KR100975785B1 - Laser assisted direct imprint lithography - Google Patents

Abstract

According to the invention, the features can be imprinted directly onto the surface of the solid substrate. In particular, the substrate provides a mold 20 having a molding surface 21 to imprint a pattern, placing the molding surface adjacent to or against the substrate 24 surface 25 to be imprinted, and the surface Imprinting directly into the desired pattern by steps of irradiating the substrate surface with radiation to soften or liquefy. The molding surface is pressed into a softened or liquefied surface to directly imprint the substrate. The substrate can be any of a wide variety of solid materials, such as semiconductors, metals or polymers. In a preferred embodiment, the substrate is silicon, the laser is a UV laser and the mold is transparent to UV radiation to allow irradiation of the silicon workpiece through the transparent mold. By using this method, the coatings were directly imprinted into silicon large-scale patterns with sub-10 nanometer resolution within a processing time of sub-250 nanoseconds. This method may be used with a flat molding surface to planarize the substrate.

Imprint Lithography, Molding Patterns, Laser Radiation, Lamp Radiation

Description

Laser assisted direct imprint lithography

Government's expressed interest

The present invention was made with government support under DARPA (N66001-98-1-8900) and ONR (N00014-01-1-0741). The government has certain rights in the invention.

Cross Reference to Related Applications

This application claims the priority of US Provisional Application No. 60 / 364,653, filed by Stephen Chou on March 15, 2002, entitled "A Method for Direct Imprint of Microstructures in Materials."

This application is a partial continuing application of US Patent Application No. 1 0 / 244,276, filed September 16, 2002 by Stephen Chou, entitled "Lithographic Method For Molding Pattern With Nanoscale Features," which was issued October 29, 2001. US patent application Ser. No. 09 / 107,006, filed on June 30, 1998, filed by Stephen Chou on June 30, 1998, now part of US Patent Application No. 10 / 046,594, filed on October 30, 2001. US Patent Application No. 08 / 558,809 (current US Patent No. 5,772,905, issued June 30, 1998) filed by Stephen Chou on November 15, 1995 Insist on priority. All of the above related applications are incorporated herein by reference.

This application is a partial continuing application of US Patent Application No. 1 0 / 140,140, filed by Stephen Chou on May 7, 2002 entitled "Fluid Pressure Imprint Lithography," entitled "Fluid Pressure Imprint Lithography." United States Patent Application No. 09 / 618,174, filed by Stephen Chou on July 18 (currently US Patent No. 6,482,742, issued November 19, 2002).

FIELD OF THE INVENTION

FIELD OF THE INVENTION The present invention relates to imprint lithography, and more particularly to laser assisted direct imprint lithography (LADI), in which laser radiation allows direct imprinting of a mold on a substrate surface. This process is particularly useful for imprinting nanoscale features directly on solid substrates.

Methods of patterning small features onto substrates are very important for manufacturing many electronic, magnetic, mechanical and optical devices as well as devices for biological and chemical analysis. Such methods are used, for example, to define the features and configurations of microcircuits and the structure and operating features of planar optical waveguides and associated optical devices.

Optical lithography is a conventional method of patterning such features. A thin layer of photoresist is applied to the substrate surface and selected portions of the resist are exposed to the pattern of light. The resist is then developed to reveal the desired pattern of exposed substrate for further processing such as etching. The difficulty with this process is that its resolution is limited by the wavelength of the light scattered in the resist and the substrate and the thickness and properties of the resist. As a result, optical lithography is becoming increasingly difficult as the desired feature size becomes smaller. Moreover, applying, developing and removing resists is a relatively slow step, which limits the processing speed.

