JP2007288159A - Method of flattening silicon film surface during and after processing through sequential lateral crystallization method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a system and method for reducing the surface roughness of a polycrystalline or monocrystalline thin film produced by a sequential lateral solidification process. <P>SOLUTION: The system comprises: 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 to obtain a fluence required to perfectly melt the thin film in the thickness direction or lower; a beam equalizer 144; a sample stage 170 for melting a portion corresponding to the laser pulses of the thin film; a parallel motion means for controllably moving in parallel the position of the sample stage 170 relative to the laser pulses; and a computer 100 for performing the generation of the excimer pulses and modulation of the fluence in accordance with the relative position of the sample stage 170, and thereby processing the polycrystalline or monocrystalline thin film by sequentially moving the sample stage 170 in parallel with the laser pulses. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

(Background of the Invention)
I FIELD OF THE INVENTION The present invention relates to semiconductor processing technology, and more particularly to semiconductor processing that can be performed at low temperatures.

II Description of 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 (heat treatment) technology is disclosed in James Im et al., "Crystalline Si Films for integrated Active-Matrix Liquid-Crystal Displays", 11 MRS Bulletin 39 (1996). In systems used to perform excimer laser annealing, the excimer laser beam is shaped into a long beam, typically up to 30 cm long and 500 μm wide or wider. The shaped beam is scanned over a sample of amorphous silicon to promote melting of the sample and formation of polycrystalline silicon as the sample resolidifies.

  The use of conventional excimer laser annealing techniques to produce polycrystalline or single crystal silicon is problematic for several reasons. First, the silicon produced by the process is usually a random microstructure with small grain size and / or non-uniform grain size, resulting in poor and non-uniform devices. This can reduce the manufacturing yield. Second, the processing technology required to obtain an acceptable level of performance requires that manufacturing throughput to produce polycrystalline silicon remain low. Also, these processes generally require a controlled environment and pre-heating of the amorphous silicon sample, further reducing the processing rate. Lastly, the fabricated films generally have an unacceptable degree of surface roughness, which can be a problem for the performance of microelectronic devices.

  There is a need in the field to produce higher quality polycrystalline and single crystal silicon at higher processing speeds. There is also a need for manufacturing techniques that reduce the surface roughness of such polycrystalline and single crystal silicon thin film films used in the fabrication of higher quality devices such as flat panel displays.

(Summary of Invention)
An object of the present invention is to provide a technique for planarizing the surface of polycrystalline and single crystal thin film semiconductors.

  A further object of the present invention is to provide a surface planarization technique applicable as a post-processing step for polycrystalline and single crystal thin film semiconductors produced during SLS (Sequential Lateral Solidification) process. There is.

  It is a further object of the present invention to provide a surface planarization technique that can be applied as a processing step during the production of polycrystalline and single crystal thin film semiconductors in a sequential lateral crystallization process.

  It is a further object of the present invention to provide techniques for producing high quality semiconductor devices that are useful in the production of displays and other products.

  In order to achieve these objectives, as well as other objectives that will become apparent with reference to the following description, the present invention provides surface roughness of polycrystalline and single crystal thin film previously produced by a sequential lateral crystallization process. A system and method for reducing the likelihood is provided. In one configuration, the system causes an excimer laser that generates a plurality of excimer laser pulses of a predetermined flux amount (fluence) and the flux amount of the excimer laser pulses to completely melt the thin film in the thickness direction. An energy density modulator that controllably modulates the flux amount to be equal to or less than a flux amount necessary for the above, a beam homogenizer (homogenizer) that uniformizes the modulated laser pulse in a predetermined plane, and A sample stage (sample stage) for receiving a uniform laser pulse and performing partial melting in a thickness direction of a portion corresponding to the laser pulse of a polycrystalline or single crystal thin film, and the sample stage Translation means for controllably translating the relative position with respect to the laser pulse, and the generation of the excimer pulse and the modulation of the flux amount are controlled by Performed in accordance with the position, therefore, by sequentially translating the sample stage relative to the laser pulse, which comprises a computer for processing polycrystalline or single crystal thin film. The excimer laser is preferably an ultraviolet excimer laser that generates ultraviolet excimer laser pulses.

