WO2000002089A1

WO2000002089A1 - Method of making optical replicas by stamping in photoresist and replicas formed thereby

Info

Publication number: WO2000002089A1
Application number: PCT/US1999/014792
Authority: WO
Inventors: Thomas J. Suleski
Original assignee: Digital Optics Corporation
Priority date: 1998-07-02
Filing date: 1999-06-30
Publication date: 2000-01-13
Also published as: AU4963799A; EP1110124A1; ATE418090T1; EP1110124B1; CA2336467A1; US6027595A; DE69940118D1; CA2336467C; WO2000002089A9

Abstract

Optical structures are replicated in photoresist on a substrate using a stamp. The transfer of the pattern into the liquid photoresist and the provision on the substrate can be achieved using manual pressures. Various techniques may be used to remove air from the liquid photoresist. The stamp is removed once the liquid photoresist is fully solidified. These structures in solidified photoresist may serve as optical elements or may be accurately transferred into the substrate. The stamp may be for an entire wafer.

Description

METHOD OF MAKING OPTICAL REPLICAS BY STAMPING IN PHOTORESIST AND REPLICAS FORMED THEREBY

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention is directed to a method of making optical replicas by

stamping in liquid photoresist and the replicas formed thereby. The stamped photoresist

may be used to further transfer the pattern into the substrate through photolithographic

processes.

Description of Related Art

Replication is a key factor to achieving affordable, mass produced optical

elements. Replication is particularly of interest for micro-optical elements and for

diffractive optical elements having multiple discrete levels, for which many masks are

needed to create such elements photolithographically.

One known replication method is disclosed in U.S. Patent No. 5,279,689 entitled

"Method for Replicating Holographic Elements," which is hereby incorporated by

reference in its entirety. In the '689 patent, a layer of embossable material is formed on

a substrate. Then a stamper is positioned on the layer and pressed into the surface of the

layer. The layer is then cared and the stamper removed, thereby forming the holographic optical element. However, this layer is subject to shifting on and peeling from the

substrate.

An attempt to address the problems of the '689 patent is disclosed in U.S. Patent

No. 5,575,878 entitled "Method for Making Surface Relief Profilers," which is hereby

incorporated by reference in its entirety. In the '878 patent, a dry photopolymer layer is

applied to a glass substrate having a thin layer of polymethylmethacrylate (PMMA)

thereon. The dry photopolymer layer is then stamped with a glass master to create the

desired pattern therein. The dry photopolymer layer is then cured and then the pattern

therein is transferred to the glass substrate by dry etching. In the '878 patent, the PMMA

layer is used as an adhesion promoter on the glass substrate so that the dry photopolymer

layer will adhere to the substrate, while allowing the glass master to be lifted therefrom.

However, several problems still exist with either method. First, the photopolymer

disclosed in the ' 878 patent degrades during etching, particularly during the faster etching

processes, most likely at least in part due to high temperatures in the etching chamber.

Therefore, the pattern may not be accurately transferred to the substrate. When trying to

create multiple elements simultaneously on a wafer level, while the PMMA helps the dry

photopolymer layer adhere, due to the increased surface area of the wafer, part of the dry

photopolymer layer may still be pulled away from the substrate, resulting in inaccurate

transfer of the pattern. If the photopolymer layer itself was to serve as the relief structure

on either a glass or a plastic substrate, as in the '689 patent, even with the PMMA layer, when trying to create multiple elements simultaneously on a wafer level, the stresses

resulting when dicing the wafer result in the photopolymer layer peeling or shifting from

the substrate even during creation. Finally, the use of the dry photopolymer requires high

pressures to be applied in order to transfer the pattern into the dry photopolymer.

SUMMARY OF THE INVENTION

The present invention is therefore directed to a method of creating optical replicas

and the replicas made thereby which substantially overcomes one or more of the problems

due to the limitations and disadvantages of the related art.

It is an object of the present invention to replicate structures in photoresist. It is a

further object of the present invention to create replicated structures on a wafer level.

