Abstract:
System and methods for processing an amorphous silicon thin film sample into a single or polycrystalline silicon thin film are disclosed. 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 fluence of the excimer laser pulses, a beam homoginizer for homoginizing modulated laser pulses in a predetermined plane, a mask for masking portions of the homoginized modulated laser pulses into patterned beamlets, a sample stage for receiving the patterned beamlets to effect melting of portions of any amorphous silicon thin film sample placed thereon corresponding to the beamlets, translating means for controllably translating a relative position of the sample stage with respect to a position of the mask and a computer for controlling the controllable fluence modulation of the excimer laser pulses and the controllable relative positions of the sample stage and mask, and for coordinating excimer pulse generation and fluence modulation with the relative positions of the sample stage and mask, to thereby process amorphous silicon thin film sample into a single or polycrystalline silicon thin film by sequential translation of the sample stage relative to the mask and irradiation of the sample by patterned beamlets of varying fluence at corresponding sequential locations thereon.

Description:
This is a division of application Ser. No. 09/390,537, filed Sep. 3, 1999. 
    
    
     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. For example, in James Im et al., “Crystalline Si Films for Integrated Active-Matrix Liquid-Crystal Displays,” 11 MRS Bullitin 39 (1996), an overview of conventional excimer laser annealing technology is presented. In such a system, an excimer laser beam is shaped into a long beam which is typically up to 30 cm long and 500 microns 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 silicon is problematic for several reasons. First, the polycrystalline silicon generated in the process is typically small grained, of a random microstructure, and having a nonuniform grain sizes, therefore resulting in poor and nonuniform devices and accordingly, low manufacturing yield. Second, in order to obtain acceptable performance levels, the manufacturing throughput for producing polycrystalline silicon must be kept low. Also, the process generally requires a controlled atmosphere and preheating of the amorphous silicon sample, which leads to a reduction in throughput rates. Accordingly, there exists a need in the field to generate higher quality polycrystalline silicon at greater throughput rates. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide techniques for growing large grained polycrystalline or single crystal silicon structures using energy-controllable laser pulses. 
     A further object of the present invention is to utilize small-scale translation of a silicon sample in order to grow large grained polycrystalline or single crystal silicon structures on the sample. 
     Yet another object of the present invention is to provide techniques for growing location controlled large grained polycrystalline or single crystal silicon structures which yield planarized thin silicon films. 
     Yet a further object of the present invention is to provide techniques for growing large grained polycrystalline or single crystal silicon structures at low temperatures, for example at room temperature, and without preheating. 
     A still further object of the present invention is to provide techniques for coordinated attenuation of laser fluence. 
     In order to achieve these objectives as well as others that will become apparent with reference to the following specification, the present invention provides an excimer laser for generating a plurality of excimer laser pulses of a predetermined fluence, an energy density modulator for controllably modulating fluence of the excimer laser pulses, a beam homoginizer for homoginizing modulated laser pulses in a predetermined plane, a mask for masking portions of the homoginized modulated laser pulses into patterned beamlets, a sample stage for receiving the patterned beamlets to effect melting of portions of any amorphous silicon thin film sample placed thereon corresponding to the beamlets, translating means for controllably translating a relative position of the sample stage with respect to a position of the mask and a computer for controlling the controllable fluence modulation of the excimer laser pulses and the controllable relative positions of the sample stage and mask, and for coordinating excimer pulse generation and fluence modulation with the relative positions of the sample stage and mask, to thereby process amorphous silicon thin film sample into a single or polycrystalline silicon thin film by sequential translation of the sample stage relative to the mask and irradiation of the sample by patterned beamlets of varying fluence at corresponding sequential locations thereon. 
     In a preferred arrangement, the excimer laser is a ultraviolet excimer laser, and the energy density modulator includes a rotatable wheel, two or more beam attenuators circumferentially mounted on the wheel, and a motor, for controllably rotating the wheel such that each sequential pulse emitted by the laser passed through one of the two or more beam attenuators. Advantageously, the beam attenuators are capable of producing at least two different levels of fluence attenuation. 
     In an alternative arrangement, the energy density modulator includes a multilayer dialectic plate that is rotatable about an axis perpendicular to a path formed by the excimer pulses, in order to variably fluence modulate the excimer pulses in dependance of an angle formed between the excimer pulse path and the axis of rotation. A compensating plate is advantageously provided to compensate for a dialectic induced shift in the beam path, 
     In another alternative arrangement, the energy density modulator includes one or more beam attenuators and a translating stage for controllably translating the one or more beam attenuators such that each sequential pulse emitted by the laser passes through one of the one or more beam attenuators or passes through the energy density modulator without passing through any of the one or more beam attenuators. The translating stage is movable in both a direction parallel to a path formed by the excimer pulses and a direction perpendicular to the path, and the beam attenuators are positionable such that the excimer pulses pass through one of the one or more beam attenuators or through none of the one or more beam attenuators. 