  In one configuration, the beam homogenizer is operable to shape a laser pulse with top-hat characteristics in both the x and y directions. The energy density modulator attenuates the flux amount of the excimer laser pulse to about 25% to 75% of a threshold value for completely dissolving the polycrystalline or single crystal thin film in the thickness direction. Enable operation.

(Description of preferred embodiments)
The translation stage includes a translation unit in the X direction and a translation unit in the Y direction. These units are coupled to the computer and coupled to each other, and two perpendicular to the path formed by the laser pulse. It is advantageous to be able to move in orthogonal directions and be controllable by the computer, the computer controllably translating the sample in both directions which can be translated under the control of the computer. . Further, the beam homogenizer is operable to shape the laser pulse in a top hat type characteristic in both the x and y directions, and the translation means includes the polycrystalline or single crystal thin film, Operable to translate in two directions perpendicular to the direction of the laser pulse, so that the homogenized laser pulse overlaps little by little in the two directions of the polycrystalline or single crystal thin film. Incidently enter the region.

  The modified configuration of the present invention provides a system and method for processing an amorphous silicon thin film sample into a single crystal or polycrystalline silicon thin film with reduced surface roughness. In one configuration, the method is performed on an amorphous silicon thin film sample having a thickness sufficient to withstand shrinkage and expansion during melting and resolidification of the thin film silicon during a sequential lateral crystallization process. Forming a rigid cap layer. The method includes the steps of generating an excimer laser pulse train, modulating each excimer laser pulse in the pulse train to a predetermined flux amount in a controllable manner, and modulating and controlling the flux amount in the pulse train. A step of uniformizing each laser pulse in a predetermined plane, and masking a part of each laser pulse in the pulse train that has been uniformized to control the flux amount, thereby patterning and controlling the flux amount. Generating a pulse train of beams, irradiating an amorphous silicon thin film sample with a pulse train of small beams patterned by controlling the amount of flux, and performing partial melting in the thickness direction of the sample; The sample is sequentially translated in a controllable manner with respect to each pulse of a small beam patterned by controlling the amount of the flux. Treating a silicon thin film sample into a single crystal or polycrystalline silicon thin film with reduced surface roughness; removing the cap layer from the treated single crystal or polycrystalline silicon thin film; and It has.

  The accompanying drawings constitute a part of this specification and illustrate the preferred embodiment of the present invention and serve to explain the principles of the invention.

Embodiments of the present invention will be described below with reference to the drawings.
The present invention provides a technique for planarizing the surface of polycrystalline and single crystal thin film semiconductors. In a preferred embodiment, this surface planarization technique is applied as a post-processing step to polycrystalline and single crystal thin film semiconductors produced during the sequential lateral crystallization process, or in the sequential lateral crystallization process. Applies as a processing step in the production of polycrystalline or single crystal thin film semiconductors. Therefore, in order to fully understand these techniques, the sequential side solidification process must first be understood.

  The sequential lateral crystallization process is a technique that produces a large grain silicon structure by unidirectional narrow translation of a silicon sample between successive pulses emitted by an excimer laser. As each pulse is absorbed by the sample, a small region of the sample is completely melted in the thickness direction and enters the crystal region produced by the previous pulse in the pulse set and resolidifies.

  A particularly advantageous sequential lateral crystallization process and apparatus for carrying out this process are disclosed in the Applicant's 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 ", which is incorporated herein by reference. Although the foregoing disclosure has been made with reference to a specific technique described in the applicant's co-pending patent application, other sequential lateral crystallization techniques are readily applicable for the present invention. Is clear.

  FIG. 1 illustrates a system described as a preferred embodiment in the applicant's co-pending patent application, which includes an excimer laser 110, an energy density modulator 120 that rapidly changes the energy density of the laser beam 111, and Beam attenuator / shutter 130, optical devices 140, 141, 142 and 143, beam homogenizer 144, lens systems 145, 146 and 148, masking system 150, and lens systems 161, 162 and 163 An incident laser pulse 164, a thin film silicon film sample 170, a sample translation stage (stage) 180, a graphite block 190, support systems 191, 192, 193, 194, and a management computer 100. Mask in masking system 150 under 100 instructions 10 movement of, either by movement of the sample translation stage 180 performs parallel movement in the X and Y directions of the silicon sample 170.