These and other objects may be realized by a method of making an optical replica

including contacting liquid photoresist with a stamp having a pattern therein, thereby

replicating the pattern in the photoresist, providing the liquid photoresist on a substrate

opposite a side of the liquid photoresist contacting the stamp, solidifying the liquid

photoresist; and removing the stamp from solidified photoresist. The providing may

include applying the liquid photoresist on the substrate and the contacting may include

stamping the liquid photoresist with the stamp. Alternatively, the contacting may include

applying the liquid photoresist on the stamp and the providing may include pressing the

liquid photoresist and the substrate together. The pattern in the solidified photoresist may be transferred into the substrate. The

substrate may be a glass substrate, including silicon. The transferring may include

anisotropic etching the pattern in the solidified photoresist into the glass substrate. The

substrate may be a wafer. The wafer may be diced after the stamp is removed. The stamp

may be a wafer. The stamp may be made of an elastomeric, polymeric material, which

may be molded to form the pattern therein. The elastomeric, polymeric material may be

polydimethylsiloxane. Alternatively, the stamp may be made of glass which is treated

with a release agent, e.g., a fluorinated trichlorosilane, before contacting the liquid

photoresist. At least part of the solidifying may occur before the liquid photoresist is

provided on the substrate.

The method may further include removing air bubbles from the liquid photoresist.

Such removal may include contacting the substrate with the stamp having the liquid

photoresist thereon at an angle to allow the air to escape while the substrate is brought

into contact. When the liquid photoresist is provided on the substrate, such removal may

include, when using an elastomeric, polymeric material for the stamp, bowing the stamp

such that the stamp contacts the liquid photoresist first in the center and then pressing out

to the edges to allow the air to escape through the edges or may include tightly clamping

one side of the stamp to the substrate, loosely clamping the other side of the stamp to the

substrate, and tightening the loose clamp until tight, allowing the air to escape out of the

loosely clamped side. Such air removal is particularly advantageous when replicating

optics on a larger scale, such as on a wafer level. These and other objects are further realized by an optical replica made in

accordance with any of the above methods.

These and other objects of the present invention will become more readily

apparent from the detailed description given hereinafter. However, it should be

understood that the detailed description and specific examples, while indicating the

preferred embodiments of the invention, are given by way of illustration only, since

various changes and modifications within the spirit and scope of the invention will

become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other object, features and advantages of the present invention will

become readily apparent to those skilled in the art from the detailed description which

follows and from the accompanying drawings, in which:

FIG. 1 shows schematic illustrations of the replication process in accordance with

the present invention using photoresist and etching the pattern into the substrate;

FIG. 2 shows schematic illustrations of the replication process, including an

embodiment for forming the stamp, in accordance with another embodiment of the

present invention using photoresist and etching the pattern into the substrate to form

multiple replicas from the same stamp; and

FIGS. 3 A-3C illustrates three methods for removing air from a liquid photoresist

prior to solidification. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

While the present invention is described herein with reference to illustrative

embodiments for particular applications, it should be understood that the present

invention is not limited thereto. Those having ordinary skill in the art and access to the

teachings provided herein will recognize additional modifications, applications, and

embodiments within the scope thereof and additional fields in which the invention would

be of significant utility without undue experimentation.

Typically when attempting to replicate an element, the materials examined for use

in the replicating process are those specifically designed for such replication. However,

the present invention is based on the requirement that the replicated structure be

accurately transferred into a glass substrate. As used herein, the term "glass" is to include

glass, silica, fused quartz, and other inorganic substances, such as Si, GaAs, that have

good optical properties and are durable.

With the emphasis being on the accurate transfer of the pattern into the substrate,

particularly when creating structures having small features and large aspect ratios

requiring the use of anisotropic techniques, including fast etching processes such as

inductively coupled plasma etching, the present invention involves first determining

materials suitable for use with such techniques. Therefore, the present invention is

directed to selecting liquid photoresist as a suitable material for use with such techniques

and determining how to replicate in liquid photoresist. As used herein, the term "photoresist" is to include liquid photopolymers which, after solidification, allow a high

fidelity transfer into a substrate by etching.