     In still another alternative arrangement, the energy density modulator includes one or more movable beam attenuators being controllably moved such that each sequential pulse emitted by the laser passes through one or more of the one or more beam attenuators or passes through the energy density modulator without passing through any of the one or more beam attenuators. 
     In one preferred arrangement, the translating means includes a mask translating stage that is translatable in both orthogonal directions that are perpendicular to a path formed by the homoginized beams, and a translating stage motor for controllably translating the mask translating stage in both of the translatable directions under control of the computer. In an alternative arrangement, the translating means includes the sample translating stage, and has an X direction translation portion and a Y direction translation portion permitting movement in two orthogonal directions that are perpendicular to a path formed by the patterned beamlets and being controllable by the computer for controllably translating the sample in both of the translatable directions under control of the computer. The sample translation stage may additionally include a Z direction translation portion, for permitting movement of the sample in a direction parallel to the path formed by the patterned beamlets. Most preferably, the entire system is mounted on a granite block to stabilizing the sample from ambient vibration. 
     The present invention also provides methods for processing an amorphous silicon thin film sample into a single or polycrystalline silicon thin film. In a preferred technique, the method includes the steps of generating a plurality of excimer laser pulses of a predetermined fluence; controllably modulating the fluence of the excimer laser pulses; homoginizing the modulated laser pulses in a predetermined plane; masking portions of the homoginized 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. 
    
    
     The accompanying drawings, which are incorporated and constitute part of this disclosure, illustrate several 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 in accordance with a preferred embodiment of the present invention; 
     FIG. 2 a  is an illustrative diagram of an energy density modulator suitable for use in the system of FIG. 1; 
     FIG. 2 b  is an illustrative diagram taken along section A-A′ of FIG. 2 a;    
     FIG. 3 is a graph showing an illustrative fluence profile of the laser beam pulses shaped by the energy density modulator of FIG. 2; 
     FIGS. 4-6 are illustrative diagrams of alternative energy density modulators suitable for use in the system of FIG. 1; 
     FIG.  7 . is an illustrative diagram of a masking system suitable for use in the system of FIG. 1; 
     FIG. 8 is an illustrative diagram of a sample translation stage suitable for use in the system of FIG. 1; 
     FIG.  9 . is an illustrative diagram showing the formation and avoidance of a mount in the region where two crystals meet; 
     FIG. 10 is a flow diagram illustrating the basic steps implemented in the system of FIG. 1; and 
     FIG. 11 is a flow diagram illustrating the steps implemented in the system of FIG. 1 with energy-density modulation. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a preferred embodiment of the present invention will be described. An excimer laser  110 , which may be Lambda Physik Model LPX315i, generates a laser beam  110  which passes through an energy density modulator  120 , described with greater particularity below. While the Lambda Physik excimer laser generates an ultraviolet beam at a wavelength of 308 nm, more powerful excimer lasers or excimer lasers generating beams at other wavelengths may be utilized. In accordance with the present invention, the modulator  120  acts to rapidly change the energy density of laser beam  110 . The excimer laser  110  and energy density modulator  120  are each linked by a standard computer interface connection  101  to a computer  100  to effect control of the energy density modulator  120  in accordance with the timing of laser pulses generated by laser  110 . The modulated laser beam  121  is directed through beam attenuator and shutter  130 , which permit fine control over the fluence of the modulated laser beam. 
     The fluence controlled laser beam  131  is directed to reflecting surface  140  and through telescoping lenses  141  and  142  to become incident on reflecting surface  143 , where it is directed through beam homogenizer  144 . The telescoping lenses, which may be two plano-convex lenses  141  and  142  or other well known lens configurations, act to shape the laser beam to match the requirements of beam homogenizer  144 . The beam homogenizer, which may be a Microlas beam homogenizer  144 , causes the laser beam to gain nearly uniform fluence in the plane of homogenization. The homogenized beam  146  is passed through condensing lens  145 , reflected by reflecting surface  147  and passed through field lens  148 , which collimates the beam. 
