCT/US00/07479, filed Mar. 21, 2000, each of from which priority is claimed.


NOTICE OF GOVERNMENT RIGHTS
The U.S. Government has certain rights in this invention pursuant to the terms of the Defense Advanced Research Project Agency award number N66001-98-1-8913.


BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to techniques for semiconductor processing, and more particularly to semiconductor processing which may be performed at low temperatures.
II. Description of the Related Art
In the field of semiconductor processing, there have been several attempts to use lasers to convert thin amorphous silicon films into polycrystalline films. An overview of conventional excimer laser annealing technology is presented by James Im et al. in “Crystalline Si Films for Integrated Active-Matrix Liquid-Crystal Displays,” 11 MRS Bulletin 39 (1996). In systems used for carrying out excimer laser annealing, an excimer laser beam is shaped into a long beam which is typically up to 30 cm long and 500 micrometers or greater in width. The shaped beam is scanned over a sample of amorphous silicon to facilitate melting thereof and the formation of polycrystalline silicon upon resolidification of the sample.
The use of conventional excimer laser annealing technology to generate polycrystalline or single crystal silicon is problematic for several reasons. First, the silicon generated in the process is typically small grained, of a random microstructure, and/or has non-uniform grain sizes, which result in poor and non-uniform devices that lead to low manufacturing yield. Second, the processing techniques needed to obtain acceptable performance levels require that the manufacturing throughput for producing polycrystalline silicon be kept low. Also, these processes generally require a controlled atmosphere and preheating of the amorphous silicon sample, which lead to a further reduction in throughput rates. Finally, the fabricated films generally exhibit an unacceptable degree of surface roughness that can be problematic for performance of microelectronic devices.
There exists a need in the field to generate higher quality polycrystalline silicon and single crystal silicon at greater throughput rates. As well, there exists a need for manufacturing techniques that reduce the surface roughness of such polycrystalline and single crystal silicon thin films to be used in the fabrication of higher quality devices, such as flat panel displays.
SUMMARY OF THE INVENTION
An object of the present invention is to provide techniques for planarizing the surfaces of polycrystalline and single crystal thin film semiconductors.
A further object of the present invention is to provide surface planarization techniques that may be applied as a post processing step to polycrystalline and single crystal thin film semiconductors that are produced during a sequential lateral solidification process.
Yet a further object of the present invention is to provide surface planarization techniques that may be applied as a processing step during the production of polycrystalline and single crystal thin film semiconductors in a sequential lateral solidification process.
Yet another object of the present invention is to provide techniques for the fabrication of high quality semiconductors devices useful for fabricating displays and other products.
In order to achieve these objectives as well as others that will become apparent with reference to the following specification, the present invention provides systems and methods for reducing surface roughness of a polycrystalline or single crystal thin film that had previously been produced by the sequential lateral solidification process. In one arrangement, the system includes an excimer laser for generating a plurality of excimer laser pulses of a predetermined fluence, an energy density modulator for controllably modulating the fluence of the excimer laser pulses such that the fluence is below that which is required to completely melt the thin film, a beam homogenizer for homogenizing modulated laser pulses in a predetermined plane, a sample stage for receiving homogenized laser pulses to effect partial melting of portions of the polycrystalline or single crystal thin film corresponding to the laser pulses, translating means for controllably translating a relative position of the sample stage with respect to the laser pulses, and a computer for coordinating the excimer pulse generation and fluence modulation with the relative positions of the sample stage to thereby process the polycrystalline or single crystal thin film by sequential translation of the sample stage relative to the laser pulses. The excimer laser is preferably an ultraviolet excimer laser for generating ultraviolet excimer laser pulses.
In one arrangement, the beam homogenizer is operable to shape laser pulses with a tophat profile in both the x and y directions. The energy density modulator is operable to attenuate fluence of the excimer laser pulses to approximately 25% to 75% of the full melt threshold of the polycrystalline or single crystal thin film.
