This invention relates to methods and systems for processing thin film material, and more particularly to forming large-grained, grain-boundary-location controlled thin films from amorphous or polycrystalline thin films using laser irradiation. In particular the present invention relates to methods and systems for the production of integrated thin film transistors.
In recent years, various techniques for crystallizing or improving the crystallinity of an amorphous or polycrystalline semiconductor film have been investigated. This technology is used in the manufacture of a variety of devices, such as image sensors and active-matrix liquid-crystal display (AMLCD) devices. In the latter, a regular array of thin-film transistors (TFT) is fabricated on an appropriate transparent substrate, and each transistor serves as a pixel controller.
Sequential lateral solidification (SLS) using an excimer laser is one method for fabricating high quality polycrystalline films having large and uniform grains. A large-grained polycrystalline film can exhibit enhanced switching characteristics because the number of grain boundaries in the direction of electron flow is reduced. SLS processing controls the location of grain boundaries. U.S. Pat. Nos. 6,322,625, 6,368,945, 6,555,449 and 6,573,531 issued to Dr. James Im, the entire disclosures of which are incorporated herein by reference, and which are assigned to the common assignee of the present application, describe such SLS systems and processes.
In an SLS process, an initially amorphous or poly crystalline film (for instance, a continuous wave (CW)-processed Si film, an as-deposited film, or solid-phase crystallized film) is irradiated by a narrow laser beamlet. The beamlet is formed by passing a laser beam through a patterned mask, which is projected onto the surface of the film. The beamlet melts the precursor film, and the melted film then recrystallizes to form one or more crystals. The crystals grow primarily inward from edges of the irradiated area towards the center. After an initial beamlet has crystallized a portion of the film, a second beamlet irradiates the film at a location less than the lateral growth length from the previous beamlet. In the newly irradiated film location, crystal grains grow laterally from the crystal seeds of the polycrystalline material formed in the previous step. As a result of this lateral growth, the crystals attain high quality along the direction of the advancing beamlet. The elongated crystal grains are generally perpendicular to the length of the narrow beam and are separated by grain boundaries that run approximately parallel to the long grain axes.
Although the resultant polycrystalline films have elongated grains with enhanced mobilities, the many iterative steps of irradiation and translation result in low throughput rates. There is a need to increase the throughput rates in the processing of semiconductor materials without sacrificing the quality of the processed materials.
In one aspect, the present invention provides a single scan irradiation technique for crystallization of thin films. In other aspects, the present invention provides methods and systems for processing amorphous or polycrystalline precursor films to produce films of higher crystallinity. The present invention also provides polycrystalline films of varying degrees of crystallinity and crystalline grain orientation.
In one aspect of the present invention, a method is provided for processing a selected region of a film in a single laser scan across the selected film region. The method includes translating a film in a first direction, e.g., y-direction, while simultaneously translating a mask in a second direction, e.g., the x-direction and/or y-direction. The mask projects a patterned laser beam or a set of patterned laser beams (hereinafter referred to as a “a patterned laser beam”) onto the film surface. The mask pattern and the translation pathways and speeds for the film and mask are selected so that the selected film region is substantially completely crystallized in a single scan of the film region by the laser.
By “completely crystallized” it is meant that the selected region of the film possesses the desired microstructure and crystal orientation, so that no further laser scanning of the region is required. In some instances, the entire surface area of the selected regions is crystallized. In other instances, bands or islands of the selected regions are crystallized. A film region is considered “completely crystallized” if the desired level of crystallization is achieved in the selected region of the film.
According to one aspect of the invention, a method of crystallizing a film includes generating a plurality of laser beam pulses, directing the plurality of laser beam pulses through a mask to generate a plurality of patterned laser beams, and irradiating a portion of a selected region of a film with one of the plurality of patterned beams having an intensity that is sufficient to melt the irradiated portion of the film, wherein the irradiated portion of the film crystallizes upon cooling. The film moves along a first translation pathway and the mask moves along a second translation pathway while successive portions of the selected region are irradiated with patterned beams, such that the selected region of the film are substantially completely crystallized in a single traversal of patterned beams over the selected region of the film.
In one or more embodiments of the present invention, the process provides constant or oscillating motion of the mask in one direction, e.g., along the x-axis, while the film is continuously advanced in another direction, e.g., along the y-axis. The resultant polycrystalline film possesses columns of elongated grains having long grain boundaries whose locations are controlled by the spatially confined melting and crystallization of the film.