One reason photoresist is so advantageous is that while transfer of a pattern into

a substrate may be conceptually easy, in practice it is difficult to achieve with high

fidelity. There has been much work conducted regarding the control of photoresist when

using photoresist to transfer a pattern into a substrate. For example, while the respective

etch rates of the material having the pattern and the substrate may not theororetically

matter as long as they are known, in practice the relative etch rates of these materials

needs to be comparable, e.g., a ratio between approximately 0.5 and 2. If the substrate has

an etch rate which is much slower than the etch rate of the patterned material, the aspect

ratios of the pattern will have to be much bigger and the patterned material much thicker.

The large aspect ratio is difficult to make and the thicker the material, the lower the

resolution. If the substrate has an etch rate which is much faster than the etch rate of the

patterned material, any lack of uniform thickness or other imperfection will be amplified

in the transfer of the pattern.

The use of photoresist for transferring a pattern into a substrate is sufficiently

understood that the above noted parameters can be controlled to allow the accurate

transfer of the pattern. It is known how to make photoresist sufficiently maintain its

shape throughout the etching process. Therefore, the present invention is directed to

determining how to replicate in photoresist to obtain all the attendant advantages of both replication and the use of photoresist, particularly for transferring a pattern therein into

a substrate.

In a preferred embodiment, the replicating of the present invention is based on the

discovery that elastomeric, polymeric materials do not stick to liquid photoresist, either

before or after curing. Elastomeric, polymeric materials are elastic enough to closely

conform to even small features, e.g., sub-micron, but not so elastic as to deform when

pressed against a stamping surface with unacceptable fidelity for many applications. In

particular, there are a variety of suitable elastomeric polymeric materials, especially

silicone polymers, epoxy polymers, and acrylate polymers. A preferred material within

the silicone polymers is polydimethylsiloxane (PDMS). PDMS does not adhere to liquid

photoresist, either before or after curing of the liquid photoresist.

The use of elastomeric, polymeric materials, in particular PDMS, generally in

replicating optical elements is disclosed in U.S. Patent No. 5,512,131 entitled "Formation

of Microstamped Patterns on Surfaces and Derivative Articles" which is hereby

incorporated by reference in its entirety. The ' 131 patent discloses forming lenses by

molding elastomeric, polymeric material and using the resultant element made of PDMS

as the lens. The '131 patent also discloses forming a diffraction grating by

micromachining the elastomeric, polymeric material and using this as the diffraction

grating itself or as a stamp to create diffraction gratings on other materials. The stamping process in the ' 131 patent involves applying a self-assembled

monolayer (SAM) to a PDMS stamp and coating a substrate with metal. Upon stamping

the metal on the substrate with the PDMS stamp, the non-recessed portions of the PDMS

stamp will transfer the SAM to the metal. This SAM serves as a "mask"such that portions

of the metal not covered by the SAM are further processed. While this processing may

include etching, the SAM only serves as a blocking mask, so any degrading thereof does

not affect the final product as long as it covers the appropriate portions. Thus, the pattern

itself is not transferred into the substrate.

The ' 131 patent fails to teach that the elastomeric, polymeric materials set forth

therein freely release from liquid photoresist. The embossable materials typically

employed in both the ' 131 patent and other related art are sub-optimal for use during

anisotropic etching.

The present invention, by recognizing that the elastomeric, polymeric materials,

in addition to having suitable deformation characteristics recognized in the ' 131 patent,

have the further advantageous property of being extremely easy to release from liquid

photoresist. Thus, in accordance with the present invention, photoresist can be used as a

replication material using, for example, elastomeric, polymeric materials, in particular

PDMS, as a master element to transfer a pattern therein to the liquid photoresist. The desired pattern may be transferred to the PDMS by molding with a master,

e.g., a glass master or a master having microstructures in photoresist, on which the

desired pattern has been generated, typically photolithographically . For example, PDMS,

which is a mixture of two liquids, the silicone elastomer and a curing agent, is poured

over an optical element master. The PDMS will cure over time at room temperature, but

heat may be used to speed up the curing process. When the PDMS stamp is formed by

molding with a glass master, a release agent such as a fiuorinated trichlorosilane release

agent, e.g., (tridecafluoro- 1 , 1 ,2,2-tetrahydro-octyl)- 1 -trichlorosilane can be used to allow

easy separation of the glass and the PDMS. Alternatively, the desired pattern may be

micromachined into the elastomeric, polymeric material to form the stamp.