The translating stage advantageously includes an X direction translation portion and a Y direction translation portion, each being coupled to the computer and to each other and permitting movement in two orthogonal directions that are perpendicular to a path formed by the laser pulses, and being controllable by the computer for controllably translating the sample in both of said translatable directions under control of said computer. Also, the beam homogenizer is operable to shape said laser pulses with a tophat profile in both the x and y directions, and the translating means is operable to translate the polycrystalline or single crystal thin film in two directions orthogonal to a direction of said laser pulses such that sequential homogenized laser pulses are incident on slightly overlapping regions of the polycrystalline or single crystal thin film in the two directions.
In an alternative arrangement, the present invention provides for systems and methods for processing an amorphous silicon thin film sample into a single or polycrystalline silicon thin film having a reduced surface roughness. In one arrangement, the method includes forming a rigid cap layer on an amorphous silicon thin film sample having sufficient thickness to withstand contractions and expansions during melting and resolidification of the silicon thin film during the sequential lateral solidification process. The method also includes generating a sequence of excimer laser pulses; controllably modulating each excimer laser pulse in the sequence to a predetermined fluence; homogenizing each modulated laser pulse in the sequence in a predetermined plane; masking portions of each homogenized fluence controlled laser pulse in the sequence to generate a sequence of fluence controlled pulses of patterned beamlets, irradiating the amorphous silicon thin film sample with the sequence of fluence controlled patterned beamlets to effect melting of portions thereof; controllably sequentially translating the sample relative to each of said fluence controlled pulse of patterned beamlets to thereby process the amorphous silicon thin film sample into a single or polycrystalline silicon thin film having a reduced surface roughness; and removing said cap layer from the processed single or polycrystalline silicon thin film.
The accompanying drawings, which are incorporated and constitute part of this disclosure, illustrate a preferred embodiment of the invention and serve to explain the principles of the invention.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a functional diagram of a system for performing the sequential lateral solidification process preferred to implement a preferred process of the present invention;
FIG. 2 is a chart showing the surface profile of a typical film which has been processed by the sequential lateral solidification system of FIG. 1;
FIG. 3 is a functional diagram of a preferred system for planarizing the surface of a polycrystalline or single crystal thin film semiconductor produced during a sequential lateral solidification process in accordance with the present invention;
FIGS. 4a and 4b are illustrative diagrams of a crystallized silicon film to be processed by the system of FIG. 3 using a narrow beam;
FIG. 5 is an illustrative diagram of a crystallized silicon film to be processed by the system of FIG. 3 using a wide beam;
FIGS. 6–7 are charts showing the surface profile of a typical film before and after processing by the system of FIG. 3;
FIG. 8 is an illustrative diagram of a cross section of a crystallized silicon film processed by the system of FIG. 1 in accordance with a second embodiment of the present invention;
FIG. 9 is a chart showing the surface profile of a typical film which has been processed in accordance with the second embodiment of the present invention.
FIG. 10 is a flow diagram illustrating the steps implemented in the system of FIG. 3 in accordance with the first embodiment of the present invention; and
FIG. 11 is a flow diagram illustrating steps implemented in the system of FIG. 1 in accordance with the second embodiment of the present invention.



DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention provides techniques for planarizing the surfaces of polycrystalline and single crystal thin film semiconductors. In the preferred embodiments, the surface planarization techniques are applied as a post processing step to polycrystalline and single crystal thin film semiconductors that are produced during a sequential lateral solidification process, or as a processing step during the production of polycrystalline and single crystal thin film semiconductors in a sequential lateral solidification process. Accordingly, in order to fully understand those techniques, the sequential lateral solidification process must first be appreciated.
The sequential lateral solidification process is a technique for producing large grained silicon structures through small-scale unidirectional translation of a silicon sample in between sequential pulses emitted by an excimer laser. As each pulse is absorbed by the sample, a small area of the sample is caused to melt completely and resolidify laterally into a crystal region produced by the preceding pulses of a pulse set.
