WO2018156080A1 - Reconfigurable gray scale photomasks - Google Patents

Reconfigurable gray scale photomasks Download PDF

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Publication number
WO2018156080A1
WO2018156080A1 PCT/SG2018/050079 SG2018050079W WO2018156080A1 WO 2018156080 A1 WO2018156080 A1 WO 2018156080A1 SG 2018050079 W SG2018050079 W SG 2018050079W WO 2018156080 A1 WO2018156080 A1 WO 2018156080A1
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Prior art keywords
phase change
patterns
thin film
gray scale
chalcogenide thin
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PCT/SG2018/050079
Other languages
French (fr)
Inventor
Qian Wang
Jinghua Teng
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Agency For Science, Technology And Research
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Publication of WO2018156080A1 publication Critical patent/WO2018156080A1/en

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Classifications

    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
    • G03F7/004—Photosensitive materials
    • G03F7/0042—Photosensitive materials with inorganic or organometallic light-sensitive compounds not otherwise provided for, e.g. inorganic resists
    • G03F7/0043—Chalcogenides; Silicon, germanium, arsenic or derivatives thereof; Metals, oxides or alloys thereof
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F1/00—Originals for photomechanical production of textured or patterned surfaces, e.g., masks, photo-masks, reticles; Mask blanks or pellicles therefor; Containers specially adapted therefor; Preparation thereof
    • G03F1/50—Mask blanks not covered by G03F1/20 - G03F1/34; Preparation thereof
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F1/00—Originals for photomechanical production of textured or patterned surfaces, e.g., masks, photo-masks, reticles; Mask blanks or pellicles therefor; Containers specially adapted therefor; Preparation thereof
    • G03F1/54—Absorbers, e.g. of opaque materials
    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
    • G03F1/00—Originals for photomechanical production of textured or patterned surfaces, e.g., masks, photo-masks, reticles; Mask blanks or pellicles therefor; Containers specially adapted therefor; Preparation thereof
    • G03F1/68—Preparation processes not covered by groups G03F1/20 - G03F1/50
    • G03F1/76—Patterning of masks by imaging

