Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Forming nonplanar surface
Reexamination Certificate
2001-03-05
2003-06-24
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Forming nonplanar surface
C430S312000, C430S313000, C430S394000
Reexamination Certificate
active
06582890
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to photolithographic preparation of multilayer and/or high aspect ratio microstructures. More specifically, the invention relates to the preparation of such microstructures using electromagnetic radiation of multiple wavelengths.
BACKGROUND
“Nanotechnology” refers to nanometer-scale manufacturing processes, materials and devices, as associated with, for example, nanometer-scale lithography and nanometer-scale information storage. See, for example,
Nanotechnology,
ed. G. Timp (New York: Springer-Verlag, 1999), and
Nanoparticles and Nanostructured Films,
ed. J. H. Fendler (Weinheim, Germany: Wiley-VCH, 1998). Nanometer-scale components find utility in a wide variety of fields, particularly in the fabrication of microelectromechanical systems (commonly referred to as “MEMS”). Such systems include, for example, micro-sensors, micro-actuators, micro-instruments, micro-optics, and the like. Many MEMS fabrication processes exist, and tend to fall into the two categories of surface micro-machining and bulk-micromachining. The latter technique involves formation of microstructures by etching directly into a bulk material, typically using wet chemical etching or reactive ion etching (“RIE”). Surface micro-machining involves fabrication of microelectromechanical systems from films deposited on the surface of a substrate, e.g., from thin layers of polysilicon deposited on a sacrificial layer of silicon dioxide present on a single crystal silicon substrate (this technique is commonly referred to as the “thin film polysilicon process”).
An exemplary surface micro-machining process is known as “LIGA.” See, for example, Becker et al. (1986), “Fabrication of Microstructures with High Aspect Ratios and Great Structural Heights by Synchrotron Radiation Lithography Galvanoforming, and Plastic Moulding (LIGA Process),”
Microelectronic Engineering
4(1):35-36; Ehrfeld et al. (1988), “1988 LIGA Process: Sensor Construction Techniques via x-Ray Lithography,”
Tech. Digest from IEEE Solid
-
State Sensor and Actuator Workshop,
Hilton Head, S.C.; Guckel et al. (1991)
J. Micromech. Microeng.
1: 135-138. A related process is termed “SLIGA,” and refers to a LIGA process involving sacrificial layers. LIGA is the German acronym for X-ray lithography (“lithographic”), electrodeposition (“galvanoformung”) and molding (“abformtechnik”), and was developed in the mid-1970's. LIGA involves deposition of a relatively thick layer of an X-ray resist on a substrate, e.g., metallized silicon, followed by exposure to high-energy X-ray radiation through an X-ray mask, and removal of the irradiated resist portions using a chemical developer. The mold so provided can be used to prepare structures having horizontal dimensions—i.e., diameters—on the order of microns. The technique is now used to prepare metallic microcomponents by electroplating in the recesses (i.e., the developed regions) of the LIGA mold. See, e.g., U.S. Pat. No. 5,190,637 to Guckel et al. and U.S. Pat. No. 5,576,147 to Guckel et al.
Typically, complex three-dimensional microcomponents may be formed in part by successive application of LIGA or other lithographic techniques. First, a mold is prepared by depositing a layer of resist on a substrate, exposing the layer of resist to radiation through a patterned mask, and removing the resist layer according to the pattern. The mold is filled with a filler material, e.g., a metal, that will eventually become the three-dimensional component or a portion thereof. To ensure that no voids are included in the final microcomponent, the mold may be “overfilled,” i.e., excess filler material is used in filling the mold. Polishing is then carried out to expose a surface on which another mold may be formed by using the above-described process. By controlling the thickness of each resist layer and using an appropriate sequence of masks, a desired complex shape may be formed. However, this technique is not easily adaptable for large scale production because of the difficulty in carrying out the series of lithographic, filling and polishing steps, particularly in view of the size of such microcomponents.
There are a number of advantages associated with the use of deep X-ray lithography techniques such as LIGA in the preparation complex three-dimensional microcomponents. In addition, since X-ray photons are short wavelength particles, diffraction effects are absent for mask dimensions above 0.1 micrometer. Moreover, because X-ray photons are absorbed by atomic processes, standing wave problems, which can limit exposures of thick photoresist by electromagnetic radiation having long wavelengths are not problematic for X-ray exposures.
Ordinarily, LIGA requires a synchrotron that yields high flux densities, several watts per square centimeter, as the source of X-ray photons. Such sources generate X-rays with excellent collimation to produce thick photoresist exposures without any horizontal “run-out” as is described below. Locally exposed patterns therefore result in vertical photoresist walls if a developing system with very high selectivity between exposed and unexposed photoresist is available. However, the use of X-ray technology is also an inherent drawback in ordinary LIGA processes. For example, the dangers of X-ray exposure to living tissue are well known. In addition, X-ray technology and associated specialized equipment, X-ray masks and synchrotrons in particular, involve great cost due to the high-degree of expertise required to design, manufacture, operate and to maintain such equipment. For example, X-ray masks typically require hours to make while production of ordinary UV lithography masks require only a few minutes. Thus, there are strong economic incentives to find an alternative source or wavelength of electromagnetic radiation in order to carry out LIGA or other photolithographic processes for thick film applications.
One possible alternative to using X-ray technology is to employ ultraviolet-wavelength-based photolithography. Such photolithography is commonly practiced in semiconductor manufacturing processes and has been characterized. See, e.g., Flack et al. (1998), “Characterization of Ultra-thick Photoresists for MEMS Application s Using a 1× Stepper,”
SPIE conference on Materials and Device Characterization in Micromachining,
3512:296-315, and Löchel et al. (1994), “Galvanoplated 3D Structures for Micro Systems,”
Microelectronic Engineering
23:455-459. Consequently, equipment associated with ultraviolet photolithography is much less expensive and much more commercially available than synchrotrons and other X-ray equipment as a whole. However, as a general rule, ordinary photolithographic techniques using ultraviolet radiation cannot achieve the resolution and the aspect ratios associated with ordinary LIGA process using X-ray technology.
Advanced photolithographic techniques have been proposed in semiconductor processing in order to achieve definition of fine lines with high aspect ratios. Such techniques may involve the use of multilayer resists as described in Ghandi,
VLSI Fabrication Principles, Silicon and Gallium Arsenide
687-690 (2
nd
ed., John Wiley & Sons, 1994). This section of the textbook describes that there are two situations in which two layers of photoresist may be used. The first situation involves providing a first resist layer with high radiation sensitivity on a second resist layer having a low radiation sensitivity, exposing both resist layers to optical radiation and developing the layers. As described, this situation tends to result in significant undercutting to the thicker resist layer and is therefore unsuitable for most LIGA or other applications for producing microcomponents having high aspect ratios. The second situation also require two resist layers, a first resist layer sensitive to electromagnetic radiation of a first wavelength but opaque to a second wavelength and a second resist layer sensitive to electromagnetic radiation of the second wavelength. The first resist layer is applied on the secon
Dentinger Paul Michael
Krafcik Karen Lee
Huff Mark F.
Reed & Eberle LLP
Sagar Kripa
Sandia Corporation
Wu Louis L.
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