A particularly advantageous sequential lateral solidification process and an apparatus to carry out that process are disclosed in our co-pending patent application Ser. No. 09/390,537, filed Sep. 3, 1999, entitled “Systems and Methods using Sequential Lateral Solidification for Producing Single or Polycrystalline Silicon Thin Films at Low Temperatures,” the disclosure of which is incorporated by reference herein. While the foregoing disclosure is made with reference to the particular techniques described in our co-pending patent application, it should be understood that other sequential lateral solidification techniques could readily be adapted for use in the present invention.
With reference to FIG. 1, our co-pending patent application describes as a preferred embodiment a system including excimer laser 110, energy density modulator 120 to rapidly change the energy density of laser beam 111, beam attenuation and shutter 130, optics 140, 141, 142 and 143, beam homogenizer 144, lens system 145, 146, 148, masking system 150, lens system 161, 162, 163, incident laser pulse 164, thin silicon film sample 170, sample translation stage 180, granite block 190, support system 191, 192, 193, 194, 195, 196, and managing computer 100 X and Y direction translation of the silicon sample 170 may be effected by either movement of a mask 710 within masking system 150 or by movement of the sample translation stage 180 under the direction of computer 100.
As described in further detail in our co-pending application, an amorphous silicon thin film sample is processed into a single or polycrystalline silicon thin film by generating a plurality of excimer laser pulses of a predetermined fluence, controllably modulating the fluence of the excimer laser pulses, homogenizing the modulated laser pulses in a predetermined plane, masking portions of the homogenized modulated laser pulses into patterned beamlets, irradiating an amorphous silicon thin film sample with the patterned beamlets to effect melting of portions thereof corresponding to the beamlets, and controllably translating the sample with respect to the patterned beamlets and with respect to the controlled modulation to thereby process the amorphous silicon thin film sample into a single or polycrystalline silicon thin film by sequential translation of the sample relative to the patterned beamlets and irradiation of the sample by patterned beamlets of varying fluence at corresponding sequential locations thereon.
While the sequential lateral solidification process is highly advantageous to produce single crystal or large grained polycrystalline silicon thin films, the produced crystals often exhibit a surface roughness due to the irrative nature of the melting and resolidification inherent in the crystal growth process. Thus, as shown in FIG. 2, a 200 nm thick crystal will exhibit variations in height throughout the length of the crystal. In FIG. 2, a height of 0 indicates the optimal height in a 200 nm thick crystal, and heights varying from 175 to 225 nm are shown to be common throughout the length of the crystal. Note the large bump 210 near the crystal boundary, where crystal thickness exceeds the optimal 200 nm thickness by 350 nm.
Referring to FIGS. 3 and 4, a first embodiment of the present invention will now be described. FIG. 3 illustrates a post processing system embodiment for planarizing polycrystalline and single crystal thin film semiconductors produced by the sequential lateral solidification process. The system includes an excimer laser 310, beam attenuator and shutter 320, reflecting plate 330, telescoping lenses 331, 332, reflecting plate 333, beam homogenizer 340, condensing lens 345, reflecting plate 347, field lense 350, sample 360, sample translation stage 370, optical table 380, and managing computer 300. A preferred laser 310, attenuator 320, telescoping lenses 332, 332, homogenizer 340, and sample translation stage 370 that is movable in two orthogonal directions are each described in the co-pending patent application Ser. No. 09/390,537. The table 380 may be as described in that patent document, or may be an ordinary table. It is preferable that the homogenized beam 346 be shaped with a tophat profile in both the x and y directions, and essential that the beam energy density is below that required to completely melt the sample 360.
With reference to FIGS. 4a and 4b, the sample 360 is shown in greater detail. Since the sample in this embodiment has already been processed, it already includes a large number of single crystal regions, shown illustratively as chevron shaped crystals 365. The homogenized beam 346 is shown incident upon a portion 361 of sample 360 to induce partial melting thereof.