While PDMS is useful for many applications, there is concern that the flexible

nature of the PDMS may introduce distortions to the pattern. When extremely high

precision is required, the glass master itself may serve as the stamp, preferably with the

use of a release agent, preferably a fiuorinated tricholorosilane release agent noted above.

Obviously, such a pattern on a glass stamp would be the negative of the desired pattern.

The following description sets forth different examples of manners in which a

stamp may be used to replicate optical elements in photoresist. While a simple binary

element is shown as being replicated in one example and a grating is replicated in another

example, it is to be understood that any optical element, including refractives,

refractive/diffractive hybrids, diffractives having multiple discrete levels, diffractives having continuous contours etc., could be replicated in accordance with any of the

methods the present invention. Any photolithographic techniques, including gray scale

masks and reflow of photoresist, may be used to generate a master which is used as a

mold to form the stamp or to generate the stamp itself.

Further, whil e the creation of a single optical element is illustrated in the following

examples, it is to be understood that the present invention is particularly advantageous

when creating multiple optical elements simultaneously on a wafer level, and the master

may be a wafer master, not just a single element master. Finally, while the substrate is

shown as being planar in the figures, it is to be understood that when using the

elastomeric, polymeric master, the substrate could also have a non-planar surface, such

as a lens.

The following reference to "positive" and "negative" photoresist is different from

that in conventional lithography where a photoresist is masked, exposed and developed.

In contrast, in the present invention, the role of the liquid photoresist is to serve as a

material that can be molded into the desired shape or pattern. The distinction between

positive and negative photoresist for the stamping application of the present invention is

in terms of how the photoresist, which is stamped when in liquid form, is cured or

solidified. For a positive photoresist, the liquid photoresist is solidified by a thermal

process, e.g., baking, or by other appropriate processes, such as electron bombardment,

depending on the exact material used. Examples of positive photoresist which can be used with the present invention include SHIPLEY 3813, SHIPLEY 1400 series, AZ 5209.

For a negative photoresist, the liquid photoresist is solidified by exposure to ultraviolet

light. Examples of negative photoresist which can be used with the present invention

include FUTURREX NR8. A further difference between these types of photoresist is that

the positive photoresist needs an adhesion promoter to stick to the glass substrate, while

a negative photoresist does not. The solidifying of both types of liquid photoresist and the

adhesion promoter for the positive photoresist are all conventional.

As shown in FIG. 1, a layer of uncured photoresist 12 is applied to an embossing

stamp 10. Excess photoresist 12 is removed from the embossing stamp 10 using, for

example, a block 14 of, for example, PDMS. The embossing stamp 10 filled with the

photoresist 12 is brought into contact with a substrate 16. Manual pressure alone is

sufficient to achieve this contact. With the embossing stamp 10 still in place, the

photoresist 12 is cured by, for example, exposure to actinic radiation, when the

photoresist 12 is a negative photoresist, or by thermal processes when the photoresist 12

is a positive photoresist, such curing being known to those of skill in the art. The

radiation, if used, can be delivered through either the embossing stamp 10 or the substrate

16.

After the photoresist 12 is cured, the embossing stamp 10 is removed, leaving the

negative of the pattern, in the embossing stamp 10 transferred in the photoresist 12 on the

substrate 16. For some applications, a replica 18 formed by the patterned photoresist 12 on the substrate 16 may serve as the final optical element itself. When the replica 18 is

the end product, the substrate material is not important as long as photoresist adheres to

it. Optics made and used directly in photoresist are less expensive to produce than

elements which transfer the pattern in the photoresist into glass, since the transfer process

is eliminated. However, photoresist optics have many of the same problems as plastic

optics, i.e., are more susceptible to abrasion, thermal effects, chemical effects, and laser

damage than glass optics.

Preferably, the patterned photoresist 12 is then transferred into the substrate 16

made of glass in a conventional manner, for example, using anisotropic etching, such as

reactive ion etching (RIE), chemically assisted ion beam etching (CAIBE), inductively

coupled plasma etching, and ion milling. This transfer may include hard baking the

photoresist depending on the transfer process used. This transfer of the pattern from the

photoresist 12 to the substrate 16 results in a replica 19, in which the substrate 16 has a

patterned surface 17.

