WO2015127031A1

WO2015127031A1 - Sequential laser firing for thin film processing

Info

Publication number: WO2015127031A1
Application number: PCT/US2015/016552
Authority: WO
Inventors: James S. Im; Miao Yu
Original assignee: The Trustees Of Columbia University In The City Of New York
Priority date: 2014-02-19
Filing date: 2015-02-19
Publication date: 2015-08-27

Abstract

The present disclosure is directed to methods and systems for processing a thin film. An exemplary method can include irradiating a first region of the thin film with a first laser pulse with a first energy density sufficient to partially melt the first region and cause crystal grain lateral growth from a seed region within the first region after the first laser pulse and irradiating the first region of the thin film with at least one sequential laser pulse with a time interval after the first laser pulse and a second energy density sufficient to delay crystal nucleation and extend lateral crystal growth within the first region without partially melting the film.

Description

SEQUENTIAL LASER FIRING FOR THIN FILM PROCESSING

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit under 35 U.S. C. 119(e) to U.S. Provisional

Application No. 61/941,795, entitled "METHODS AND SYSTEMS FOR SEQUENTIAL LASER FIRING TO INDUCE EXTENDED LATERAL GROWTH SOLIDIFICATION," filed on

February 19, 2014, and to U.S. Provisional Application No. 61/941,796, entitled "METHODS AND SYSTEMS FOR COMPLETE MELT CRYSTALLIZATION WITH SEQUENTIAL FIRING," filed on February 19, 2014, and to U.S. Provisional Application No. 61/941,790, entitled "METHODS AND SYSTEMS FOR LARGE GRAIN EXCIMER LASER ANNEALING," filed on February 19, 2014, the contents of all are incorporated by reference herein in their entirety.

TECHNICAL FIELD

[0002] The present disclosure relates to systems and methods for thin film processing. BACKGROUND

[0003] Excimer laser annealing ("ELA") is a technique for processing thin films by partially melting portions of the film using laser radiation. ELA systems are described in United States Patent No. 8,479,681 entitled "Single-Shot Semiconductor Processing System and Method Having Various Irradiation Patterns," the entire contents of which are incorporated by reference.

[0004] Advanced Excimer Laser Annealing ("AELA") is a relatively new ELA technique whereby portions, e.g., target areas, of a large area thin film can be irradiated and partially melted to induce crystallization. AELA differs from ELA in that AELA requires less irradiations per target area of film. AELA systems are described in PCT Publication Number WO2013172965 entitled "Advanced Excimer Laser Annealing for Thin Films." These AELA systems can eliminate many of the "edge issues" experienced in ELA systems.

[0005] Recent excimer laser systems can have multiple separate laser tubes. For example some systems can have two, four, or more separate laser tubes. For standard ELA processes, the separate lasers can be fired/used simultaneously in order to uniformly irradiate a thin film for laser crystallization processes. Sequential firing of laser tubes using two laser tubes has been described previously in United States Patent Application No. 13/892,904 entitled "Systems and Methods for Non-Periodic Pulse Sequential Lateral Solidification," and U.S. Patent Application No. 13/505,961 entitled "Systems and Methods for Non-Periodic Pulse Partial Melt Film Processing," the entire contents of each are incorporated by reference herein.

SUMMARY

[0006] According to aspects of the disclosure, a method of processing a thin film is provided. The method can include the steps of irradiating a first region of the thin film with a first laser pulse with a first energy density sufficient to partially melt the first region and cause crystal grain lateral growth from a seed region within the first region after the first laser pulse, and irradiating the first region of the thin film with at least one sequential laser pulse with a time interval after the first laser pulse and a second energy density sufficient to delay crystal nucleation and extend lateral crystal growth within the first region without partially melting the film.

[0007] According to alternative aspects of the disclosure, a method of processing a thin film can include the steps of irradiating a first region of the thin film with a first laser pulse with a first energy density sufficient to partially melt the first region and cause crystal grain lateral growth from a seed region with the first region after the first laser pulse and irradiating the first region of the thin film with a second laser pulse with a first time interval after the first laser pulse and a second energy density sufficient to delay crystallization of the thin film without partially melting the film. The method can also include the steps of irradiating the first region of the thin film with a third laser pulse with a second time interval after the second laser pulse and a second energy density sufficient to delay crystallization of the thin film without partially melting the film, and irradiating the first region of the thin film with a fourth laser pulse with a third time interval after the third laser pulse and a second energy density sufficient to delay crystallization of the thin film without partially melting the film.

