WO2007123819A2

WO2007123819A2 - Method for making lens features

Info

Publication number: WO2007123819A2
Application number: PCT/US2007/008433
Authority: WO
Inventors: Gerard Schmid; Nicholas A. Stacey; Douglas J. Resnick; Ronald D. Voisin; Lawrence J. Myron
Original assignee: Molecular Imprints, Inc.
Priority date: 2006-04-18
Filing date: 2007-04-03
Publication date: 2007-11-01
Also published as: US20070243655A1; US7547398B2; TW200744831A; WO2007123819B1; WO2007123819A3; TWI338615B

Abstract

A process describes a combined use of anisotropic dry etching and isotropic wet etching to produce features. The mask protecting material (301) to be etched is exposed through lithography. An anisotropic dry etch is used to generate a trench through the hole (303). The depth of this etch will increase the overall lenslet aspect ratio which is generated in the next, wet etch.

Description

Method for making lens features

[0001] Nano-fabrication involves the fabrication of very small structures, e.g., having features on the order of nanometers or smaller. One area in which nano- fabrication has had a sizeable impact is in the processing of integrated circuits. As the semiconductor processing industry continues to strive for larger production yields while increasing the circuits per unit area formed on a substrate, nano-fabrication becomes increasingly important. Nano-fabrication provides greater process control while allowing increased reduction of the minimum feature dimension of the structures formed. Other areas of development in which nano-fabrication has been employed include biotechnology, optical technology, mechanical systems and the like. [0002] An exemplary nano-fabrication technique is commonly referred to as imprint lithography. Exemplary imprint lithography processes are described in detail in numerous publications, such as United States patent application publication 2004/0065976 filed as United States patent application 10/264,960, entitled, "Method and a Mold to Arrange Features on a Substrate to Replicate Features having Minimal Dimensional Variability"; United States patent application publication 2004/0065252 filed as United States patent application 10/264,926, entitled "Method of Forming a Layer on a Substrate to Facilitate Fabrication of Metrology Standards"; and United States patent number 6,936,194, entitled "Functional Patterning Material for Imprint Lithography Processes," all of which are assigned to the assignee of the present invention and all of which are incorporated by reference herein. The imprint lithography technique disclosed in each of the aforementioned United States patent application publications and United States patent includes formation of a relief pattern in a polymerizable layer and transferring a pattern corresponding to the relief pattern into an underlying substrate. The substrate may be positioned upon a motion stage to obtain a desired position to facilitate patterning thereof. A template is employed spaced-apart from the substrate with a formable liquid present between the template and the substrate. The liquid is solidified to form a solidified layer that has a pattern recorded therein that is conforming to a shape of the surface of the template in contact with the liquid. The template is then separated from the solidified layer such that the template and the substrate are spaced-apart. The substrate and the solidified layer are then subjected to processes to transfer, into the substrate, a relief image that corresponds to the pattern in the solidified layer.

[0003] There is a need for a an efficient and precise method for making lens features in a template or in a substrate which method is not complex. This is achieved by the methods of the independent claims, whereas preferred embodiments are characterized in the sub-claims. Thereby, a self-aligned process for fabricating imprint templates containing variously etched features.

[0004] Preferred embodiments of the invention are now described with reference to the drawings in which:

[0005] Fig. 1 is a simplified side view of a lithographic system having a template spaced-apart from a substrate;

[0006] Fig. 2 illustrates a method of patterning a substrate in accordance with the prior art; [0007] Fig. 3 illustrates a method of patterning a substrate in accordance with the present invention; and

[0008] Figs. 4a-4n illustrate a process flow for creating variously etched features on an imprint template.

[0009] The present invention comprises a process that enables coplanarization of the structures that have been created in multiple independent etch steps. The various etches are performed independently by selectively exposing only certain patterns to particular etching conditions. After these structures have been created, it is possible that the various structures will exist at different planes/elevations relative to the template surface. At this point, the elevations of the various structures are adjusted independently by selectively exposing "higher" structures to an anisotropic etch that reduces the overall elevation of the structures, while preserving the structural topography. These and other embodiments are described more fully below.