More recent attempts to pattern small features include nanoimprint lithography in which the nanoofeatured molding surface is imprinted with a resist or resist-like material (polymer) and the imprinted pattern is removed to selectively expose the substrate surface. to be. This process removes the limitation on the resolution caused by the wavelength of light, but nevertheless has limitations imposed by the use and processing of resists or polymers. Further details regarding nanoimprint lithography are described in the Applicant's US Patent No. 5,772,905, issued June 30, 1998 and entitled "Fluid Pressure Imprint Lithography," dated November 19, 2002. US Pat. No. 6,482,742 issued by US Pat. Both of these patents '905 and' 742 are incorporated herein by reference.

The electronics and optical communications industries, as well as companies introducing new nanotechnology fields, continue to find new ways to produce microcircuits, optical devices and new nanoscale structures. In particular, they are looking for faster processes for patterning substrates with larger resolution patterns.

According to the invention, the features can be imprinted directly onto the surface of the solid substrate. In particular, the substrate provides a mold having a molding surface to imprint a pattern, disposing the molding surface adjacent to or opposite the substrate surface to be imprinted, and irradiating the substrate surface with radiation to soften or liquefy the surface. The steps are imprinted directly into the desired pattern. The molding surface is pressed on the softened or liquefied surface to directly imprint the substrate. The substrate can be any of a wide variety of solid materials, such as semiconductors, metals or polymers. In a preferred embodiment, the substrate is silicon, the laser is a UV laser and the mold is transparent to UV radiation to allow irradiation of the silicon workpiece through the transparent mold. By using this method, the coatings were directly imprinted into silicon large-scale patterns with sub-10 nanometer resolution within a processing time of sub-250 nanoseconds. This method may use a flat molding surface to planarize the substrate.

1 is a schematic flowchart showing steps involved in laser assisted direct imprint lithography.

2A-2E illustrate a mold and a substrate at various process steps of FIG. 1.

3 is a graph plotting the reflectance of a HeNe laser beam from a silicon surface as a function of time when the silicon surface is irradiated.

4 shows a scanning electron micrograph of an imprinted silicon grating.

5A shows a scanning electron micrograph of a sectional view of a quartz mold.

5B shows a scanning electron micrograph of a cross-sectional view of a silicon substrate imprinted by the method of FIG. 1.

6 shows an atomic force micrograph of an isolated silicon square pattern produced by the method of FIG. 1.

7 shows a scanning electron micrograph of a quartz mold according to two uses of the method of FIG. 1.

The advantages, features and various further features of the invention will appear more fully upon consideration of the exemplary embodiments shown in the accompanying drawings which are described in detail below.

It is to be understood that these drawings are provided for the purpose of illustrating the concepts of the invention and are not drawn to scale except to the graph.

Referring to the drawings, FIG. 1 is a schematic block diagram showing the steps involved in directly laser-imprinting a desired pattern onto a surface of a substrate without intervening resist or polymeric material. The first step shown in block A is to provide a mold having a molding surface to imprint the desired pattern.

The next step (block B) is to place the mold such that the molding surface is adjacent to the substrate surface to be imprinted. The molding surface may be pressed against the substrate surface.

2A shows a mold 20 having a molding surface 21 comprising one or more protruding features 22 and one or more concave regions 23. The mold 20 is placed near the substrate 24 having the surface 25 to be imprinted. The molding surface 21 is adjacent to the surface 25.

Substrate 24 may be any material that softens or liquefies in response to laser radiation and returns to its original state when laser exposure ends. The mold 20 is advantageously constructed of a material transparent to laser radiation such that the substrate surface 25 can be irradiated through the mold.

Molding surface 21 may have microscale or nanoscale features. Advantageously, this surface comprises a pattern of nanofeatures having at least one feature having a minimum dimension of less than 200 nanometers, for example the lateral dimension of the overhanging feature 22 can be less than 200 nanometers. . Advantageously, the molding surface also provides a mold depth of less than 250 nanometers, ie the distance between the outside of the protruding features 22 and the inside of the adjacent concave region 23 is less than 250 nanometers, preferably 5 And may range from 250 nanometers. However, to planarize the substrate surface, it is advantageous for the molding surface to be flat.