Definitions

  • the present invention generally relates to photolithography, and more particularly relates to reconfigurable multi-level gray scale photomasks for three- dimensional photolithography.
  • Photolithography is an essential element of present technology nodes of the semiconductor industries. Thus, due to mass production of semiconductor devices like integrated circuits or micro-electro-mechanical systems requiring the use of photomasks, photolithography is a driving force in semiconductor industry growth.
  • a method for fabricating gray scale photomasks includes optically writing gray scale photomasks in a phase change chalcogenide thin film using tightly focused femtosecond laser pulses to form patterns in the phase change chalcogenide thin film including a plurality of submicron sized crystallized marks.
  • a gray scale photomask includes rewriteable phase change chalcogenide thin film including a plurality of submicron sized crystallized marks forming patterns in the phase change chalcogenide thin film.
  • a system for fabricating gray scale photomasks includes rewriteable phase change chalcogenide thin films and a laser setup.
  • the laser setup emits a tightly focused pulsed laser beam with controllable pulse numbers onto the phase change chalcogenide thin film to form patterns in the phase change chalcogenide thin film including a plurality of submicron sized crystallized marks, the patterns forming gray scale photomasks.
  • FIG. 1 depicts a system for fabricating photomasks in accordance with a present embodiment
  • FIG. 1A depicts an illustration of optically writing photomasks in a phase change chalcogenide thin film using femtosecond laser pulses
  • FIG. IB depicts a magnified view of a submicron sized crystallized mark formed in the phase change chalcogenide thin film using the femtosecond laser pulses
  • FIG. 1C depicts a graph of the change of reflectance of phase change chalcogenide thin film in response to the number of femtosecond laser pulses.
  • FIG. 2 depicts steps in a method for fabricating three-dimensional silicon structures with gray scale photomasks
  • FIG. 2A depicts a reflection optical image of a gray scale photomask patterned in a phase change chalcogenide thin film in accordance with the present embodiment
  • FIG. 2B depicts a transmission optical image of the gray scale photomask of FIG. 2A
  • FIG. 2C depicts a positive photoresist pattern for etching in a conventional mask aligner system
  • FIG. 2D depicts three-dimensional silicon patterned by reactive ion etch (RIE) in accordance with the multi-level photoresist mask of FIG. 2C
  • FIG. 2E depicts a surface profiler characterization of an etched silicon surface using the gray scale photomask of FIGs. 2A and 2B.
  • RIE reactive ion etch
  • FIG. 3 depicts two examples of gray scale photomask transmission patterns, wherein FIG. 3A depicts an optical transmission image of GeSbTe (GST) grating gray scale photomask patterns with a fine structure, and FIG. 3B depicts an optical transmission image of multi-step gray scale photomask patterns.
  • GST GeSbTe
  • FIG. 4 depicts the optical reflection images of photoresist patterns on silicon
  • FIG. 4A depicts an optical reflection image of grating photoresist patterns on silicon using a GST gray scale photomask in accordance with the present embodiment
  • FIG. 4B depicts an optical reflection image of multi-step photoresist patterns on silicon.
  • FIG. 5 depicts the optical reflection images of silicon patterns after four minutes of etching
  • FIG. 5A depicts an optical reflection image of silicon grating patterns after etching for four minutes with RIE using a GST gray scale photomask in accordance with the present embodiment
  • FIG. 5B depicts an optical reflection image of silicon multi-step patterns after etching for four minutes through patterned photoresist.
  • System and methods in accordance with the present embodiment use a femtosecond laser to directly write - erase-rewrite patterns with either binary transmittance or multi-level gray scale transmittance on chalcogenide thin films for use as photomasks thereby providing a low cost, high resolution grayscale photomask solution.
  • the patterns in the phase change chalcogenide thin film are fabricated by tightly focusing the femtosecond laser pulses and by controlling degrees of crystallization in the phase change chalcogenide thin films by tuning the energy dosage and the number of stimulation optical pulses of the tightly focused femtosecond laser pulses.
  • an illustration 100 depicts a system for fabricating photomasks in accordance with a present embodiment.
  • Femtosecond laser pulses 102 are tightly focused onto a phase change chalcogenide thin film 104, including a chalcogenide layer 106 sandwiched between two capping and buffering layers 108, 110, mounted on glass 112.
  • the chalcogenide layer 106 is composed of Ge 2 Sb 2 Te 5 having a thickness of approximately seventy nanometers. It will be apparent to those skilled in the art, that other chalcogenide materials and other layer thicknesses can be used for the chalcogenide layer 106.
  • the insulative layers 108, 110 depicted in the illustration 100 are composed of seventy nanometer thick ZnS-Si0 2 layers, those skilled in the art will realize that other thin layer thermal-conductive materials can be used.
  • the chalcogenide thin film 104 is deposited on the glass 112.
  • the glass 112 is transparent to allow the light energy of the laser pulses 102 to pass through to the chalcogenide thin film 104.