     The collimated beam  149  passes through masking system  150 , which will be described in greater detail below. Patterned beamlets  151  are output from the masking system  150 , reflected by reflecting surface  160 , passed through an eye lens  161 , reflected off reflecting surface  162  and passed through an objective lens  163 . Alternatively, the beamlets could be directed to an objective lens  163  without intermediate reflections. The objective lens  162  acts to demagnify and focus the pattern beam  151 . 
     The focused pattern beam  164  is incident on a thin silicon film sample  170 , such as a film of amorphous or randomly grained polycrystalline silicon ranging from 100 Angstroms to greater than 5000 Angstroms, deposited on a substrate. In accordance with the preferred invention, the sample  170  is preferably kept at room temperature and does not have to be pre-heated. As further described below, the focused pattern beam  164  is used to laterally solidify the thin silicon sample  170  into a single or uniformly grained polycrystalline film. 
     The silicon film sample  170  is placed on top of a sample translation stage  180 , to be described in greater detail below, which in turn rests on a granite block  190 . The granite block  190  is supported by support system  191 ,  192 ,  193 ,  194 ,  195 ,  196  which is actively controllable to minimize vibration of the granite block  190  which may be caused by movement in the floor. The granite block must be precision manufactured to have a flat surface, and preferably is of a high grade such as Laboratory Grade AA per federal specification GGG-P-463c. The support system may be a commercially available system from Technical Manufacturing Corporation, which is pneumatically controlled to inhibit the passing of vibrations to the granite block  190 . Cross supports between legs  191 ,  192 ,  193 ,  194 ,  195 ,  196  may be included for further stability. 
     Referring next to FIGS. 2 a  and  2   b , the energy density modulator  120  is described in more detail. FIG. 2 a  shows a side view of the energy beam modulator  120 , which includes a metal wheel  210 , motor  220 , and a plurality of beam attenuators  230 . The motor  220  is a standard stepper motor and includes an encoder so that the motor  220  is able to provide computer  100  with feedback regarding the angular position of wheel  210 , and accordingly, the position of each beam attenuator  230 . Each beam attenuator  230  is attached to the wheel  220  to permit rotation thereof. Commercially available beam attenuators fabricated from a dielectric coated piece of silicon dioxide or magnesium floride are suitable for use as beam attenuators  230 . 
     For the laser beam generated by the Lambda Physik Model LPX315i, which is approximately 1.5×3 cm, a suitable wheel  210  may be approximately 10-20 cm in diameter and include at least 10 beam attenuators  230 , each being at least slightly larger than 1.5×3 cm, circumscribing the wheel  210 . FIG. 2 b  is a diagram taken along cross section A-A′ of FIG. 2 a ., showing ten beam attenuators  230 . The number of beam attenuators  230  chosen will depend on the desired grain size, with longer grains requiring more excimer pulses to manufacture, and accordingly, more beam attenuators. 
     In operation, the energy density modulator  120  and excimer laser  110  are operated in a synchronized manner under the control of computer  100  to achieve the desired attenuation of each laser pulse emitted by the excimer laser  110 . For example, if ten excimer pulses are required to properly irradiate a small region of the silicon sample  170  and the excimer laser emits laser pulses at 100 Hz, the wheel  210  would be rotated at ten revolutions per second, or 600 rpm, in synchronicity with the laser pulse emissions. In this example, each laser pulse would be incident on a different beam attenuator  230  when each attenuator was in a position substantially corresponding to the beam path. In accordance with the present invention, the last laser pulses of the pulse set are attenuated in order to planarize the thin silicon film being irradiated. Accordingly, in the foregoing example, the first seven beam attenuators  230  would cause no or little beam attenuation, while the eighth, ninth and tenth beam attenuators would cause increasing fluence attenuation of the incident beam pulses  111 . 
     Thus, as shown in FIG. 3, the energy density modulator  120  changes the fluence profile of the laser beam pulses emitted by excimer laser  110 . If the excimer laser is emitting laser beam pulses having a fluence of 300 mJ/cm2, the energy density modulator  120  may be set up to freely pass the first seven pulses, to attenuate the eight pulse to 250 mJ/cm2, the ninth pulse to 200 mJ/cm2, and to completely block the tenth pulse. Of course, the foregoing is merely an example and those skilled in the art will appreciate that other fluence profiles are easily achievable by changing the number of attenuators on wheel  210  and the dielectric coating on each attenuator. 