In a preferred embodiment, the substrate is silicon, the laser is a UV laser and the mold is fused quartz. A typical mold may be composed of 1 millimeter thick fused quartz with patterns having portions in the size range from less than 10 nanometers up to tens of microns. Other advantageous substrates include other semiconductors, metals, semimetals, polymers and ceramics.

The substrate surface layer to be molded may be a multilayer structure of thin films of material selected from semiconductors, metals, semimetals, polymers and insulators. This surface layer may also consist of a plurality of surface regions, which are different surface regions formed of different materials.

The third step (block C) is to irradiate the substrate surface to be molded with radiation to soften or liquefy the surface.

2B illustrates a preferred arrangement for irradiation where radiation 26 from a source (not shown) passes through the mold 20 to irradiate the surface 25 of the substrate. Laser radiation, which may be laser radiation, forms a softened or liquefied surface area 27 on the substrate 24.

Useful radiation sources include lasers (ultraviolet, infrared and visible light) and lamps (narrow band and wide band spectrum). The laser radiation is advantageously in the range from 1 nanometer to 100 micrometers. The lamp radiation is advantageously in the range from 1 nanometer to 50 micrometers. The irradiation can advantageously be a pulse with a duration in the range of 1 nanosecond to 10 seconds.

The next step, shown in block D of FIG. 1, is to imprint the substrate by pressing the molding surface into a softened or liquefied surface area.

Such compaction may be performed by mechanical compaction, as disclosed in the referenced US Pat. No. 5,772,905, by hydraulic pressure as described in US Pat. No. 6,482,742, or by any other method of applying a force. Pressure may be applied at the start before irradiation. One advantageous attempt is to squeeze the mold against the substrate and to irradiate the substrate surface with a plurality of pulses during the squeeze process.

2C shows a substrate having a molding surface 21 that imprints the surface area 27.

To complete the imprint process, irradiation is terminated by switching off the source, the surface area is slightly cooled, and the molding surface is removed from the substrate.

2D illustrates that the mold 20 is removed from the substrate 24, leaving the imprinted mold surface 28 on the substrate surface 25.

Hereinafter, the present invention can be more clearly understood by considering the following specific embodiments.

Three patterns: (1) grating at 140 linewidth, 110 nm deep and 300 nm spacing; (2) 10 nm wide and 15 nm deep lines (generated due to the trenching effect in reactive ion etching during the molding process), (3) a series of experiments using molds with rectangles of length and width tens of microns and 110 nm deep Were performed. All three patterns were imprinted directly into silicon in the same LADI process.

First, as described above, the quartz mold was brought into contact with the silicon substrate. Two large crimp plates were used to provide the force to compress the mold against the substrate. The silicon wafer was placed on the lower plate by a mold on top of the wafer. Also on the top of the mold was a top plate made of fused quartz and thus transparent to the laser beam. The two plates were pressed together by increasing the pressure provided by the screws between the two large plates. The preferred level of pressure applied to contact the mold and the substrate was approximately 1.7x10 ⁶ Pa or about 17 atmospheres. These pressure levels produce imprint depths of ~ 110nm at imprint times of ~ 250ns. Preferably, pressure is applied before the introduction of the laser pulse.

Next, a laser pulse was applied to the mold-substrate assembly. In one experiment, a single XeCl excimer laser pulse with a wavelength of 308 nm, pulse duration of 20 ns at FWHM, and fluence of 1.6 J / cm ² was applied to the assembly. The laser passing through the quartz mold is absorbed by the mold with very little energy. The transmittance of the mold to laser radiation was measured and found to be ˜93%. The difference in diffraction index at the two mold-air interfaces that caused a total radiation reflection of -7% indicating the mold itself does not absorb laser radiation.

A single laser pulse irradiates the silicon surface and induces melting and liquefaction of the silicon for about 250 ns. Advantageously, melting of the substrate surface can be self-monitored during the LADI process by measuring the time-varying reflectivity of the laser beam from the substrate surface. Reflectance is the ratio of the energy of the waveform reflected from the surface to the energy possessed by the waveform striking the surface. When silicon melts, it changes from semiconductor to metal, and the surface reflectivity of silicon to visible light increases by a factor of two.