  • a magnified view 140 depicts a zoomed optical image 142 of a submicron sized crystallized mark 144 created by the femtosecond laser pulses 102 in the phase change chalcogenide thin film 104.
  • FIG. 1C depicts a graph 170 where a per cent change in reflectance of the phase change chalcogenide thin film 104 is plotted along a y-axis 172 in response to a number of femtosecond laser pulses 102 plotted along a x-axis 174.
  • phase change chalcogenide thin film 104 can be fabricated into multi-level gray scale photomasks for three- dimensional photolithography wherein multi-level gray scale transmittance patterns in the phase change chalcogenide thin film 104 control optical transmission of each state in the gray scale photomasks.
  • an illustration 200 depicts an optical microscope image 202 of a reflection optical image of a gray scale photomask 204 patterned in a phase change chalcogenide thin film in accordance with the present embodiment.
  • FIG. 2B depicts an illustration 220 of an optical microscope image 222 of a transmission optical image of the gray scale photomask 224 patterned in the phase change chalcogenide thin film 104 in accordance with the present embodiment
  • FIG. 2C depicts an illustration 240 of an optical microscope image 242 of a positive photoresist pattern 244 as a gray scale photomask.
  • FIG. 2D depicts an illustration 260 of an optical microscope image 262 of an etched Si pattern 264 by conventional reactive ion etching (RIE) in accordance with the gray scale photoresist mask 244 and having a RIE exposure time of four minutes. It can be seen from the illustration 260 that after four minutes of exposure to RIE, the gray scale photoresist pattern is transferred into three-dimensional silicon structures.
  • RIE reactive ion etching
  • a graph 280 depicts a surface profiler characterization 282 of the silicon surface 264 etched using the gray scale photomask 204, 224.
  • a depth profile legend 284 multi-level gray scale transmittance patterns of the phase change chalcogenide thin film 106 of the gray scale photomask 204, 224 allows formation of three-dimensional microstructures on the silicon surface as shown by the varying depths of microstructures indicate by the silicon surface profiler characterization 282.
  • gray scale masks are optically written in the thin Ge 2 Sb 2 Te 5 film 106 with tightly focused femtosecond laser pulses 102 forming submicron sized crystallized marks 144.
  • the significant change in optical properties of the crystallized mark 144 allows formation of multilevel gray scale mask patterns.
  • Desired optical transmission of each state in the resulting gray scale photomask is achieved by controlling the degree of crystallization in the phase change material (e.g., the thin Ge 2 Sb 2 Te 5 film 106) through tuning the energy dosage and the number of stimulations pulses.
  • the gray scale photomasks can then be used for fabrication of three- dimensional patterns by transferring the initial grayscale mask onto a photoresist layer as the etching mask.
  • a key advantage of phase change material based gray scale photomasks are that they are optically rewritable, which makes it feasible to realize quick and inexpensive reconfiguration for on-line trial and error.
  • FIG. 3A depicts an optical reflection image 300 of period grating GST gray scale photomask patterns 302, while FIG. 3B depicts an optical reflection image 350 of multi-step gray scale photomask patterns 352.
  • FIGs. 4A and 4B show optical reflection images of photoresist patterns on silicon when using the gray scale photomask 352.
  • FIG. 4A depicts an optical reflection image 400 of period grating photoresist patterns 402, while FIG. 4B depicts an optical reflection image 450 of multi-step photoresist patterns 452 after two minutes' exposure when GST gray scale photomask 352 is used for patterning the photoresist in accordance with the present embodiment.
  • FIGs. 5 A and 5B demonstrate the optical reflection images of silicon patterns after four minutes of etching when using gray scale photoresist masks 452.
  • FIG. 5A depicts an optical reflection image 500 of silicon patterns 502 while
  • FIG. 5B depicts an optical reflection image 550 of multi-step Si patterns 552.
  • the present embodiment provides low cost gray scale photomask fabrication where high resolution and reconfigurable gray scale photomasks can be directly optically written onto thin phase change chalcogenide film 106 using a tightly focused femtosecond laser pulses 102 forming patterns of submicron sized crystallized marks 144.
  • the gray scale photomasks in accordance with the present embodiment can be used for fabricating three-dimensional microstructures on semiconductor wafers, micro lens arrays and other micro-optic structures, and functional surfaces such as anti-microbial, hydrophilic and hydrophobic surfaces.
  • Gray scale photomasks in accordance with the present embodiment can make three-dimensional microstructures of continuously varying surface height profiles with low cost and fast speed.
  • the method of fabricating gray scale lithography masks and rewritable masks in accordance with the present embodiment uses phase change chalcogenide thin film and femtosecond laser pulses to directly write-erase-rewrite two-dimensional patterns or gray scale patterns on the chalcogenide thin film as photomasks.
  • Photomasks in accordance with the present embodiment are rewritable and therefore convenient to reconfigure for online trial and error.