For a 200 nm thick silicon thin film, the full melt threshold is approximately 600 mJ/cm2. Thus, to induce sufficient partial melting of the portion 361, a beam 346 having an energy that is approximately 25% to 75% of the full melt threshold should be utilized. If the beam is more energetic, energy fluctuations inherent in excimer lasers create the possibility of causing a full melt of the sample region 361. If the beam is less energetic, the sample portion 361 will not melt sufficiently to satisfactorily planarize.
As shown in FIG. 4b, the sample 360 includes a silicon oxide base layer 400 and a silicon layer 410. In accordance with the present invention, the outer surface of silicon layer 410 is caused to melt to a depth 420. Upon resolidification, the rough surface 430 is reformed in a more planarized manner.
While a single homogenized beam pulse having an energy that is approximately 25% to 75% of the full melt threshold is sufficient to induce partial melting of the region 361, it is preferred that multiple beam pulses are caused to irradiate every such region. Each subsequent beam pulse will induce partial melting of the region 361, which upon resolidification will exhibit a more planarized surface. Thus, the use often beam pulses per region 361 will produce a far smoother surface 430 than would the use of a single pulse.
Returning to FIG. 4a, the sample stage 370 is translated, under the control of computer 300, from right to left to cause the homogenized beam 346 to scan the sample 360 from left to right 450 on the top of sample 360. The stage 370 is then moved in an orthogonal direction (shown as the Y direction) to realign the sample at a new position 460, and translation in the opposite direction is began 470. This processes is repeated until the entire surface of sample 360 has been scanned by the homogenized beam 346.
When the sample stage is translated in the Y direction, it may be advantageous to align the homogenized beam to slightly overlap a previously scanned region of the sample 360. Thus, if the region 361 is 1.2×1.2 cm, Y direction translation of 1.15 cm may be utilized to avoid edge effects caused by irregularities in the homogenized beam. Likewise, it is advantageous to cause a slight overlap with X-direction translation is being effected.
While the foregoing has been described with respect to a tophat profile square homogenized beam, beams of other shapes may be utilized. Thus, as shown in FIG. 5, a wide homogenized beam 500 which is sufficiently wide to eliminated the need for X direction translation may be utilized, with the benefit of necessitating less movement by the translation stage 360, and adoringly, greater throughput. Likewise, a beam that is shaped with a Gaussian profile in the X direction could be utilized if greater overlaps between X translations are performed.
As shown in FIG. 6–7, the results of the process described with reference to FIGS. 3–4a are illustrated. The profile of a sample 360 fabricated in accordance with the sequential lateral solidification process is shown in FIG. 6a. The sample exhibits surface irregularities of +/−25 nm from the optimal 200 nm height. As shown in FIG. 6b, after post processing with a single laser pulse in accordance with the present invention, those surface irregularities are markedly reduced. These results are alternatively illustrated in FIG. 7, where it is shown >100% decrease in surface roughness caused by post processing in accordance with the invention herein.
Referring next to FIG. 8, a second embodiment of the present invention will now be described. In this embodiment, the surface of silicon thin film is kept planarized through the employment of a rigid cap layer during the sequential lateral solidification process. Thus, FIG. 8 shows a thin silicon sample formed of an approximately 50–200 nm thick amorphous silicon layer 810 deposited on a silicon oxide base layer 820. The sample is capped with a thick second silicon oxide layer 820, approximately 2 microns thick, which is substantially rigid. The cap layer must be sufficiently thick to withstand the contractions and expansions during melting and resolidification of the silicon layer during the sequential lateral solidification process.
The sample with cap layer 830 are then used in place of sample 170 in the lateral solidification process, a complete description of which is contained in the above mentioned patent application Ser. No. 09/390,537. After such processing, the cap layer 830 is removed from the sample by traditional wet or dry etching techniques. As shown in FIG. 9, the results of the process described with reference to FIG. 8 is illustrated.