     In an alternative embodiment of the energy density modulator  120  shown with reference to FIG. 4, greater flexibility in configuring the attenuation profile is achieved where the attenuator is coated with a multi-layered dielectric suitable for variable transmittance depending on the incident angle of the incident beam pulse. Thus, a variable beam attenuator  410  controlled by step motor  411  is positioned to receive incident beam pulses  111  and to attenuate the beam in accordance with the angle theta. The attenuated beam  121  is brought back onto the beam axis by passing through a compensating plate  420  which is moved by motor  412  so that the beam attenuator  410  and compensating plate  420  are at opposing angles with respect to the beam axis. As with the embodiment shown in FIG. 2, the beam attenuator is rotated under the control of computer  100  to synchronize the timing of the laser beam pulses emitted by the laser  110  and the attenuation caused by the energy density modulator  120 . 
     Referring next to FIGS. 5 a  and  5   b , a translational energy density modulator suitable for use in the system of FIG. 1 will now be described. As shown in FIG. 5 a , the embodiment includes a translating stage  510  to which several beam attenuators  520 ,  530 ,  540  of varied attenuation are attached. In operation, the translating stage would be positioned by computer  100  such that the laser beam  111  does not pass through any attenuator during the initial excimer pulses. Towards the end of a pulse cycle, the translating stage is moved so that the beam  111  passes through attenuations  520 ,  530 ,  540 , to become an increasingly attenuated beam  121 . The computer synchronizes the movement of the translating stage  510  such that each sequential excimer pulse passes through the center of the respective attenuations. One drawback of this embodiment is that the stage will be positioned to maximally attenuate the first pulse of the immediately succeeding pulse cycle. However, this drawback can be overcome if the attenuating stage includes two orthogonal degrees of freedom, as shown in FIG. 5 b . The beam translators  550  can be positioned to intersect the beam path when attenuation is desired towards the end of a pulse cycle, and moved out of the way of the beam path  560  prior to initiation of a succeeding cycle, as indicated by the y direction, and translated in the X direction to an initial position while non attenuated beams are desired. 
     Referring next to FIG. 6, a movable multi-plate energy density modulator suitable for use in the system of FIG. 1 will now be described. The embodiment includes several beam attenuators, each of which is movable to be positioned either in the path  610 ,  620 ,  630  of incident beam  111  or outside of the beam path  611 ,  621 ,  631 . The attenuators may be movable in a direction perpendicular to the beam path, or may be pivoting or rotatable to move in and out of the beam path. In operation, computer  100  positions each attenuator such that the laser beam  111  does not pass through any attenuator during the initial excimer pulses. Towards the end of a pulse cycle, the computer causes the attenuators to be moved so that the beam  111  passes through one or more attenuations to become an increasingly attenuated beam  121  in accordance with the desired modulation profile. The computer synchronizes the movement of the attenuators such that each sequential excimer pulse passes through the center of all attenuators that are placed in the beam path. It should be understood that in operation, it may be desirable to pass several excimer pulses through the same attenuation, or to vary the attenuation scheme in any other manner to achieve the desired attenuation profile. 
     Referring next to FIG. 7, the masking system  150  is described in more detail. Homogenized and columnated beam  149 , passes through mask  710  which contains a pattern thereon. The mask  710  may be a chrome or dielectric coated fused silica slab, and should include a pattern, such as an array of slits or chevrons, which have been etched from the coating. The mask  710  rests upon an open frame XY translation stage  720  which is controlled by X and Y axis motors  730  under the direction of computer  100 . Movement of the XY translation stage  720  permits crystal growth within the silicon sample  170 , as will be described below. Alternatively, the mask could rest upon a fixed open frame stage, with beam translation being effected by the sample translator  180 . As described in detail in commonly assigned co-pending application Ser. No. 09/200,533, filed on Nov. 27, 1996, the disclosure of which is incorporated by reference herein, the slit array mask enables the production of large grained polycrystalline silicon having a substantially uniform grain structure, while the chevron array mask enables the production of location controlled, large single crystal silicon regions. 
     Referring next to FIG. 8, the sample translation stage  180  is described in more detail. The stage may include a linear motor/air bearing translation stage, for example an Aerotech ATS 8000 model stage. Thus, the Aerotech stage includes X and Y direction translators  810 ,  820 , and is controllable by computer  100 . A separate Z direction translator  830 , also controllable by computer  100 , is preferably included. The silicon sample  170  is rested on the Z direction translator  830  in the path of masked beam  164 . 
     In operation, the computer  100  controls the movement of either the sample translation stage  180  or the mask translation stage  720  in accordance with the timing of the pulses generated by excimer laser  110  to effect the desired crystal growth in silicon sample  170 . Either the sample  170  is moved with respect to the incident pulse  164 , or alternatively, the location of the pulse  164  is moved with respect to the sample  170  through mask translation stage. 