Abstract

Methods and systems for fabricating gray scale photomasks and a gray scale photomask are provided. The method includes optically writing gray scale photomasks in a phase change chalcogenide thin film using tightly focused femtosecond laser pulses to form patterns in the phase change chalcogenide thin film including a plurality of submicron sized crystallized marks. The gray scale photomask includes rewriteable phase change chalcogenide thin film including a plurality of submicron sized crystallized marks forming patterns in the phase change chalcogenide thin film.

Description

RECONFIGURABLE GRAY SCALE PHOTOMASKS
PRIORITY CLAIM
[0001] This application claims priority from Singapore Patent Application No. 10201701425U filed on 22 February 2017.
TECHNICAL FIELD
[0002] The present invention generally relates to photolithography, and more particularly relates to reconfigurable multi-level gray scale photomasks for three- dimensional photolithography.
BACKGROUND OF THE DISCLOSURE
[0003] Photolithography is an essential element of present technology nodes of the semiconductor industries. Thus, due to mass production of semiconductor devices like integrated circuits or micro-electro-mechanical systems requiring the use of photomasks, photolithography is a driving force in semiconductor industry growth.
[0004] However, conventional photolithography uses two-dimensional photomasks. Semiconductor wafer processing may involve many two-dimensional layers which are each built onto the wafer by a deposition or epitaxial growth step to lay down a new layer followed by a photomask step to pattern the layer. As semiconductor integrated circuit designs are incorporating three-dimensional design elements, it may take as many as twenty to thirty two-dimensional photomask steps to build up a single layer incorporating three-dimensional circuit elements. This issue is aggravated when a three-dimensional microstructure includes a continuously varying surface height profile. [0005] In addition, conventional photomasks are one-time use binary chrome masks which are configured for only one use. While this is not a drawback in mass production of a single semiconductor wafer design, smaller runs of semiconductor processing including research trial and error semiconductor fabrication is very expensive as each new modification of a design will require new photomasks.
[0006] Thus, what is needed is a photomask for three-dimensional photolithography and similar applications which facilitates both multi-level gray scale photolithographic applications and reconfigurability. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure.
SUMMARY
[0007] According to at least one embodiment of the present invention, a method for fabricating gray scale photomasks is provided. The method includes optically writing gray scale photomasks in a phase change chalcogenide thin film using tightly focused femtosecond laser pulses to form patterns in the phase change chalcogenide thin film including a plurality of submicron sized crystallized marks.
[0008] According to another embodiment of the present invention, a gray scale photomask is provided. The gray scale photomask includes rewriteable phase change chalcogenide thin film including a plurality of submicron sized crystallized marks forming patterns in the phase change chalcogenide thin film.
[0009] According to a further embodiment of the present invention, a system for fabricating gray scale photomasks is provided. The system includes rewriteable phase change chalcogenide thin films and a laser setup. The laser setup emits a tightly focused pulsed laser beam with controllable pulse numbers onto the phase change chalcogenide thin film to form patterns in the phase change chalcogenide thin film including a plurality of submicron sized crystallized marks, the patterns forming gray scale photomasks.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to illustrate various embodiments and to explain various principles and advantages in accordance with a present embodiment.
[0011] FIG. 1, comprising FIGs. 1A, IB and 1C, depicts a system for fabricating photomasks in accordance with a present embodiment, wherein FIG. 1A depicts an illustration of optically writing photomasks in a phase change chalcogenide thin film using femtosecond laser pulses, FIG. IB depicts a magnified view of a submicron sized crystallized mark formed in the phase change chalcogenide thin film using the femtosecond laser pulses, and FIG. 1C depicts a graph of the change of reflectance of phase change chalcogenide thin film in response to the number of femtosecond laser pulses.
[0012] FIG. 2, comprising FIGs. 2A to 2E, depicts steps in a method for fabricating three-dimensional silicon structures with gray scale photomasks, wherein FIG. 2A depicts a reflection optical image of a gray scale photomask patterned in a phase change chalcogenide thin film in accordance with the present embodiment, FIG. 2B depicts a transmission optical image of the gray scale photomask of FIG. 2A, FIG. 2C depicts a positive photoresist pattern for etching in a conventional mask aligner system, FIG. 2D depicts three-dimensional silicon patterned by reactive ion etch (RIE) in accordance with the multi-level photoresist mask of FIG. 2C, and FIG. 2E depicts a surface profiler characterization of an etched silicon surface using the gray scale photomask of FIGs. 2A and 2B.