[0010] Referring to Fig. 1, a system 8 to form a relief pattern on a substrate 12 includes a stage 10 upon which substrate 12 is supported and a template 14, having a patterning surface 18 thereon. In a further embodiment, substrate 12 may be coupled to a substrate chuck (not shown), the substrate chuck (not shown) being any chuck including, but not limited to, vacuum and electromagnetic.

[0011] Template 14 and/or mold 16 may be formed from materials including, but not limited to, rased-silica, quartz, silicon, organic polymers, siloxane polymers, borosilicate glass, fluorocarbon polymers, metal, and hardened sapphire. As shown, patterning surface 18 comprises features defined by a plurality of spaced-apart recesses 17 and protrusions 19. However, in a further embodiment, patterning surface 18 may be substantially smooth and/or planar. Patterning surface 18 may define an original pattern that forms the basis of a pattern to be formed on substrate 12. [0012] Template 14 may be coupled to an imprint head 20 to facilitate movement of template 14, and therefore, mold 16. In a further embodiment, template 14 may be coupled to a template chuck (not shown), the template chuck (not shown) being any chuck including, but not limited to, vacuum and electromagnetic. A fluid dispense system 22 is coupled to be selectively placed in fluid communication with substrate 12 so as to deposit polymeric material 24 thereon. It should be understood that polymeric material 24 may be deposited using any known technique, e.g., drop dispense, spin-coating, dip coating, chemical vapor deposition (CVD), physical vapor deposition (PVD), and the like. [0013] A source 26 of energy 28 is coupled to direct energy 28 along a path 30. Imprint head 20 and stage 10 are configured to arrange mold 16 and substrate 12, respectively, to be in superimposition and disposed in path 30. Either imprint head 20, stage 10, or both vary a distance between mold 16 and substrate 12 to define a desired volume therebetween that is filled by polymeric material 24. [0014] Referring to Fig. 1, typically, polymeric material 24 is disposed upon substrate 12 before the desired volume is defined between mold 16 and substrate 12. However, polymeric material 24 may fill the volume after the desired volume has been obtained. After the desired volume is filled with polymeric material 24, source 26 produces energy 28, e.g., broadband energy that causes polymeric material 24 to solidify and/or cross-link conforming to the shape of a surface 25 of substrate 12 and patterning surface 18, defining a patterned layer 50 on substrate 12. [0015] The broadband energy may comprise an actinic component including, but not limited to, ultraviolet wavelengths, thermal energy, electromagnetic energy, visible light and the like. The actinic component employed is known to one skilled in the art and typically depends on the material from which imprinting layer 12 is formed. Control of this process may be regulated by a processor 32 that is in data communication with stage 10, imprint head 20, fluid dispense system 22, source 26, operating on a computer readable program stored in memory 34.