     In order to grow large grained silicon structures, small-scale translation occurs in between each excimer pulse of a pulse set until the final pulse of the pulse set has been absorbed by the sample  170 . As each pulse is absorbed by the sample, a small area of the sample is caused to melt and resolidify into a crystal region initiated by the initial pulse of a pulse set. Of course, the number of pulses in a pulse set will define the size of the grain that can be produced, with more pulses enabling the growth of larger sized crystals. Thus, since crystal structures having lengths varying from approximately 0.5 microns to 2 microns may be produced from a single pulse, it should be understood that crystal structures which obtain lengths of 10s of microns may be generated by a suitable set of pulses. 
     In order to avoid surface protrusions near the last place in the crystal to solidify, the last pulses of a pulse set are attenuated. Referring to FIGS. 9 a - 9   c , when a silicon thin film  900  is irradiated with an excimer laser pulse having sufficient energy to effect complete melt through of the film  900 , a liquid area  912  is formed in between two solid areas  911 ,  912 . Two crystal fronts  921 ,  922  form and grow into the narrowing liquid area  922  until all liquid silicon has crystallized as part of crystals  930 ,  931 . Since silicon is denser when in the liquid phase than when in the solid phase, the volume of the silicon film increases as the silicon solidifies, thereby forming a mount  933  where the two crystals  930 ,  931  meet  932 . 
     In accordance with the present invention, one or more attenuated pulses are applied to the thin film, either after the long polycrystalline or single crystalline silicon structures have been formed or near the end of the formation of such structures. Referring to FIGS. 9 d - 9   f , when an attenuated laser pulse is applied to the silicon film  940  in the region of a mount  941 , the top surface of the film is liquified  952 , leaving neighboring crystals  950 ,  951  adjoining at the bottom surface of the film. Since there can be no lateral crystal growth of crystals  950 ,  951 , the crystals grow upward  960 ,  961  to form crystal boundary which either has no mount or a far less pronounced mount  962  than mount  941 . 
     Referring next to FIGS. 10 and 11, the steps executed by computer  100  to control the crystal growth process in accordance with the present invention will now be described. FIG. 10 is a flow diagram illustrating the basic steps implemented in the system of FIG.  1 . The various electronics of the system shown in FIG. 1 are initialized  1000  by the computer to initiate the process. A thin silicon film 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  100 . Next, the sample translation stage is moved into an initial position  1015 , which may include an alignment with respect to reference features on the sample. 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 fully melt the silicon sample in accordance with the particular processing to be carried out. 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 desired microstructure of the sample. The shutter is opened  1040  to expose the sample to irradiation and accordingly, to commence the sequential lateral solidification process. 
     Sample translation and irradiation continues until that the desired crystallization has been competed  1050 ,  1051 , at which time the computer closes the shutter and stops translation  1055 ,  1060 . If other areas on the sample have been designated for crystallization, the sample is repositioned  1065 ,  1066  and the crystallization process is repeated on the new area. If no further areas have been designated for crystallization, the laser is shut off  1070 , the hardware is shut down  1075 , and the process is completed  1080 . Of course, if processing of additional samples is desired or if the present invention is utilized for batch processing, steps  1005 ,  1010 , and  1035 - 1065  can be repeated on each sample. 
     FIG. 11 is a flow diagram illustrating the steps implemented in the system of FIG. 1 with energy-density modulation. Steps  1100 - 1140 , and  1150 - 1180  are identical to those described above with reference to FIG. 10 for steps  1000 - 1040 , and  1050 - 1080 . In order to implement energy-density modulation, the attenuation of the excimer laser pulses are modulated  1145  in a predefined manner so as to be synchronized with both the timing of the laser pulses emitted by the laser and the instantaneous position of the silicon sample being irradiated. In connection with the foregoing, the ability to vary the rate at which beam attenuators are moved to impact energy density modulation over the attenuation-modulation cycle may be desirable to achieve greater flexibility 
     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 the foregoing describes a sample sitting on a translation stage, it may be advantageous to place the sample within a vacuum chamber or a chamber with a controlled atmosphere, such as one housing an inert gas, with the chamber lying atop the translation stage. Other types of homogenizers may be utilized, such as a fly&#39;s eye homogenizer. Instead of using a stacked XY translator, more precise translators having additional degrees of freedom may be utilized. Moreover, in order to insure that the excimer pulses are properly focused on the sample, an active focusing system may be utilized. 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.