[0013] FIG. 3, comprising FIGs. 3A and 3B, depicts two examples of gray scale photomask transmission patterns, wherein FIG. 3A depicts an optical transmission image of GeSbTe (GST) grating gray scale photomask patterns with a fine structure, and FIG. 3B depicts an optical transmission image of multi-step gray scale photomask patterns.
[0014] FIG. 4, comprising FIGs. 4A and 4B, depicts the optical reflection images of photoresist patterns on silicon, wherein FIG. 4A depicts an optical reflection image of grating photoresist patterns on silicon using a GST gray scale photomask in accordance with the present embodiment and FIG. 4B depicts an optical reflection image of multi-step photoresist patterns on silicon.
[0015] And FIG. 5, comprising FIGs. 5 A and 5B, depicts the optical reflection images of silicon patterns after four minutes of etching, wherein FIG. 5A depicts an optical reflection image of silicon grating patterns after etching for four minutes with RIE using a GST gray scale photomask in accordance with the present embodiment and FIG. 5B depicts an optical reflection image of silicon multi-step patterns after etching for four minutes through patterned photoresist.
[0016] Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been depicted to scale. DETAILED DESCRIPTION
[0017] The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description. It is the intent of the present embodiment to present a system of fabricating gray scale photolithography masks, including rewritable photolithography masks, by using phase change chalcogenide thin films and femtosecond laser. System and methods in accordance with the present embodiment use a femtosecond laser to directly write - erase-rewrite patterns with either binary transmittance or multi-level gray scale transmittance on chalcogenide thin films for use as photomasks thereby providing a low cost, high resolution grayscale photomask solution. The patterns in the phase change chalcogenide thin film are fabricated by tightly focusing the femtosecond laser pulses and by controlling degrees of crystallization in the phase change chalcogenide thin films by tuning the energy dosage and the number of stimulation optical pulses of the tightly focused femtosecond laser pulses.
[0018] Referring to FIG. 1A, an illustration 100 depicts a system for fabricating photomasks in accordance with a present embodiment. Femtosecond laser pulses 102 are tightly focused onto a phase change chalcogenide thin film 104, including a chalcogenide layer 106 sandwiched between two capping and buffering layers 108, 110, mounted on glass 112.
[0019] In accordance with one aspect of the present embodiment, the chalcogenide layer 106 is composed of Ge2Sb2Te5 having a thickness of approximately seventy nanometers. It will be apparent to those skilled in the art, that other chalcogenide materials and other layer thicknesses can be used for the chalcogenide layer 106. In addition, while the insulative layers 108, 110 depicted in the illustration 100 are composed of seventy nanometer thick ZnS-Si02 layers, those skilled in the art will realize that other thin layer thermal-conductive materials can be used.
[0020] During writing and erasing in accordance with the present embodiment, the chalcogenide thin film 104 is deposited on the glass 112. The glass 112 is transparent to allow the light energy of the laser pulses 102 to pass through to the chalcogenide thin film 104.
[0021] The femtosecond laser pulses 102 to form patterns in the phase change chalcogenide thin film 104 by creating a plurality of submicron sized crystallized marks in the chalcogenide layer 106. Referring to FIG. IB, a magnified view 140 depicts a zoomed optical image 142 of a submicron sized crystallized mark 144 created by the femtosecond laser pulses 102 in the phase change chalcogenide thin film 104.
[0022] The pattern creating the photomask that is created by directly writing onto the phase change chalcogenide thin film 104 with the femtosecond laser pulses 102 is controllably written onto the phase change chalcogenide thin film 104 by varying the intensity and the number of pulses of the femtosecond laser. FIG. 1C depicts a graph 170 where a per cent change in reflectance of the phase change chalcogenide thin film 104 is plotted along a y-axis 172 in response to a number of femtosecond laser pulses 102 plotted along a x-axis 174. A trace 176 on the graph 170 shows that the change of reflectance of a crystallized mark 144 in the phase change chalcogenide thin film 104 changes from zero per cent to one hundred per cent from a first pulse to about one hundred pulses. Thus, direct writing on the phase change chalcogenide thin film 104 as one hundred femtosecond laser pulses take about 10"6 second. [0023] In accordance with the present embodiment, the phase change chalcogenide thin film 104 can be fabricated into multi-level gray scale photomasks for three- dimensional photolithography wherein multi-level gray scale transmittance patterns in the phase change chalcogenide thin film 104 control optical transmission of each state in the gray scale photomasks. Referring to FIG. 2A, an illustration 200 depicts an optical microscope image 202 of a reflection optical image of a gray scale photomask 204 patterned in a phase change chalcogenide thin film in accordance with the present embodiment. FIG. 2B depicts an illustration 220 of an optical microscope image 222 of a transmission optical image of the gray scale photomask 224 patterned in the phase change chalcogenide thin film 104 in accordance with the present embodiment