In another aspect of the invention, a method for processing a film includes generating a plurality of laser beam pulses, directing the plurality of laser beam pulses through a mask having a mask pattern with a length l, a width w, and a spacing between adjacent patterns d to generate a plurality of patterned laser beams, wherein each of the patterned beams has a length l′, a width w′ and a spacing between adjacent patterned beams d′, and irradiating a portion of a region of the film with one of the plurality of patterned beams. The mask and patterned beam proportions, e.g., w and w′, l and l′, and d and d′, are related by the demagnification factor of the projection optics. The patterned beam has an intensity that is sufficient to melt an irradiated portion, and the irradiated portion crystallizes upon cooling. The film is moved at constant velocity in a y-direction, and the mask is moved in the x-direction while the film is irradiated with the patterned beam. The patterned beam is advanced a distance of about l′/n-δ in the y-direction from the one irradiation pulse to the next, where δ is a value selected to form an overlap in adjacent irradiated portions and a distance of about λ′, where λ′=w′+d,′ in the x-direction over a selected number of irradiation pulses n.
In one or more embodiments of the present invention, n is in the range of 2 to about 25. In one or more embodiments of the present invention, n is 2, the y-translation distance of the patterned beam is about l′/2-δ, and the x-translation of the patterned beam distance is about λ′/2.
In one or more embodiments of the present invention, the process provides constant or oscillating motion of the patterned beam in one direction, e.g., along the x-axis, while the patterned beam is also continuously advanced in another direction, e.g., along the y-axis. The resultant polycrystalline film possesses columns of elongated grains having long grain boundaries (substantially aligned with the length of the column) whose locations are controlled by the spatially confined melting and crystallization of the film.
According to one embodiment of the present invention, the x- and y-translations of the patterned beam are selected to provide columns of elongated polycrystalline material that are positioned at an angle relative to the axes of translation.
In another aspect of the invention, a device includes a polycrystalline thin film having columns of elongated crystal grains separated by location-controlled grain boundaries that are tilted at an angle theta (θ) with respect to an edge of the thin film substrate, wherein θ is greater than 0° and ranges up to about 45°. Theta (θ) is referred to as the “tilt angle” of the column of crystallized material.
In another aspect of the invention, a device containing a polycrystalline thin film transistor (TFT) includes a polycrystalline thin film defined by x- and y-axes. A TFT device is located in the thin film and is substantially aligned with the x- and y-axes of the thin film. The polycrystalline thin film has a periodic polycrystalline structure including columns of elongated crystal grains. A column is substantially aligned with the x- and y-axes of the film; however, the elongated crystals within the column contain location controlled grain boundaries that are oriented at an angle with respect to the x- and y-axes of the film. The TFT device is positioned at an angle with respect to the grain boundaries of the elongated crystals. In one or more embodiments, the number of long grain boundaries in each thin film transistor device remains substantially uniform.
In one aspect, the method avoids the need to scan the same area of the substrate multiple times in order to fully crystallize the film. It also provides a simpler and time-efficient process that translates the system in the x- and y-directions during laser irradiation. Furthermore, it provides grains that are elongated beyond their characteristic lateral growth length, thereby further improving the crystalline properties of the polycrystalline film.
Various features of the present invention can be more fully appreciated with reference to the following detailed description of the invention when considered in connection with the following drawing, in which like reference numerals identify like elements. The following figures are for the purpose of illustration only and are not intended to be limiting of the invention, the scope of which is set forth in the claims that follow.
Sequential lateral solidification is a particularly useful lateral crystallization technique because it is capable of grain boundary location-controlled crystallization and provides crystal grains of exceptionally large size. Sequential lateral solidification produces large grained structures through small-scale translation of a thin film between sequential pulses emitted by a pulsed laser. As the film absorbs the energy of each pulse, a small area of the film melts completely and recrystallizes laterally from the solidus/melt interface to form a crystalline region. By “lateral crystal growth” or “lateral crystallization,” as those terms are used herein, it is meant a growth technique in which a region of a film is melted to the film/surface interface and in which recrystallization occurs in a crystallization front moving laterally across the substrate surface. “Characteristic lateral growth length,” as that term is used herein, means the length of unimpeded lateral growth of a crystalline grain in a single irradiation step under set irradiation conditions and sample configuration.
The thin film may be a metal or semiconductor film. Exemplary metals include aluminum, copper, nickel, titanium, gold, and molybdenum. Exemplary semiconductor films include conventional semiconductor materials, such as silicon, germanium, and silicon-germanium. Additional layers situated beneath or above the metal or semiconductor film are contemplated. The additional layers can be made of silicon oxide, silicon nitride and/or mixtures of oxide, nitride or other materials that are suitable, for example, for use as a thermal insulator to protect the substrate from overheating or as a diffusion barrier to prevent diffusion of impurities from the substrate to the film.
Referring to
The stage 170 is a precision x-y stage that can accurately position the substrate 160 under the beam 140. The stage 170 can also be capable of motion along the z-axis, enabling it to move up and down to assist in focusing or defocusing the image of the mask 180 produced by the laser beam 140 at the location 165. In another embodiment of the method of the present invention, it is possible for the stage 170 to also be able to rotate.