[0016] The above-mentioned may be further employed in imprint lithography processes and systems referred to in United States patent 6,932,934 entitled "Formation of Discontinuous Films During an Imprint Lithography Process;" United States patent application publication 2004/0124566, filed as United States patent application 10/194,991 entitled "Step and Repeat Imprint Lithography Processes;" and United States patent application publication 2004/0188381, filed as United States patent application 10/396,615, entitled "Positive Tone Bi-Layer Imprint Lithography Method"; and United States patent application publication 2004/0211754, filed as United States patent application 10/432,642, entitled Method of Forming Stepped Structures Employing Imprint Lithography," all of which are incorporated by reference herein. [0017] A template often used for the above-mentioned method of patterning a substrate is fused silica, and features often defined in this template comprise a so-called binary relief structure. Such structures, which are used widely in semiconductor processing, diffractive optics, etc., are prepared by well known techniques employed by the photomask industry, involving anisotropic dry etching of exposed fused silica using chromium to mask or protect areas that are not to be etched. [0018] While the above-mentioned method may be desired for the fabrication of a binary relief image, there is growing interest in the use of nanoimprinting for the fabrication of continuous tone relief images, such as those required by refractive optical elements. An example of a commercially relevant need for such elements is in the imaging sensor area where arrays of such elements ("lenslet" arrays) are used to gather light and increase sensor sensitivity. [0019] While several methodologies exist in principal to fabricate continuous tone structures in fused silica, many are complex in their execution, requiring careful process control in order to be able to deliver the required structures. In contrast, isotropic etching, which is commonly achieved through so-called wet etching, naturally provides a continuous tone etch that lends itself to the fabrication of refractive optics. In isotropic etching, the etch rate at all surfaces is equal in rate in all directions. Fig. 2 shows how this can be made to make a lenslet. A layer of etch mask, 201 , is patterned so that a small area, 203, of substrate body, 202, can be exposed to the etching chemistry. When the opening 203 is a small circle, isotropic wet-etching processes can produce a spherical lenslet. [0020] It may be apparent in this process there is a fundamental restriction in wet etching; since the etch depth will be the same as the lateral etch extent in either direction, the lens shape is constrained by the following equation:

Lens depth = 0.5 x (lens width - hole width)

[0021] This means that the lens may not be deeper than half its width (even with no hole to etch through). However, it is very possible that for some applications lenses will be required where the lens depth needs to be greater than is achievable by the method described.

[0022] Referring to Fig. 3, there is illustrated a schematic of a methodology that describes a combined use of anisotropic dry etching and isotropic wet etching to produce features unobtainable by wet etching alone.

[0023] The mask protecting material 301 to be etched is exposed through lithography. In the case of fused silica 302, chromium is typically used for this mask 301. The exposed area may be a round hole 303, although this need not be necessarily the case. [0024] Next, an anisotropic dry etch is used to generate a trench through the hole 303. The depth of this etch will increase the overall lenslet aspect ratio which is generated in the next, wet etch. A next step of the process involves removal of the chromium layer, although there may be other process steps required that relate to other aspects of fabricating the template that could be inserted into this process. [0025] By using the above procedure, wet etching can be used to create refractive elements with much greater control than is afforded by the use of wet etching alone. The simplicity of this technique compared with other methods for generating continuous tone lenses makes this approach very attractive, and while there are still restrictions placed on the lens design using this methodology, this technology is very well suited for delivering lenslet arrays of the kind required for imaging sensors and the like, where precise optical lens shape requirements are somewhat relaxed.

[0026] Furthermore, herein described is a process for creating an imprint template containing various topological structures that are created in multiple etch steps, which might be mutually incompatible. Some or all of these structures are patterned into a hard mask material (e.g., chromium) in a single lithography step to achieve best registration among the structure types; the various structure types are thus "self-aligned." The various structures are then etched independently and sequentially by selectively masking off the regions (e.g., with photoresist) that are not to be etched during a particular step. For example, one etch step might be a dry etch to create alignment structures, and a subsequent etch might consist of wet etching to create three-dimensional structures. These steps are mutually incompatible because a wet etch will degrade the alignment structures, and vice versa. After these different etch steps, the various structures might exist at different heights above the surface of the template. It is then possible to selectively reduce the height of the higher features by an anisotropic dry etch that substantially preserves the structures while reducing their elevation relative to other structures on the template surface.

[0027] Creation of topography on an imprint template typically occurs by first transferring a pattern in a photoresist material into a thin "hard mask" layer, e.g., chromium. The hard mask pattern may then be transferred into the underlying template material (e.g., fused silica) by etching. Depending on the geometry of the desired template topography, a fused silica etching step may be one of several types of wet (liquid) or dry (plasma) etching processes. In general, different types of etching steps are mutually incompatible: an etch step that works well to create one type of structure will generally degrade other types of structures. It is possible to perform multiple lithography steps to protect/mask certain structures and expose other structures at the various etch steps; however, the resulting structures will not necessarily be coplanar at the end of this iterative process because the various etch steps might etch to different depths. However, it is often necessary for the structures to exist at the same plane to enable imprinting with a uniform and/or thin residual layer, described further below.