Referring to FIG. 10, the steps executed by computer 300 to control both the sequential lateral solidification process of FIG. 1 and the surface planarization process implemented with respect to FIG. 3 will be described. The various electronics of the system are initialized 1000 by the computer 300 to initiate the process. A sample is then loaded onto the sample translation stage 1005. It should be noted that such loading may be either manual or robotically implemented under the control of computer 300. Next, the sample is processed in accordance with the sequential lateral solidification process using the apparatus of FIG. 1 1010. The processed sample is positioned for planarization 1015. The various optical components of the system are focused 1020 if necessary. The laser is then stabilized 1025 to a desired energy level and reputation rate, as needed to partially melt the sample in accordance with the teachings of the present invention. If necessary, the attenuation of the laser pulses is finely adjusted 1030.
Next, translation of the sample is commenced 1035 at a predetermined speed and in a predetermined direction, in accordance with the previously sequential lateral solidification processed regions of the sample. The shutter is opened 1040 to expose the sample to irradiation and accordingly, to commence the planarization process.
Sample translation and irradiation continues until planarization has been competed 1045, 105, at which time the computer closes the shutter and stops translation 1055, 1060. If other areas on the sample have been designated for planarization, the sample is repositioned 1065, 1066 and the process is repeated on the new area. If no further areas have been designated for planarization, the laser is shut off 1070, the hardware is shut down 1075, and the process is completed 1080.
Referring next to FIG. 11, the steps executed by computer 100 to control the crystal growth process with the surface planarization steps implemented with respect to FIG. 1 will be described. FIG. 10 is a flow diagram illustrating the basic steps implemented in the system of FIG. 1 using a capped sample as illustrated in FIG. 8. An oxide layer is deposited on a base 1100. A silicon layer is then deposited on the oxide buffer layer 1110, and a cap oxide is deposited at the top layer of the sample 1120.
Next, the sample is processed in accordance with the sequential lateral solidification process using the apparatus of FIG. 1 1030. After processing, the cap oxide is removed, e.g., by a dilute hydrofluoric acid solution.
The foregoing merely illustrates the principles of the invention. Various modifications and alterations to the described embodiments will be apparent to those skilled in the art in view of the teachings herein. For example, while removal of the cap layer had been disclosed with respect to use of a dilute hydrofluoric acid solution, the cap layer may be removed by any. conventional technique such as dry etching. It will thus be appreciated that those skilled in the art will be able to devise numerous systems and methods which, although not explicitly shown or described herein, embody the principles of the invention and are thus within the spirit and scope of the invention.

## Claims

1. A method for processing an amorphous silicon thin film sample into a single or polycrystalline silicon thin film having a reduced surface roughness, comprising the steps of:
(a) forming a rigid cap layer on said amorphous silicon thin film sample having sufficient thickness to withstand contractions and expansions during melting and resolidification of said silicon thin film;
(b) generating a sequence of excimer laser pulses;
(c) controllably modulating each excimer laser pulse in said sequence to a predetermined fluence;
(d) homogenizing each modulated laser pulse in said sequence in a predetermined plane;
(e) masking portions of each homogenized fluence controlled laser pulse in said sequence to generate a sequence of fluence controlled pulses of patterned beamlets;
(f) irradiating said amorphous silicon thin film sample with said sequence of fluence controlled patterned beamlets to effect melting of portions thereof corresponding to each fluence controlled patterned beamlet pulse in said sequence of pulses of patterned beamlets;
(g) controllably sequentially translating said sample relative to each of said fluence controlled pulse of patterned beamlets to thereby process said amorphous silicon thin film sample into a single or polycrystalline silicon thin film; and
(h) removing said cap layer from said single or polycrystalline silicon thin film.

2. The method of claim 1, wherein said excimer laser pulses comprise ultraviolet excimer laser pulses.

3. The method of claim 1, wherein said step of forming a rigid cap layer on said amorphous silicon thin film sample comprises forming a silicon oxide layer on said amorphous silicon thin film sample.

4. The method of claim 1, wherein said step of forming a rigid cap layer on said amorphous silicon thin film sample comprises forming a silicon oxide layer approximately 2 microns thick on said amorphous silicon thin film sample.