A thin film is processed into a polycrystalline 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 intensity profile of the laser pulse plane, masking each homogenized laser pulse to define patterned laser beams, irradiating the thin film with the laser beams to effect melting of portions thereof, and controllably and continuously translating the sample and the mask to move the patterned beam across the substrate surface. The laser pulse frequency and the movement (speed and direction) of the sample are adjusted so that the areas of sequential irradiation of the sample overlap from one irradiation/crystallization cycle to the next, so as to provide for the lateral crystal growth that gives rise to large grains. Pulse frequency and stage and mask position may be coordinated and controlled by a computer. Systems and methods for providing continuous motion sequential lateral solidification are provided in U.S. Pat. No. 6,368,945, which is incorporated in its entirety by reference.
The dimensions of the mask features may depend on a number of factors, such as the energy density of the incident laser pulse, the duration of the incident laser beam pulse, the thickness of the semiconductor thin film, the temperature and thermal conductivity of the substrate. From the standpoint of processing efficiency, the width of the slit-shaped mask feature is as large as possible so as to maximize surface coverage. However, width is also determined by the desire to completely melt the thin film throughout its thickness and to avoid nucleation within the melted portions during crystallization. By way of example only, the mask feature are of a dimension sufficient to create a beam dimension in the range of about 0.5-1 mm long, about two to five micrometers (μm) wide, and a slit spacing of about one to three micrometers (μm). The actual mask dimensions are a function of the demagnification factor (discussed above).
Processing according to one or more embodiments of the present invention is described with reference to
After the first translation, the illumination pattern is at position 315a (shown in
Upon reaching the predetermined endpoint, a single scan or translation of the sample region is complete, and the sample is moved along arrows 380 to a new start position at a new column 395a on the film and irradiation and translation in the reverse direction, e.g., along irradiation path 340, 345, etc. is carried out. It will be appreciated that the sample is now translated continuously in the negative y-direction. In this manner, the entire surface of the thin film is irradiated without disruption of the pulsed laser. Although
The substrate is advanced a distance in the positive y-direction and the mask is translated a distance in the positive x-direction along irradiation paths 320 and 325, respectively, to arrive at a second position 315a with respect to the stationary laser beams. The timing of the laser pulse is controlled such that the translation of the mask and the substrate are complete at the time of the next laser beam pulse.
As shown in
During and after crystallization of regions 450 (the time scale of solidification is about 1-10 μsec, while the interval between two laser pulses is a few msec), the substrate advances a distance in the y-direction and the mask is translated a distance in the negative x-direction along irradiation paths 320a and 325a, respectively, to arrive at a third position 315b with respect to the stationary laser beams, as shown in
The substrate continues to advance along the irradiation path shown in
Upon complete crystallization of column 395, the sample is translated along path 380 to a new position corresponding to column 395a of the sample. See
In one or more embodiments, the above process is characterized by mask oscillation between two x-axis positions, e.g., +x and −x, while the sample is continuously advanced in the same direction along the y-axis, e.g., +y. The exact mode of translation is not of great importance to the invention, so long as the distances in the x- and y-directions are carried out in coordination with the laser pulse frequency. Curves 610, 615 illustrate the x-position of the mask with time during stepwise (
In one or more embodiments of the present invention, the mask is translated stepwise or continuously in a constant x-direction, while the sample is continuously moved in a y-direction. Curves 630, 635 show x-position of the mask with time during a stepwise (
Although the x-axis oscillation introduces an additional process control step, it avoids the need to scan the same area of the substrate multiple times in order to fully crystallize the semiconductor film. Furthermore, it provides grains that are elongated beyond their characteristic lateral growth length, thereby further improving the crystalline properties of the polycrystalline film.
The foregoing examples describe an irradiation cycle consisting of two laser pulses and two x,y-translations, i.e., a cycle consisting of n pulses and n x,y-translations, where n=2. It is within the scope of the present invention to process a thin film using any number of sample and mask translation sequences to traverse an x-distance of about λ and a y-distance of about l′ (recall that l′ is related to l, the length of the mask feature, by a demagnification factor of the laser optics). The value for n can vary widely, and can range, for example, from n=2-100. Without being bound by any mode or outcome of operation, higher n-values tend to provide films of higher crystallinity, longer grains and fewer grain boundaries.