An alternate method is shown in FIG. 2, in which a blazed grating is formed. A

glass master 20 having the pattern to be transferred to a substrate receives an elastomeric,

polymeric material thereon. When the elastomeric, polymeric material is removed from

the glass master 20, it is ready to serve as the embossing stamp 22. The photoresist 24

may either be applied to the embossing stamp 22 or to a substrate 26. The embossing stamp 22 is brought into contact with the substrate 26, thereby casting the photoresist

against the substrate. As before, sufficient contact may be realized by manual pressure.

As discussed above in connection with FIG. 1, the photoresist is then fully cured

and the stamp 22 is removed to form an optical element 28 by the patterned photoresist

on a substrate may serve as the final optical element itself. When the replica 28 is the end

product, the substrate material is not important as long as photoresist adheres to it.

Preferably, the pattern in the photoresist 24 may be transferred into the glass

substrate 26 in a conventional manner, for example, using anisotropic etching. This

transfer of the pattern from the photoresist layer 24 to the glass substrate 26 results in the

replica 29 in which the glass substrate 26 has a patterned surface 27. As shown in FIG.

2, a single embossing stamp may be used numerous times to form the same element as

the original glass master 20. When the glass master 20 itself is to serve as the embossing

stamp, the pattern therein will obviously be a negative of the one to be transferred to the

substrate.

As noted above, the use of the elastomeric, polymeric master or a glass stamp

treated with a release agent allows photoresist to be patterned and then cured on a glass

substrate, while easily removing the master, i.e., removal such that the pattern in the

photoresist layer and the photoresist layer/glass substrate bond are not affected. This

patterned photoresist may then be transferred to the glass substrate in known manners, or the replica consisting of the patterned photoresist on a substrate may be used as the

optical element itself.

Each of the manners of providing the liquid photoresist having the pattern therein

to the substrate discussed above has its advantages. The application of the photoresist to

the stamp reduces problems with air bubbles which may arise when pressing the stamp

into a photoresist layer. As shown in FIG. 3 A, when the photoresist 12 is applied to the

stamp 10, the stamp 10 may contact the substrate 16 at an angle to allow air bubbles in

the liquid photoresist to escape as the remainder of the stamp 10 having the photoresist

12 is brought into contact. Further, the application of the photoresist to the stamp allows

the photoresist to be at least partially cured or solidified before bringing it into contact

with the substrate. This solidification allows gases which are released during

solidification to escape, avoiding attendant gas trapping problems associated therewith.

However, it is difficult to control the thickness of the photoresist 12 when applied

to the stamp 10. Since it is often desirable for the photoresist 12 to be as thin a layer as > possible while still receiving the pattern, it can be advantageous to provide the photoresist

12 on the substrate 16, as shown in FIGS. 3B and 3C. Thicker layers of photoresist result

in much longer etching times, leading to increased expense, increased likelihood that the

patterned material will degrade due to the increased exposure to the etching process, and

increased inaccuracies due to deviations in etch rate across the element. The thickness of the photoresist provided on the substrate 16 can be accurately controlled in a conventional

manner.

When a stamp 30 is made of an elastomeric, polymeric material, it is flexible.

Therefore, such a flexible stamp 30 may be bowed to allow a central portion 30b thereof

to stamp the photoresist 12 first, and then release to allow outer portions 30a, 30c to

stamp the photoresist 12. Any air in the photoresist 12 is thus allowed to escape from the

periphery as the stamp 30 is brought into full contact with the photoresist 12.

Alternatively, when a stamp 32 is made of any material, adjustable clamps 34, 36

clamping the stamp 32 to the substrate 16 may be used for allowing a differential in

contact amount across the stamp, thereby allowing the air to escape where the stamp is

not yet in contact with the substrate. For example, the clamp 34 may be tightly clamped

so as to provide full contact of an outer portion 32a of the stamp 32 while the clamp 36

may be loosely clamped to allow air to escape from the photoresist. The clamp 36 is

tightened to provide full contact of both the middle portion 32b and the opposite outer

portion 32c, while allowing air to escape until full contact is achieved.