[0008] According to alternative aspects of the disclosure, a method of processing a thin film can include the steps of irradiating a first region of the thin film with a first laser pulse having a first energy density that is sufficient to melt the first region and upon crystallization, generate a defect- free crystal sub-region and a defective crystal sub-region, and irradiating the first region of the thin film with a second laser pulse having a second energy density that is sufficient to melt the defective crystal sub-region without melting the defect- free crystal sub-region. According to aspects of the disclosure a time interval between the first laser pulse and the second laser pulse is longer than a time interval for a single, melting, crystallization, and solidification cycle of the thin film. BRIEF DESCRIPTION OF FIGURES

[0009] FIG. 1 depicts a prior art beam irradiation profile using four laser tubes simultaneously.

[0010] FIG. 2 depicts a sequential firing method whereby instead of firing the four laser tubes simultaneously, the laser tubes are fired sequentially, according to some aspects of the present disclosure.

[0011] FIG. 3 depicts beam profiles of a sequential firing method where the beam profiles differ, according to some aspects of the present disclosure.

[0012] FIG. 4 is a depiction of an irradiation region, according to some aspects of the present disclosure.

[0013] FIG. 5 is a depiction of an irradiation region, according to some aspects of the present disclosure.

[0014] FIG. 6 depicts a flow chart of an exemplary method for irradiating a region of a film using a multi-tube laser, according to some aspects of the present disclosure.

[0015] FIG. 7 depicts a sequential firing method whereby two laser tubes are fired sequentially, according to some aspects of the present disclosure.

[0016] FIG. 8 A and 8B depict an irradiation process occurring on a region of a thin film by showing cross sections of the thin film at various stages of the irradiation process, according to some aspects of the present disclosure.

[0017] FIG. 9 A and 9B are overhead views of a thin film during an irradiation process, according to aspects of the present disclosure.

[0018] FIG. 10 depicts a flow chart of an exemplary process for irradiating a region of a film using a multi-tube laser, according to some aspects of the present disclosure.

[0019] FIG. 11 depicts a plot of energy density distribution for ELA pulses, according to an embodiment of the present disclosure.

[0020] FIG. 12 depicts a plot of energy density distribution for ELA pulses, according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0021] According to embodiments of the present disclosure, using a four laser tube AELA system, the four laser tubes are fired sequentially to produce specific heating profiles. In some embodiments, the first pulse can cause melting of the thin film, while the subsequent pulses can control the temperature of the thin film to allow longer lateral crystallization growth, for example, by delaying the occurrence of nucleation. Accordingly, controlled timing of laser firing and the beam profile can be optimized to maximize lateral growth distance. Through this process high mobility, uniform thin film transistors ("TFTs") can be formed. These TFTs can be used, for example, for displays. While a four laser tube and/or a four laser AELA system is used as an example, any number of lasers and/or laser tubes can be used.

[0022] Prior art systems use the multiple separate laser tubes to fire sequential firing of laser pulses. In those systems, the sequential pulses can either extend the melting phase of the target area, when they overlap or can result in sequential melting phases of the target area, separated by crystallization and solidification. In addition, in those systems the crystallization starts after the irradiation ends. However, those systems do not allow complete control of the lateral grown distance. Accordingly, laser systems are required that provide tight control to maximize the lateral growth distance.

[0023] FIG. 1 depicts a prior art beam irradiation profile using four laser tubes simultaneously. The x-axis corresponds to time, while the y-axis corresponds to the intensity of the laser pulse. The resultant beam profile 100 is the combination of a first beam profile 102, a second beam profile 104, a third beam profile 106, and a fourth beam profile 108. If there are non-uniformities in each of the first through fourth beam profiles, the combination of the beams into one can average out those non-uniformities.