[0024] As a demonstration of a one- step photolithography of three-dimensional silicon structure with gray scale photomask, FIG. 2C depicts an illustration 240 of an optical microscope image 242 of a positive photoresist pattern 244 as a gray scale photomask. FIG. 2D depicts an illustration 260 of an optical microscope image 262 of an etched Si pattern 264 by conventional reactive ion etching (RIE) in accordance with the gray scale photoresist mask 244 and having a RIE exposure time of four minutes. It can be seen from the illustration 260 that after four minutes of exposure to RIE, the gray scale photoresist pattern is transferred into three-dimensional silicon structures.
[0025] Referring to FIG. 2E, a graph 280 depicts a surface profiler characterization 282 of the silicon surface 264 etched using the gray scale photomask 204, 224. As can be seen from the silicon surface profiler characterization 282 and a depth profile legend 284, multi-level gray scale transmittance patterns of the phase change chalcogenide thin film 106 of the gray scale photomask 204, 224 allows formation of three-dimensional microstructures on the silicon surface as shown by the varying depths of microstructures indicate by the silicon surface profiler characterization 282.
[0026] Therefore, in accordance with the present embodiment, gray scale masks are optically written in the thin Ge2Sb2Te5 film 106 with tightly focused femtosecond laser pulses 102 forming submicron sized crystallized marks 144. The significant change in optical properties of the crystallized mark 144 allows formation of multilevel gray scale mask patterns. Desired optical transmission of each state in the resulting gray scale photomask (e.g., the gray scale photomask 204, 224) is achieved by controlling the degree of crystallization in the phase change material (e.g., the thin Ge2Sb2Te5 film 106) through tuning the energy dosage and the number of stimulations pulses.
[0027] The gray scale photomasks can then be used for fabrication of three- dimensional patterns by transferring the initial grayscale mask onto a photoresist layer as the etching mask. A key advantage of phase change material based gray scale photomasks are that they are optically rewritable, which makes it feasible to realize quick and inexpensive reconfiguration for on-line trial and error.
[0028] Referring next to FIGs. 3A and 3B, other patterns using as the gray scale photomask are depicted. FIG. 3A depicts an optical reflection image 300 of period grating GST gray scale photomask patterns 302, while FIG. 3B depicts an optical reflection image 350 of multi-step gray scale photomask patterns 352. FIGs. 4A and 4B show optical reflection images of photoresist patterns on silicon when using the gray scale photomask 352. FIG. 4A depicts an optical reflection image 400 of period grating photoresist patterns 402, while FIG. 4B depicts an optical reflection image 450 of multi-step photoresist patterns 452 after two minutes' exposure when GST gray scale photomask 352 is used for patterning the photoresist in accordance with the present embodiment.
[0029] FIGs. 5 A and 5B demonstrate the optical reflection images of silicon patterns after four minutes of etching when using gray scale photoresist masks 452. FIG. 5A depicts an optical reflection image 500 of silicon patterns 502 while FIG. 5B depicts an optical reflection image 550 of multi-step Si patterns 552.
[0030] Thus, it can be seen that the present embodiment provides low cost gray scale photomask fabrication where high resolution and reconfigurable gray scale photomasks can be directly optically written onto thin phase change chalcogenide film 106 using a tightly focused femtosecond laser pulses 102 forming patterns of submicron sized crystallized marks 144. The gray scale photomasks in accordance with the present embodiment can be used for fabricating three-dimensional microstructures on semiconductor wafers, micro lens arrays and other micro-optic structures, and functional surfaces such as anti-microbial, hydrophilic and hydrophobic surfaces. Gray scale photomasks in accordance with the present embodiment can make three-dimensional microstructures of continuously varying surface height profiles with low cost and fast speed. The method of fabricating gray scale lithography masks and rewritable masks in accordance with the present embodiment uses phase change chalcogenide thin film and femtosecond laser pulses to directly write-erase-rewrite two-dimensional patterns or gray scale patterns on the chalcogenide thin film as photomasks. Photomasks in accordance with the present embodiment are rewritable and therefore convenient to reconfigure for online trial and error.
[0031] While exemplary embodiments have been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should further be appreciated that the exemplary embodiments are only examples, and are not intended to limit the scope, applicability, operation, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of steps and method of operation described in the exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.