  As described in more detail in the applicant's co-pending application, a plurality of excimer laser pulses of a predetermined flux amount are generated and the flux amount of the excimer laser pulses is modulated in a controllable manner. Then, the modulated laser pulse is made uniform in a predetermined plane, and a part of the modulated and uniform laser pulse is masked to form a patterned small beam, and the amorphous silicon thin film sample is patterned. Irradiating with a beam to melt the portion of the sample corresponding to the small beam and translating the sample in a controllable manner with respect to the patterned small beam and the controlled modulation. A silicon thin film sample is processed into a single crystal or polycrystalline silicon thin film, so that the patterned small beam The sample pattern amorphous silicon thin film sample is processed by sequentially translating the sample and sequentially irradiating the corresponding position on the sample with a patterned beam of variable flux. Crystalline silicon thin film.

  For the production of single crystal or large polycrystalline silicon thin film films, sequential lateral crystallization processes are very advantageous, but the crystals produced are often melted and regenerated, characteristic of the crystal growth process. The surface roughness due to repeated solidification is shown. For this reason, as shown in FIG. 2, a crystal having a thickness of 200 nm exhibits a variation in height throughout the length of the crystal. In FIG. 2, a height of 0 indicates the optimum height for a 200 nm thick crystal and the height varying from 175 nm to 225 nm is shown as normal throughout the length of the crystal. In the large protrusion 210 near the crystal boundary, the thickness of the crystal exceeds the optimum thickness of 200 nm by 350 nm.

  A first embodiment of the present invention will now be described with reference to FIGS. FIG. 3 illustrates an embodiment of a post-processing system for planarizing polycrystalline and single crystal thin film semiconductors produced by a sequential lateral crystallization process. This system includes an excimer laser 310, a beam attenuator / shutter 320, a reflecting plate 330, telephoto lenses 331 and 332, a reflecting plate 333, a beam uniformizer 340, a condenser (condensing) lens 345, A reflection plate 347, a field (field of view) lens 350, a sample 360, a sample translation table 370, an optical table 380, and a management computer 300 are provided. Each of the preferred laser 310, attenuator 320, telephoto lenses 331, 332, homogenizer 340, and sample translation table 370 movable in two orthogonal directions is a co-pending patent application Ser. No. 09 / 390,537. It is described in the application. The table 380 can be as described in this patent application or can be a normal table. Preferably, the homogenized beam 346 is shaped to a top hat shape in both the x and y directions, and the beam energy density is less than or equal to the energy density required to completely melt the sample 360 in the thickness direction. This is very important.

  The sample 360 will be described in detail with reference to FIGS. 4a and 4b. Since the sample of this example has already been processed, it already contains a number of crystal regions, which are shown schematically as chevron-shaped (mountain) crystals 365. A state in which the homogenized beam 346 is incident on a part 361 of the sample 360 and induces partial melting in the thickness direction thereof is shown.

For a 200 nm thick silicon thin film, the threshold for complete melting in the thickness direction is about 600 mJ / cm ² . For this reason, a beam 346 having an energy of about 25% to 75% of the threshold for full melting in the thickness direction should be utilized to guide sufficient partial melting of the portion 361 in the thickness direction. If the energy of the beam is larger, the energy fluctuation inherent in the excimer laser can cause complete melting of the sample region 361 in the thickness direction. If the energy of the beam is smaller, the sample portion 361 will not melt sufficiently to be satisfactorily flattened.

  As shown in FIG. 4 b, the sample 360 includes a silicon oxide base layer 400 and a silicon layer 410. According to the present invention, the outer surface of the silicon layer 410 is melted to a depth 420. Upon resolidification, the rough surface 430 is modified to become more flat.

  A homogenized single beam pulse having an energy of about 25% to 75% of the full melt threshold in the thickness direction is sufficient to induce partial melt in the thickness direction of region 361, but multiple beams It is preferred to irradiate all of these areas with a pulse. Each subsequent beam pulse leads to partial melting of the region 361 in the thickness direction, which when resolidified results in a more planar surface. Thus, the use of 10 beams per region 361 produces a much smoother surface 430 than with a single pulse.