[0024] FIG. 2 depicts a sequential firing method whereby instead of firing the four laser tubes simultaneously at the same irradiation region, the laser tubes are fired sequentially. For example, a first laser tube is fired at a first time "t_l5" corresponding to a first beam profile 202, a second laser tube is fired at time "t₂," corresponding to a second beam profile 204, a third laser tube is fired at time "t₃," corresponding to a third beam profile 206, and a fourth laser tube is fired at time corresponding to a fourth beam profile 208. In this embodiment, the beam profiles are nearly identical. Each of these beam profiles corresponds to a firing of a laser tube and can be referred to as a "shot" or a "pulse." A region to be irradiated ("target region") will receive all four shots corresponding to the four beam profiles from the four laser tubes. In some embodiments, as shown in FIG. 2, each pulse ends before the subsequent pulse begins. However, in some embodiments, a subsequent pulse can be applied while the previous pulse is still being applied.

[0025] In some embodiments, the timing and profile of the first beam can be selected to maximize the lateral growth distance of the irradiated area. In one embodiment, shown in FIG. 2 each pulse has the same beam profile.

[0026] FIG. 3 depicts beam profiles of a sequential firing method where the beam profiles are not the same for each pulse. For example, a first laser tube is fired at time "t_l5" corresponding to a first beam profile 302, a second laser tube is fired at time "t₂," corresponding to a second beam profile 304, a third laser tube is fired at time "t₃," corresponding to a third beam profile 306, and a fourth laser tube is fired at time "t₄," corresponding to a fourth beam profile 308. In this embodiment, the first beam profile 302 is different from the subsequent beam profiles 304, 306, 308. For example, in this embodiment, the first beam profile is selected to control and define the melting of the region the beam irradiates, while the subsequent three pulses can be used to extend the control the temperature of the thin film. For example, the subsequent pulses can have a profile that keeps the temperature of the film high without causing melting of the thin film. The high temperature of the thin film can delay nucleation in the thin film. As a result, the lateral growth distance can be maximized. Other combinations of pulses can be used. For example, one or more pulses that control and define the melting of the target region can be combined in a sequence with one or more pulses that extend the control the temperature of the thin film.

[0027] In some embodiments, all four beam pulses can have the same energy density.

Accordingly, because the first beam profile 302 has a greater intensity than the subsequent beam profiles, the subsequent beams can have extended pulses, that is, the beam is applied to the substrate for a longer period of time to result in the same amount of energy imparted to the substrate as the first beam profile 302.

[0028] As discussed above, the pulse profiles provided by the present disclosure can provide better films because the present methods can reduce nucleation in the irradiated regions. For example, FIG. 4 depicts an irradiation region 400, irradiated by a single pulse. Irradiation region 400 contains a seed region 402, a crystalline region 404, and a nucleation region 406. Upon irradiation, seed region 402 remains solid and functions as the seed grains for crystalline growth in the crystalline region 404. Crystalline region 404 laterally grows. However, when the film cools, instead of laterally growing, the film can nucleate, as shown in nucleation region 406. This can produce a sub-optimal film for TFTs. If a second and/or third and/or fourth "shot" or laser pulse hits the irradiation region 400 before nucleation regions begin to nucleate and while lateral crystal growth continues in crystalline region 404, the entire irradiation region can be laterally crystallized, as shown in the irradiation region 500 in FIG. 5.

[0029] In some embodiments, the four laser tubes are contained within the same laser. In some embodiments, the four laser tubes can be contained within separate lasers. In some embodiments, not all four laser tubes are used or are necessary. For example, two of the four laser tubes or three of the four laser tubes can be used. Typically, the lasers used are excimer lasers.

[0030] According to aspects of the present disclosure, the thin film can be a semiconductor film, for example silicon. Alternatively, the film can be an oxide or a metal film.

[0031] The resultant crystallized film can be used for manufacturing of TFTs for use, for example, in display devices.

[0032] FIG. 6 depicts a flow chart of an exemplary method for irradiating a region of a film using a multi-tube laser. The method can include providing a thin film and a multi-tube laser 602, firing a first tube to irradiate a region of the thin film 604, sequentially firing a second tube to irradiate the same region of the thin film 606, sequentially firing a third tube to irradiate the same region of the thin film 608, and sequentially firing a fourth tube to irradiate the same region 610.