In one experiment, a HeNe laser beam with a wavelength of 633 nm was used to measure reflectance. 3 shows the measured reflectance of the silicon surface exposed to a single 20 ns XeCl excimer laser pulse at a fluence of 1.6 J / cm ² . As shown in the graph, the reflectance of the solid silicon was about 25. Under radiation, the reflectivity of the silicon surface first rapidly increases to 25 ns, saturates for 200 ns at a reflectance of about 50 and returns to raw solid silicon reflectivity at 50 ns.

When the substrate was in the liquid state, the mold was embossed with the liquid substrate layer. The low viscosity of the molten silicon causes the molten silicon to flow into all the crevasses at high speed, filling them completely and making them fit into the mold. The melting period of silicon is nearly equivalent to 20ns, the duration of the laser pulse. In Figure 3, 25 ns, which increases the edge of the reflectance to slightly longer than the pulse duration, may be related to the RC time constant (100 MHz) of the oscilloscope, which is 10 ns.

The mold and substrate are then separated to leave a negative profile of the imprinted mold pattern on the substrate surface. Typically, the transformation from liquid to solid expands the substrate volume by about 3% in each direction that can assist in identifying the mold features of the substrate to achieve sub-10 nm resolution. It is desirable to easily separate the substrate and the mold so as not to damage the imprinted features of the substrate or the features of the mold. Experiments are aided by this separation by the fact that the quartz mold has a coefficient of thermal expansion (5 × 10 ⁻⁷ K ⁻¹ ) less than silicon (2.5 × 10 ⁻⁶ K ⁻¹ ).

5A is a scanning electron micrograph showing a cross-sectional view of a quartz mold. 4 and 5B are scanning electron micrographs each showing a front view and a cross sectional view of the imprinted features of the silicon substrate as a result of the LADI. The imprinted grating resulting from the LADI has a 140 nm length, 110 nm depth and 300 nm period, which is consistent with the features of the mold. One of the most interesting features in FIG. 5B is a ridge formed along the upper corners on the imprinted grating lines. These ridges are approximately 10 nm wide and 15 nm high. A comparison to the scanning electron micrograph of the mold in FIG. 5A indicates that these ridges emerge from the notches formed in the mold caused by the trenching effect of the reactive ion etching of the mold. The perfect transition of these 10nm ridges in the LADI process suggests that the resolution of the LADI is better than 10nm.

LADI also results in imprints of large discrete patterns. Typically, LADI can pattern mesas and trenches that are several tens of microns or larger in size. 6 shows a silicon substrate imprinted by LADI with separate square mesas having a width of 8 μm and a height of 110 nm. Successful imprints of such large patterns indicate that molten silicon can easily flow over tens of microns within nanoseconds.

7 shows a scanning electron micrograph of a quartz mold following two applications of LADI. Remarkably, the mold does not show any visible damage due to multiple LADI applications.

UV laser radiation sources are advantageous, but depending on the substrate, other sources may be used. Infrared lasers can be used to soften or liquefy many metals, and even heat lamps can be used to melt or soften many polymers.

If the surface to be imprinted comprises a plurality of layers, the combination of layers may be imprinted simultaneously. The radiation can be selected to melt a layer of material that can melt adjacent layers through heat transfer.

The pulse duration of the radiation can be selected according to the imprint process, and the pulse can be short enough so that the remainder of the substrate as well as the surface and mold of the substrate being heated are not significantly heated.

In addition, there are prefabricated patterns on the substrate, and there may be an alignment between the pattern on the substrate and the pattern on the mold.

The equipment for laser assisted direct imprint is a stage for each of the substrates and molds and a mechanical stage that produces relative movement between the substrates and the mold in x, y, z, zeta, tilt and grab (all freely possible). It may have a sortability including. The sensors could not only monitor the imprint heating pressure but also monitor the relative movement between the substrates and the mold, and the information of the imprint controller could monitor the feedback and control system.                 