  Returning to FIG. 4 a, the sample stage 370 is translated from right to left under the control of the computer 300, and the uniformed beam 346 is scanned from the left to the right 450 on the upper portion of the sample 360. Then, the table 370 is moved in the orthogonal direction (shown as the Y direction), the sample is aligned with the new position 460, and the parallel translation 470 in the reverse direction is started. This process is repeated until the entire surface of the sample 360 has been scanned with the homogenized beam 346.

  When the sample stage is translated in the Y direction, it is advantageous to match the homogenized beam so that it slightly overlaps the area scanned before the sample 360. For this reason, if the region 361 is 1.2 × 1.2 cm, then a Y-direction translation of 1.15 cm can be used to avoid edge effects caused by uniform beam irregularities. Similarly, it is advantageous to produce a slight overlap during the execution of the translation in the X direction.

  The above has been described with reference to a beam that is uniformed into a square with a top-hat type characteristic, but beams of other shapes can also be used. For this reason, as shown in FIG. 5, it is possible to use a uniform beam that is sufficiently wide to eliminate the need for translation in the X direction, and the advantage that the need for movement by the translation table 370 is reduced. And thus a greater production capacity is obtained. Similarly, when performing greater overlap between translations in the X direction, a beam shaped with Gaussian characteristics in the X direction can be used.

  6 and 7 show the results of the process described with reference to FIGS. 3 and 4a. The characteristics of the sample 360 produced by the sequential lateral crystallization process are shown in FIG. 6a. This sample shows surface irregularities with an optimum height of 200 nm to ± 25 nm. As shown in FIG. 6b, these surface irregularities can be significantly reduced after post-treatment with a single laser pulse according to the invention. These results are shown in FIG. 7, which shows a surface roughness reduction of less than 100% caused by the post-treatment according to the invention.

  Next, a second embodiment of the present invention will be described with reference to FIG. In this example, the surface of the silicon thin film is planarized by employing a rigid cap layer during the sequential lateral crystallization process. Accordingly, FIG. 8 shows a thin film silicon sample formed of an amorphous silicon layer 810 having a thickness of about 50 nm to 200 nm deposited on the base layer 800 of silicon oxide. The sample is covered with a thick second silicon oxide layer 820 having a thickness of about 2 microns, which is substantially rigid. This cap layer must be thick enough to withstand the shrinkage and expansion during melting and resolidification of the silicon layer during the sequential lateral crystallization process.

  Thus, in a sequential lateral crystallization process, a sample having a cap layer 820 is used instead of sample 170, and a complete description thereof is included in the above-mentioned patent application No. 09 / 390,537. After such processing, cap layer 820 is removed from the sample by conventional wet or dry etching techniques. FIG. 9 shows the results of the process described with reference to FIG.

  Referring to FIG. 10, the steps performed by computer 300 to control both the sequential lateral crystallization process of FIG. 1 and the surface planarization process realized with respect to FIG. 3 will be described. Various electronic circuits of the system are initialized by computer 300 (1000) to begin the process. Then, the sample is loaded on the sample translation table (1005). Such loading can be performed manually or robotically under the control of the computer 300. Next, the sample is processed 1010 by a sequential lateral crystallization process using the apparatus of FIG. Then, the processed sample is positioned (1015). If necessary, the various optical components of the system are focused (1020). The laser is then stabilized (1025) to the desired energy level and repeatability required to partially melt the sample in the thickness direction in accordance with the teachings of the present invention. If necessary, finely adjust the attenuation of the laser pulse (1030).

  Next, in parallel with the region of the sample that has been subjected to the sequential lateral crystallization process, the sample starts to move in a predetermined direction at a predetermined speed (1035). The shutter is then opened (1040) and the sample is exposed by irradiation, thereby starting the planarization process.