[0033] As discussed above, prior art systems use the multiple separate laser tubes to fire sequential firing of laser pulses. In those systems, the sequential pulses can either extend the melting phase of the target area, when they overlap, or can result in sequential melting phases of the target area, separated by crystallization and solidification. However, in those systems nucleation can happen after the end of the pulse, which results in irradiation regions with defects and short- length crystal grains. Accordingly, laser systems are required that can eliminate the regions with defects, which can result in better quality thin films.

[0034] According to aspects of the disclosure, a two "shot" laser irradiation method is provided where a region of film is irradiated sequentially with beams or pulses from two lasers. A "shot" refers to a beam or pulse of irradiation generated by a laser and directed onto a thin film to melt the thin film. The shot can be selected to either completely or partially melt the film. Additionally, the timing of the two shots can be selected to either fire the second shot while

solidification/crystallization of the film caused by the first shot is still proceeding or it can be timed to be fired when solidification has been completed. [0035] An ELA four tube system can be used for this process. This process can be used to make semiconductor films for manufacturing devices such as thin film transistors ("TFTs") for displays.

[0036] FIG. 7 depicts a sequential firing method whereby two laser tubes are fired sequentially. The x-axis corresponds to time and the y-axis corresponds to intensity. For example, a first laser tube is fired at time "t_l5" corresponding to a first beam profile 702, a second laser tube is fired at time "t₂," corresponding to a second beam profile 704. Both beams or shots can irradiate the same area of a thin film. In some embodiments, the first beam profile 702 has an energy density sufficient to induce complete melting of the material irradiated with the beam. In some

embodiments, the second beam profile 704 has an energy density sufficient to partially melt, but not completely melt the material irradiated with the beam. In some embodiments, the region irradiated with the first beam profile 702 is allowed to cool and completely crystallize before the second beam profile 704 irradiates the same region. In some embodiments, the second beam profile 704 irradiates the same region while the region is in the process of crystallizing. Exemplary energy densities for the first beam profile, which completely melts the film, can be about 350 mJ per square centimeter to about 400 mJ per square centimeter for a 50 nm film on a glass substrate. The second beam profile 704 can have an energy density of about 300 mJ per square centimeter, e.g., an energy density that does not completely melt the film. The duration of the beams can be about 30 nanoseconds to about 300 nanoseconds for extended pulses. The interval between the first and second beam can be between two nanoseconds to hundreds of nanoseconds.

[0037] FIG. 8 depicts an irradiation process proceeding on a region of a thin film by showing cross sections of the thin film at various stages of the irradiation process. FIG. 8A shows a completely melted region 802 of thin film. This can be a result of the first beam profile being selected to induce complete melting in the film. FIG. 8B shows a crystallized region of the thin film 804. This film structure can be the result of crystallization after the second shot by the second beam.

[0038] FIGS. 9A and 9B depict an overhead view of the film being processed through laser irradiation where the second shot is fired after the solidification is complete. FIG. 9A shows a crystallized region 900 after the first shot and after the film has melted, cooled, and crystallized. Crystallized region 900 has large grains, but the grains have defects 902. Once region 900 has cooled and crystallization has started, a second shot is fired, which melts again the film in specific areas only and the film re-crystallizes to form re-crystallized region 904. The beam intensity of the second shot is selected to preferentially melt the defective core region, but not the non-defective regions of the film. The areas where nucleation has occurred have a lower melting temperature than the defect-free areas. Accordingly, the sequential pulse profile can be selected such that it can cause only melting in the areas with nucleation. In this way, the defective core region 904 melts and re-crystallizes to form re-crystallized region 904, having large, and substantially defect-free grains.

[0039] FIG. 10 depicts a flow chart of an exemplary method for irradiating a region of a film using a multi-tube laser. The method can include irradiating a first region of the thin film with a first laser pulse having a first energy density that is sufficient to melt the first region and upon crystallization, generate a defect-free crystal sub-region and a defective crystal sub-region (step 1002). The exemplary method can also include waiting a time interval longer than a time interval for a single, melting, crystallization, and solidification cycle of the thin film (step 1004) and irradiating the first region of the thin film with a second laser pulse having a second energy density that is sufficient to melt the defective crystal sub-region without melting the defect- free crystal sub- region (step 1006).