The method is expected to be used in a series of steps to form complex structures. The substrate may have an existing pattern, and the pattern on the mold may be aligned with respect to the existing substrate pattern prior to imprint. The substrate can be patterned as disclosed, the layers can be added and planarized if necessary, and the added layers can be patterned in the same manner but in different configurations. Multiple successive layers can be imprinted according to the aligned patterns to form complex devices in the same manner as integrated circuits are formed. The substrate may comprise a thin layer of radiation absorbing material on a substrate that is transparent to radiation. This method can be used with a polymer coated substrate to imprint the pattern into resists and then use the patterned resist to pattern the substrates, as disclosed in US Pat. No. 5,772,905.

Moreover, this process can be used to fabricate many devices on the same substrate using step and repeated imprint and automatic alignment techniques well known in the art.

Imprinted substrates may have concave regions that will retain patterns of material added after imprint. The added material may be magnetic or conductive to form small electromagnetic components. The concave regions can even act as microscale or nanoscale molds for the production of small parts.

Microscale or nanoscale grooves can be imprinted into the substrate to select, orient, and direct macromolecules, and the imprinted peaks and recesses can be provided for attachment of indicators to be selectively attached to target molecules. have.

It is to be understood that the above embodiments illustrate only a few of the many embodiments that may represent applications of the present invention. Numerous various other arrangements can be made by those skilled in the art without departing from the spirit and scope of the invention.

Claims (45)