The removal of air from the photoresist is of particular importance when making

larger components, including forming a plurality of elements on a wafer level. Any of the

above techniques may be used with wafer stamps and wafer substrates. A further problem arises when replicating on a wafer level when dicing a wafer

containing such elements into individual portions, which may contain one or more optical

elements. The photoresist serving as the optical element itself can withstand the dicing

stresses and remain adhered to the substrate better than previous replication materials.

Further, if the pattern in the photoresist is transferred into the substrate, this adherence is

no longer a problem. However, dicing is still a messy process. The dirty by-products of

the dicing stick to photoresist and are difficult to remove. If only dicing a single wafer,

the wafer could be flipped over and covered with a material which seals elements so that

when the dicing slurry is used, it does not contact the elements. Alternately, another

wafer, with or without further optical elements, could be bonded to the wafer containing

the stamped photoresist features such that a seal is formed between the wafers around

each individual portion to be diced. Such sealing is disclosed in commonly assigned, co-

pending U.S. Application Serial No. 08/943,274 entitled "Wafer Level Integration of

Multiple Optical Elements" filed October 3, 1997, which is hereby incorporated by

reference in its entirety. This sealing would keep the photoresist clean during dicing.

In accordance with the present invention, a desired pattern is replicated in

photoresist. This replication may be achieved using an elastomeric, polymeric stamp or

a glass stamp treated with a release agent. Such a stamp easily releases from the

photoresist. The pattern may then be easily transferred into a substrate, or the photoresist

may serve as the optical element itself. Such replication, either with or without

transferring to a substrate, may conveniently be used to reproduce multiple optical elements on a wafer level. Such wafer level replication may be performed using a wafer

stamp.

Although preferred embodiments of the present invention have been described in

detail herein above, it should be clearly understood that many variations and/or

modifications of the basic inventive concepts herein taught, which may appear to those

skilled in the art, will still fall within the spirit and scope of the present invention as

defined in the appended claims and their equivalents.

Claims

What is Claimed Is:

1. A method of making an optical replica comprising:

contacting liquid photoresist with a stamp having a pattern therein, thereby

replicating the pattern in the photoresist;

providing the liquid photoresist on a substrate opposite a side of the liquid

photoresist contacting the stamp;

solidifying the liquid photoresist; and

removing the stamp .

2. The method of claim 1, wherein said providing includes applying the liquid

photoresist on the substrate and said contacting includes stamping the liquid photoresist

with the stamp.

3. The method of claim 1, wherein said contacting includes applying the liquid

photoresist on the stamp and said providing includes pressing the liquid photoresist and

the substrate together.

4. The method of claim 1, further comprising transferring the pattern in the

solidified photoresist into the substrate.

5. The method of claim 1, wherein the substrate is a glass substrate.

6. The method of claim 5, wherein the glass substrate is a silicon substrate.

7. The method of claim 5 , further comprising anisofropic etching the pattern in the

solidified photoresist into the glass substrate.

8. The method of claim 1, wherein the substrate is a wafer.

9. The method of claim 8, further comprising dicing the substrate after said

removing.

10. The method of claim 8, wherein the stamp is a wafer.

11. The method of claim 1, wherein the stamp is made of an elastomeric,

polymeric material.

12. The method of claim 11 , further comprising, before said contacting, molding

the elastomeric, polymeric material to form the pattern therein.

13. The method of claim 11, wherein the elastomeric, polymeric material is

polydimethylsiloxane.

14. The method of claim 1 , further comprising, when the stamp is made of glass,

treating the stamp with a release agent before said contacting.

15. The method of claim 14, wherein the release agent is a fiuorinated

trichlorosilane.

16. The method of claim 3 , wherein at least part of said solidifying occurs before

said pressing.

17. The method of claim 3 , wherein said pressing includes introducing the liquid

photoresist at an angle to the substrate.

18. The method of claim 2, wherein said contacting includes applying differential

initial stamping to the photoresist across the stamp.

19. The method of claim 18, wherein, when the stamp is flexible, said applying

differential initial stamping includes bending the stamp.

20. The method of claim 18, wherein said applying differential initial stamping

includes clamping one portion of the stamp more tightly than another portion of the

stamp.

21. An optical replica made in accordance with the method of claim 1.