In one or more embodiments, a process is provided to fully irradiate a subregion defined by a mask feature of dimensions w, d, and l using “n” laser irradiation pulses and “n” sets of x,y-translations, e.g., hereafter referred to as an “n-irradiation cycle.” According to an exemplary embodiment, the sample moves a distance of about l′/n-δ in the y-direction between laser pulses. Translation distance of the mask in the x-axis between laser pulses is selected such that a total distance of λ is traversed over the sum of “n” cycles. Each x-translation can be the same or different, stepped or continuous. In one or more embodiments, each x-direction translation is substantially the same and can be λ/n. The selection criteria for the length and width of the mask features are similar to those described above for a mask used in the two-cycle irradiation process of
This process is illustrated schematically in
A crystallized column can also be generated using stepwise or continuous movement of the mask in a constant x-direction. By way of example,
In an exemplary process, the sample and the mask are positioned to provide an illumination pattern at an initial position 815 (shown by dotted lines in
The mask and sample move to position 815a (shown in
In other exemplary embodiments, the irradiation/crystallization sequence is carried out over “n” cycles, as described above for the oscillating mask process. ‘n’ can range, for example, from about 2 to 100, or more.
With reference to
The crystallization pattern such as shown in
An alternative crystalline grain structure is obtained when the mask is translated in a constant x-direction.
Tilted grains find many applications in fabrication of micro-electronic devices, for example, in the formation of thin film transistors (TFT) having active channel regions with uniform performance. The performance of a TFT depends, in part, on the electron mobility of the semiconductor polycrystalline layer, which depends, in part, on the number of grain boundaries in the TFT active channel. There will be certain device orientations where optimization of device uniformity (rather than device performance) is desired. In a tilt-engineered device, each thin film device is tilted (relative to the substrate edge) so that the same number of perpendicular grain boundaries is found in the active channel region. Each device, therefore, has comparable mobilities and performances. For example, co-pending International Application Serial Number PCT/US02/27246, filed Aug. 27, 2002, and entitled “Method to Increase Device-To-Device Uniformity for Polycrystalline Thin-Film Transistors by Deliberately Misaligning the Microstructure Relative to the Channel Region,” recommends that the TFT active channel is placed at an angle relative to the long location-controlled grain boundaries of the semiconductor film. However, forming TFT active channels (which typically involves conventional semiconductor fabrication steps such as patterning and ion implantation) at such irregular angles is inefficient and not easily integratable into standard fabrication processes.
According to exemplary embodiments of the present invention, a TFT device contains active channels arranged at regular and ordered intervals, while maintaining a desired tilt angle of the location-controlled grain boundaries.
In one or more embodiments of the present invention, a film is crystallized using an inverted masked 1300 such as the one shown in
With reference to
The mask and sample are positioned for an initial irradiation. The sample is moved continuously in the y-direction a distance 1410, while the mask also is microtranslated a distance 1415 in the x- and y-directions. The sample and mask are moved at a velocity that is calculated to position the sample in the correct location relative to the mask in time for the laser pulse. The illumination pattern is indicated in
The continued translation of the sample along pathway 1430 and the microtranslation of the mask in the x- and y-directions along pathway 1435 positions the sample for the third irradiation, denoted in
In one or more embodiments of the present invention, the irradiated pattern at position overlaps slightly with adjacent column 1495a, which ensures that the full sample surface is irradiated. The overlap is selected to maximize extent of the film coverage, yet to ensure that the film surface is fully irradiated. The width of the overlap is small, and can be, for example, 0.5 μm, 1 μm, 1.5 μm, or greater.
After four laser pulses, a region of the film exemplified by circle 1560 is completely irradiated. The opaque region designated as “1”′ indicates a fifth laser irradiation where the mask has returned to its original position A. Sample translation continues in this manner along path 1420′, 1425′, 1430′, 1435′, 1440′, 1445′, etc. until the sample reaches a predetermined endpoint. In sum, the sample moves with constant velocity in the y-direction, while the mask moves in both the x- and y-directions with the appropriate microtranslations to obtain the desired crystalline film.
If the distance between adjacent opaque regions 1320 on mask 1300 is greater than two times the characteristic lateral growth length of the film, then a crystallized structure surrounded by a small-grained polycrystalline precursor film is formed. Note that under some circumstance “complete irradiation” may result in some regions having small polycrystalline grains. As noted above, complete crystallization does not require that the entire film have large grains. It merely requires that the region be crystallized to the extent desired by the process, such that the process does not require the laser to again traverse the same subregion of the film. If the separation distance is less than or equal to the characteristic lateral growth length, then adjacent crystallized structures will form abutting grains and the entire irradiated film forms contiguous large crystalline grains. This structure is illustrated in
Although various embodiments that incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that incorporate these teachings.
By way of example only, it is within the scope of the invention to define an irradiation path that traverses only a portion of a substrate. It is also apparent that the choice of (x,y) as the coordinate system is arbitrary; the process can also be conducted using another set of coordinates. The sample can also be translated in the negatively and positive-x direction during irradiation, or the laser source may be moved during operation to achieve one or more of the directional translations.
This application claims priority under 35 U.S.C. § 119(e) to U.S. Application Ser. No. 60/504,270, filed Sep. 19, 2003, and entitled “Single Scan Irradiation for Crystallization of Thin Films,” the contents of which are incorporated herein by reference.
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