In a method of imprinting a mold pattern directly onto a substrate surface: Providing a solid substrate having a surface to be imprinted, wherein the surface to be imprinted comprises a material selected from the group consisting of semiconductors, metals, semimetals, alloys, ceramics and insulators Providing a substrate; Providing a mold having a molding surface to imprint the pattern; Placing the mold near the substrate such that the molding surface is adjacent to the substrate surface to be imprinted; Irradiating the substrate surface with radiation to soften or liquefy the substrate surface; Pressing the molding surface onto the softened or liquefied substrate surface; And Removing the molding surface from the substrate to leave an imprinted pattern of the molding surface on the substrate. The method of claim 1, wherein the radiation is laser radiation. The method of claim 2, wherein the laser radiation is in a wavelength ranging from 1 nanometer to 100 micrometers. The method of claim 1, wherein the radiation is lamp radiation. The method of claim 4, wherein the lamp radiation ranges from 1 nanometer to 50 micrometers. The method of claim 1, wherein the irradiation is pulsed. The method of claim 6, wherein the irradiation is a pulse having a duration in the range of 1 nanosecond to 10 seconds. 7. The method of claim 6, wherein the irradiation is repeatedly pulsed while the mold is pressed against the substrate. The method of claim 1, wherein the radiation heats the surface layer of the substrate without heating the mold or the entire substrate. The method of claim 1, wherein the mold comprises a material transparent to the radiation. The method of claim 1, wherein the mold comprises fused quartz. The method of claim 1, wherein the molding surface comprises a pattern of projecting and recessed features having at least one feature having a minimum dimension of less than 200 nanometers. The method of claim 1, wherein the molding surface comprises a pattern of raised and concave features having a mold depth of less than 250 nanometers from the outside of the raised feature to the inside of the concave region. The method of claim 1, wherein the substrate surface is irradiated with ultraviolet laser radiation. The method of claim 1, wherein the molding surface is pressed onto the substrate surface by mechanical compression or hydraulic pressure. The method of claim 1, wherein the molding surface is flat to planarize the surface of the substrate. The method of claim 1, wherein the mold is formed of a material transparent to ultraviolet radiation, the substrate is silicon, and the substrate is irradiated with ultraviolet laser radiation through the mold. The method of claim 1, wherein the substrate surface comprises a semiconductor material. The method of claim 1, wherein the substrate surface comprises a metal or an alloy. The method of claim 1, wherein the substrate surface comprises a ceramic. The method of claim 1, wherein the substrate surface comprises a plurality of material layers to be molded. The method of claim 1, wherein the radiation is infrared radiation. The method of claim 1, wherein the substrate comprises a previously imprinted layer. The method of claim 1, further comprising filling the concave regions of the imprinted pattern with material. 10. The method of claim 1, further comprising repeating the imprint process at different locations on the same substrate. The method of claim 1, wherein the substrate comprises an existing pattern, and further comprising aligning the pattern on the mold with the existing pattern on the substrate. 10. The method of claim 1, further comprising planarizing the substrate prior to imprinting the substrate. The method of claim 1, wherein the substrate comprises silicon. The method of claim 1, wherein the substrate comprises an insulator. The method of claim 1, wherein the solid substrate comprises a plurality of layers and the surface to be imprinted comprises a thin film of semiconductor. The method of claim 1, wherein the solid substrate comprises a plurality of layers and the surface to be imprinted comprises a thin film of metal. The method of claim 1, wherein the solid substrate comprises a plurality of layers and the surface to be imprinted comprises a thin film of semimetal. The method of claim 1, wherein the solid substrate comprises a plurality of layers and the surface to be imprinted comprises a thin film of an alloy. The method of claim 1, wherein the solid substrate comprises a plurality of layers and the surface to be imprinted comprises a thin film of ceramic. The method of claim 1, wherein the solid substrate comprises a plurality of layers and the surface to be imprinted comprises a thin film of insulator. The method of claim 1, wherein the substrate comprises a plurality of layers, and the surface to be imprinted consists of a plurality of surface regions formed of different materials. 37. The method of claim 36, further comprising adding and imprinting one or more additional layers to the imprinted added layer. 37. The method of claim 36, comprising planarizing the added layer prior to imprinting the added layer. In a method of forming a composite device: Providing a substrate having a first surface to be imprinted; Imprinting the first surface in a first pattern according to the method of claim 1; Adding a layer to the imprinted surface; And Imprinting the added layer in a second pattern. In a method of forming a composite device: Providing a substrate having a surface imprinted with a first pattern; Adding a layer to the imprinted surface; And Imprinting the added layer in a second pattern according to the method of claim 1. In a method of imprinting a mold pattern directly on a solid substrate surface: Providing the solid substrate; Providing a mold having a molding surface to imprint the pattern; Placing the mold near the substrate such that the molding surface is adjacent to the substrate surface to be imprinted; Irradiating the substrate surface with radiation to soften or liquefy the substrate surface; Pressing the molding surface onto the softened or liquefied substrate surface; And Removing the molding surface from the substrate surface to leave an imprinted pattern of the molding surface on the substrate, Wherein the mold is transparent to the radiation, the substrate surface absorbs the radiation, and the substrate surface is irradiated through the transparent mold. 42. The method of claim 41 wherein the substrate surface to be imprinted is softened or liquefied without softening or liquefying the remainder of the solid substrate. 42. The mold pattern of claim 41 wherein the substrate surface is irradiated with a pulsed laser and the pulses of the laser are selected to selectively soften or liquefy the substrate surface without softening or liquefying the rest of the solid substrate. Direct imprint method. 42. The method of claim 41 wherein the solid substrate comprises silicon. In a method of directly imprinting a mold pattern onto a silicon substrate surface without intervening polymer or resist layers: Providing the silicon substrate; Providing a mold having a molding surface to imprint the pattern; Placing the mold near the substrate such that the molding surface is adjacent to the surface to be imprinted; Irradiating the substrate surface with ultraviolet radiation to liquefy the substrate surface; Pressing the molding surface onto the liquefied substrate surface; And Removing the molding surface from the substrate surface to leave an imprinted pattern of the molding surface on the substrate, Wherein the mold is transparent to the ultraviolet radiation and the radiation is absorbed on the surface to be molded without liquefying the remainder of the substrate.

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