[0040] According to aspects of the disclosure, methods and systems can use a multi-tube laser system with improved pulse to pulse characteristics to perform ELA at higher energy densities, e.g., higher than conventional ELA methods, but below the complete melting threshold, to obtain large- grained silicon using fewer laser pulses.

[0041] Films obtained through these methods can be used to produce devices, for example, thin film transistors ("TFTs"). The TFTs produced from these films can have high mobility and can be more efficient than TFTs manufactured using other types of silicon films. These TFTs can then be used for display devices. The thin films can be semiconductor films, for example, silicon films. Alternatively, the thin films can be oxide or metal films.

[0042] FIG. 11 depicts a plot of energy density distribution for ELA pulses. The y-axis corresponds to energy density and the x-axis corresponds to distribution of energy in the pulse. ELA is typically a partial melting crystallization method. Accordingly, to perform ELA, the entire distribution of the pulse should remain under the complete melting threshold 1102 of the material. For complete melting of a 50nm-thick silicon thin film, the complete melting threshold can be between 350-400mJ/cm², e.g., ~350mJ/cm². Conventional ELA pulses 1104 have a relatively wide energy distribution. Accordingly, the median energy density 1106 of a conventional ELA pulse had to remain substantially below the complete melting threshold of the film to ensure that no portion of the film was irradiated with enough energy to completely melt portions of the film. Generally, conventional ELA processes should be about 300 millijoules per square centimeter, with a distribution at full width half maximum of about 10 millijoules. However, advances in ELA systems have resulted in pulses having significantly tighter profiles as shown in pulse 1108.

Because pulse 1108 has a tighter energy distribution, the mean energy density 1110 can be significantly higher than conventional ELA pulses without resulting in complete melting of areas of the film. For example, for this new higher-energy, large-grain ELA process, the energy density is ~325mJ/cm² (just under the complete melting threshold, and closer to it than conventional ELA), with a full width half maximum distribution of a few millijoules, for example, twenty five millijoules.

[0043] FIG. 12 is a plot of energy density distribution for a conventional ELA pulse 1202 and a narrower ELA pulse 1204. As can be seen, narrower pulse 1204 has a significantly smaller energy distribution than conventional ELA pulse 1202.

[0044] Prior ELA systems, performed at a lower energy density, required 20 or more irradiations of the film to obtain appropriate crystal structure of the film. However, by increasing the average energy density of the narrower ELA pulse, less pulses, for example, two to four pulses, can be used to generate large crystal grains. Even with significantly less pulses, the grains obtained using these high energy density, narrow pulses can be two to three times the size of standard ELA 308 nm size grains.

[0045] According to aspects of the disclosure, the narrower ELA pulse and the conventional ELA pulse can have similar energy density. The narrower ELA pulse can result in a more aggressive melting and processing compared to a conventional ELA pulse. This can result in longer crystal grains compared to conventional system with similar energy. According to alternative aspects of the disclosure, using a narrower ELA pulse with lower energy compared to a conventional ELA pulse can result is crystal grains of similar quality.

[0046] According to aspects of the disclosure, systems for processing a film can include a laser source, a non-absorbing substrate for depositing the film, and memory having a set of instructions that can implement the steps of the methods described herein.

[0047] Although the disclosed subject matter has been described and illustrated in the foregoing exemplary embodiments, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the details of implementation of the disclosed subject matter may be made without departing from the spirit and scope of the disclosed subject matter.

Claims

1. A method of processing a thin film, comprising: irradiating a first region of the thin film with a first laser pulse with a first energy density sufficient to partially melt the first region and cause crystal grain lateral growth from a seed region within the first region after the first laser pulse; and irradiating the first region of the thin film with at least one sequential laser pulse with a time interval after the first laser pulse and a second energy density sufficient to delay crystal nucleation and extend lateral crystal growth within the first region without partially melting the film.