Claims

CLAIMS What is claimed is:
1. A method for fabricating photomasks, the method comprising:
optically writing photomasks in a phase change chalcogenide thin film using tightly focused femtosecond laser pulses to form patterns in the phase change chalcogenide thin film comprising a plurality of submicron sized crystallized marks.
2. The method in accordance with Claim 1 wherein forming the patterns in the phase change chalcogenide thin film comprises controlling degrees of crystallization in the phase change chalcogenide thin film by tuning an energy dosage and a number of stimulation optical pulses of the tightly focused femtosecond laser pulses.
3. The method in accordance with either Claim 1 or Claim 2 wherein the phase change chalcogenide thin film comprises a Ge2Sb2Te5 material.
4. The method in accordance with any of Claims 1 to 3 wherein the patterns comprise binary transmittance patterns.
5. The method in accordance with any of Claims 1 to 3 wherein the photomasks are gray scale photomasks and wherein the patterns comprise multi-level gray scale transmittance patterns for controlling optical transmission of each state in the gray scale photomasks.
6. The method in accordance with Claim 5 wherein the multi-level gray scale transmittance patterns comprise continuously varying transmittance for creating one or more three-dimensional microstructures having continuously varying surface height profiles.
7. The method in accordance with any of the proceeding claims further comprising:
erasing the patterns in the phase change chalcogenide thin film using additional tightly focused femtosecond laser pulses.
8. The method in accordance with Claim 7 further comprising after the erasing step:
optically rewriting photomasks in the phase change chalcogenide thin film using additional tightly focused femtosecond laser pulses to form patterns comprising a plurality of submicron sized crystallized marks in the phase change chalcogenide thin film.
9. A photomask comprising:
rewriteable phase change chalcogenide thin film comprising a plurality of submicron sized crystallized marks forming patterns in the phase change chalcogenide thin film.
10. The photomask in accordance with Claim 9 wherein the phase change chalcogenide thin film comprises a Ge2Sb2Te5 material.
11. The photomask in accordance with either Claim 9 or Claim 10 wherein the patterns comprise binary transmittance patterns.
12. The photomask in accordance with either Claim 9 or Claim 10 wherein the photomask is a gray scale photomask, and wherein the patterns comprise multilevel gray scale transmittance patterns.
13. The gray scale photomasks in accordance with Claim 12 wherein the multi-level gray scale transmittance patterns comprise one or more continuously varying transmittance for creating three-dimensional microstructures having continuously varying surface height profiles.
14. A system for fabricating photomasks, the system comprising:
rewriteable phase change chalcogenide thin films; and
a laser set up for emitting a tightly focused pulsed laser beam onto the phase change chalcogenide thin film to form patterns in the phase change chalcogenide thin film comprising a plurality of submicron sized crystallized marks, the patterns forming the photomasks.
15. The system in accordance with Claim 14 wherein the rewriteable phase change chalcogenide thin film comprises a Ge2Sb2Te5 material.
16. The system in accordance with either Claim 14 or Claim 15 wherein the laser set up emits the tightly focused pulsed laser beam onto the phase change chalcogenide thin film to form the patterns comprising binary transmittance patterns.
17. The system in accordance with either Claim 14 or Claim 15 wherein the photomasks are gray scale photo masks, and wherein the laser set up emits the tightly focused pulsed laser beam onto the phase change chalcogenide thin film to form the patterns comprising multi-level gray scale transmittance patterns.
18. The system in accordance with Claim 17 wherein the laser set up tunes an energy dosage and a number of stimulation optical pulses to control degrees of crystallization in the phase change chalcogenide thin film so as to control optical transmission of each state in the gray scale photomasks.
19. The system in accordance with either Claim 17 or Claim 18 wherein the laser set up emits the tightly focused pulsed laser beam onto the phase change chalcogenide thin film to form the multi-level gray scale transmittance patterns comprising continuously varying transmittance for creating one or more three- dimensional microstructures having continuously varying surface height profiles.
PCT/SG2018/050079 2017-02-22 2018-02-22 Reconfigurable gray scale photomasks WO2018156080A1 (en)

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