  Until flattening is completed (1045), the sample continues to be translated and irradiated (1050), at which point the computer closes the shutter (1055) and stops the translation (1060). If another region on the sample is selected for flattening (1065), the sample is repositioned (1066) and the process is repeated with the new region. If an area for further planarization has not been selected, the laser is turned off (1070), the hardware is turned off (1075), and the process is terminated (1080).

  Referring to FIG. 11, the steps performed by computer 100 along with the surface planarization steps performed with respect to FIG. 1 to control the crystal growth process will be described. FIG. 10 is a flow diagram showing the basic steps performed in the system of FIG. 1 using a capped sample as shown in FIG. An oxide layer is deposited on the base (1100). Then, a silicon layer is deposited on the oxide buffer layer (1110), and a cap oxide is deposited on the upper layer portion of the sample (1120).

  The sample of FIG. 1 is then used to process the sample according to a sequential lateral crystallization process. After treatment, the cap oxide is removed, for example by dissolution in dilute hydrofluoric acid

  The foregoing merely illustrates the principles of the invention. Various modifications and alternatives to the described embodiments will be apparent to those skilled in the art in light of the teachings herein. For example, the removal of the cap layer is disclosed with respect to the use of dissolution in dilute hydrofluoric acid, but the cap layer can be removed by any conventional technique such as dry etching. Thus, although not explicitly described herein, it will be apparent to those skilled in the art that the principles of the present invention may be embodied and thus numerous systems and methods within the scope of the present invention may be implemented by those skilled in the art.

FIG. 2 is a functional diagram of a system that performs a sequential lateral crystallization process suitable for implementing the preferred process of the present invention. FIG. 2 shows the surface properties of a regular film processed by the sequential lateral crystallization process of FIG. 1 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 crystallization process according to the present invention. FIG. 4a and 4b schematically show a crystalline silicon film to be processed by the system of FIG. 3 using a narrow beam. FIG. 4 schematically shows a crystallized silicon film to be processed by the system of FIG. 3 using a wide beam. It is a figure which shows the surface characteristic of the normal film before the process by the system of FIG. It is a figure which shows the surface characteristic of the normal film after the process by the system of FIG. FIG. 3 schematically shows a cross section of a crystallized silicon film processed by the system of FIG. 1 according to a second embodiment of the invention. It is a figure which shows the characteristic of the normal film processed by 2nd Example of this invention. FIG. 4 is a flow diagram illustrating steps performed in the system of FIG. 3 according to a first embodiment of the present invention. FIG. 6 is a flow diagram illustrating steps performed in the system of FIG. 1 according to a second embodiment of the present invention.

Claims (4)

In a method of processing a sample of an amorphous silicon thin film into a single crystal or polycrystalline silicon thin film with reduced surface roughness,
(a) forming a rigid cap layer on a sample of the amorphous silicon thin film having a thickness sufficient to withstand shrinkage and expansion during melting and resolidification of the silicon film;
(b) generating an excimer laser pulse train;
(c) controllably modulating each excimer laser pulse in the pulse train to a predetermined flux amount;
(d) homogenizing each modulated laser pulse in the pulse train in a predetermined plane;
(e) masking a part of each laser pulse that is uniformed by controlling the amount of flux in the pulse train to generate a pulse train that controls the amount of flux of the patterned small beam;
(f) irradiating the sample of the amorphous silicon thin film with a pulse train in which the flux amount of the patterned small beam is controlled to control the flux amount of the sample in the pulse train of the patterned small beam. Melting the portion corresponding to each pulse performed;
(g) The sample is sequentially translated in a controllable manner with respect to each pulse whose flux amount of the patterned small beam is controlled, thereby processing the sample of the amorphous silicon thin film, Forming a monocrystalline or polycrystalline silicon thin film;
(h) removing the cap layer from the monocrystalline or polycrystalline silicon thin film. The method of claim 1, wherein the excimer laser pulse comprises an ultraviolet excimer laser pulse. The step of forming a rigid cap layer on the sample of amorphous silicon thin film comprises the step of forming a silicon oxide layer on the sample of amorphous silicon thin film. the method of. The method of claim 1, wherein the step of forming a rigid cap layer on the sample of amorphous silicon thin film comprises forming a silicon oxide layer having a thickness of about 2 microns.

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