2. The method of claim 1, wherein the first energy density is equal to the second energy density.

3. The method of claim 1, wherein an intensity of the first laser pulse is higher than an intensity of the at least one sequential laser pulse.

4. The method of claim 1, wherein a duration of the first laser pulse is shorter than a duration of the at least one sequential laser pulse.

5. The method of claim 1, wherein the time interval between the first laser pulse and the at least one additional sequential laser pulse is shorter than a time interval for a single melting, crystallization, and solidification cycle of the thin film.

6. The method of claim 1 , wherein the time interval between the first laser pulse and the at least one sequential laser pulse is shorter than a time interval for nucleation to begin in the first region of the thin film.

7. The method of claim 1, further comprising: firing the first laser pulse from a first laser; and firing the at least one sequential laser pulse from a second laser.

8. The method of claim 1, further comprising: firing the first laser pulse from a first tube of a first laser; and firing the at least one sequential laser pulse from a second tube of the first laser.

9. A method of processing a thin film, comprising: irradiating a first region of the thin film with a first laser pulse with a first energy density sufficient to partially melt the first region and cause crystal grain lateral growth from a seed region with the first region after the first laser pulse; irradiating the first region of the thin film with a second laser pulse with a first time interval after the first laser pulse and a second energy density sufficient to delay crystallization of the thin film without partially melting the film; irradiating the first region of the thin film with a third laser pulse with a second time interval after the second laser pulse and a second energy density sufficient to delay crystallization of the thin film without partially melting the film; and irradiating the first region of the thin film with a fourth laser pulse with a third time interval after the third laser pulse and a second energy density sufficient to delay crystallization of the thin film without partially melting the film.

10. The method of claim 9, wherein the first, the second, the third, and the fourth laser pulses have the same energy density.

11. The method of claim 9, wherein an intensity of the first laser pulse is higher than an intensity of each of the second, the third, and the fourth laser pulses.

12. The method of claim 9, wherein a duration of the first laser pulse is shorter than a duration of each of the second, the third, and the fourth laser pulses.

13. The method of claim 9, wherein the time interval between each of the first laser pulse and second laser pulse, the second laser pulse and the third laser pulse, and the third laser pulse and the fourth laser pulse is shorter than a time interval for a single melting, crystallization, and

solidification cycle of the thin film.

14. The method of claim 9, wherein the time interval between each of the first laser pulse and second laser pulse, the second laser pulse and the third laser pulse, and the third laser pulse and the fourth laser pulse is shorter than a time interval for nucleation to begin in the first region of the thin film.

15. The method of claim 9, further comprising: firing the first laser pulse from a first laser; firing the second laser pulse from a second laser; firing the third laser pulse from a third laser; and firing the fourth laser pulse from a fourth laser.

16. The method of claim 9, further comprising: firing the first laser pulse from a first tube of a first laser; firing the second laser pulse from a second tube of the first laser; firing the third laser pulse from a third tube of the first laser; and firing the fourth laser pulse from a fourth tube of the first laser.

17. A method of processing a thin film, comprising: irradiating a first region of the thin film with a first laser pulse having a first energy density that is sufficient to melt the first region and upon crystallization, generate a defect- free crystal sub- region and a defective crystal sub-region; and irradiating the first region of the thin film with a second laser pulse having a second energy density that is sufficient to melt the defective crystal sub-region without melting the defect- free crystal sub-region; wherein a time interval between the first laser pulse and the second laser pulse is longer than a time interval for a single, melting, crystallization, and solidification cycle of the thin film.

18. The method of claim 17, further comprising: firing the first laser pulse from a first laser; and firing the second laser pulse from a second laser.

19. The method of claim 17, further comprising: firing the first laser pulse from a first tube of a first laser; firing the second laser pulse from a second tube of the first laser;

20. The method of claim 17, wherein upon crystallization of the defective crystal sub-region after the second pulse a defect in the defective crystal sub-region is removed.

21. The method of claim 17, wherein an intensity of the first laser pulse is higher than an intensity of the second laser pulse.

22. The method of claim 17, wherein a duration of the first laser pulse is shorter than a duration of the second laser pulse.