[0028] Figs. 4a through 4n illustrate a process in accordance with an embodiment of the present invention. [0029] Referring to Fig. 4a, a standard fused silica photomask blank 401 with a thin layer of chromium 402 is provided.

[0030] Referring to Fig. 4b, patterns 408 are created in the chromium layer 402 to define multiple structure types in a single lithography step.

[0031] Referring to Fig. 4c, certain chromium patterns 408 are masked, e.g., with resist 403.

[0032] Referring to Fig. 4d, the unmasked chromium features 408 are etch transferred into fused silica with an appropriate etch process, e.g., a reactive ion plasma etching process.

[0033] Referring to Fig. 4e, the resist 403 from the previous step illustrated in Fig. 4d is removed and the substrate is cleaned. The step associated with Fig. 4c is repeated to expose unetched chromium patterns. Photoresist 404 is deposited to mask the previously unmasked features.

[0034] Referring to Fig. 4f, an etch transfer exposes the chromium patterns now unmasked into the underlying quartz substrate 401, e.g., with a wet etch process. [0035] Referring to Fig. 4g, additional etch processing may be performed as needed to created a desired topography, e.g., wet strip the chromium 402 in exposed regions.

[0036] Referring to Fig. 4h, additional etch processing may be performed as needed to created a desired topography, e.g., a second wet etch of the fused silica 401. [0037] Referring to Fig. 4i, the resist 404 and chromium 402 may be removed as needed. At this point, the template 401 contains desired local structures, but the structures exist at different planes 406, 407 of elevation across the template 401.

[0038] Referring to Fig. 4j, the substrate 401 may be coated with another thin layer of chromium 409, and the structures that exist at a lower plane 407 of elevation may be masked, e.g., with resist 410. [0039] Referring to Fig. 4k, the chromium 409 is etched from the unmasked portion of the substrate 401.

[0040] Referring to Fig. 41, the resist 410 is removed at this point to improve the performance of the subsequent etch process, as needed. [0041] Referring to Fig. 4m, the exposed structures at the higher plane 406 of elevation may be anisotropically etched downward to a lower plane of elevation (e.g., plane 407) which might be substantially coplanar with other structures.

[0042] Referring to Fig. 4n, the chromium 409 is removed to yield the final structure. [0043] Embodiments of the present invention described above are exemplary.

Many changes and modifications may be made to the disclosure recited above, while remaining within the scope of the invention. Therefore, the scope of the invention should not be limited by the above description, but instead should be determined with reference to the appended claims along with their full scope of equivalent.

Claims

1. A method for making a microlens template comprising: performing a first wet etch of a substrate through an aperture in a mask lying over the substrate resulting in a hemisphere etched into a surface of the substrate; removing the mask; and performing a second wet etch on the hemisphere etched into the substrate.

2. The method as recited in claim 1 , wherein the substrate comprises fused silica.

3. The method as recited in claim 1 , wherein the second wet etch increases a diameter of the hemisphere.

4. The method as recited in claim 3, wherein the second wet etch does not increase a depth of the hemisphere.

5. A method for making a microlens in a substrate comprising: performing a dry etch of the substrate through an aperture in a mask; and performing a wet etch of the substrate through the aperture in the mask.

6. The method as recited in claim 5, further comprising removing the mask.

7. The method as recited in claim 5, wherein the dry etch results in a discrete hole into the substrate at a prescribed depth and having a shape similar to the aperture.

8. The method as recited in claim 7, wherein the wet etch is performed on the discrete hole.