Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2000-06-26
2002-11-19
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C378S035000, C378S034000
Reexamination Certificate
active
06482553
ABSTRACT:
This invention pertains to masks for x-ray or deep x-ray lithography, particularly to masks formed on graphite membranes or other graphite substrates.
The fabrication of x-ray lithography masks is currently a lengthy and expensive process. It is the object of this invention to provide reliable x-ray or deep x-ray masks that may be produced more quickly and less expensively than has previously been possible.
Masks for deep X-Ray lithography are typically made from fragile, micrometer-thin membranes, typically formed of Si or Ti, and absorber structures 10-15 &mgr;m in height to provide sufficient contrast. The fabrication of these masks is a multi-step, lengthy, and expensive process, typically using an intermediate x-ray mask that in turn was directly patterned with an electron beam. The intermediate mask is used to copy the pattern into a thick polymethylmethacrylate (“PMMA”) resist using soft x-rays. A metal (usually gold) is electroplated into the thick, patterned resist layer to form an absorber pattern of sufficient thickness to achieve adequate contrast when harder x-rays are used.
Masks for ultra-deep X-ray lithography are usually made in a simpler and less expensive way, for example using thick Si wafers as a substrate. Optical lithography is used to transfer a pattern into a thick UV resist to generate a resist pattern up to 50 &mgr;m thick, and the pattern is then filled with 30-35 &mgr;m of Au. However, the smallest structures achieved are typically
−
10 &mgr;m, and these masks are generally suited only for hard x-ray sources.
The three main criteria for a high quality x-ray exposure are: (1) The top-to-bottom dose ratio for exposure through the thickness of the resist should be held below approximately 5 (an approximate figure whose value depends on the material used for the resist and its thickness—it is typically smaller with the thicker resist layers used in ultra-deep X-ray lithography). (2) The dose on the bottom of the resist should be at least about 3000 J/cm
3
(again, depending on the material of the resist—the figure given is for PMMA). (3) The contrast between the exposed regions and the unexposed regions should be such that the maximum dose underneath the absorber is about 100-150 J/cm
3
(a figure dependent on the material of the resist, the resist thickness, and the development procedures used).
As a consequence, to pattern thicker resists a “harder” x-ray spectrum is typically needed to effectively expose the bottom of the resist. (Harder x-rays can be used to expose thick resists relatively uniformly, while softer x-rays can overexpose the top of a thick resist before the bottom is properly exposed). A thicker absorber pattern is needed to provide sufficient contrast with harder X-rays.
Fabrication of a mask suitable for patterning 1000 &mgr;m of PMMA can take up to six months, and generally costs several thousand dollars.
Several methods have previously been used to fabricate x-ray lithography masks. The methods differ depending on the substrates used, and the processes used to generate the absorber pattern.
A general description of procedures that have typically been used previously to generate X-ray masks is the following:
(1) Electron beam lithography or an optical pattern generator is used to transfer a pattern from a drawing onto an intermediate x-ray mask or onto an optical (typically chromium) mask.
(2) In an intermediate x-ray mask, the resist is approximately 3-4 &mgr;m thick, and the Au absorber pattern is approximately 2-2.5 &mgr;m thick. This pattern is transferred using “soft” x-rays (a few keV photon energy) into a 30-100 &mgr;m thick PMMA resist applied onto either a membrane or a substrate.
(3) In case of a chromium mask, optical lithography is used to pattern a thick optical resist onto either a membrane or a substrate, typically using UV light.
(4) In either case, following exposure a wet-chemical development process is used to remove either the exposed regions (for a positive resist) or the unexposed regions (for a negative resist).
(5) The recesses produced in the previous step are then filled with an absorber by electroplating, typically with Au. A typical height for the Au absorber is about ⅔ of the resist height.
(6) The resist is removed, and the mask is mounted onto a carrier for use in x-ray lithography.
(7) In the case of a membrane-based mask, the substrate must be mounted onto a frame or carrier prior to patterning. Such mounting requires special fixtures and extreme care in handling. In the case of a substrate-based mask, the frame or carrier may be mounted after patterning the mask, allowing the use of standard fixtures (typically vacuum chucks) and only moderate attention in handling.
For prototype development work, the time and cost required to manufacture an x-ray mask using this procedure can be excessive.
A graphite-based substrate for an x-ray or deep x-ray mask would be relatively inexpensive, would have reasonably good x-ray transmission, and could be mechanically sound. Such substrates could be used in otherwise standard or ultra-deep x-ray lithography processes.
Graphite substrates have not generally been used in x-ray masks, primarily because they have been considered too “dirty” for use in the “clean room” environment used for x-ray lithographic processes. Graphite surfaces tend to be rough and porous. Silicon substrates and membranes are more familiar to most users.
J. Göttert et al., “Lithographic Fabrication of Graphite-based X-ray Masks,” paper presented at 42nd Electron Ion and Photon Beam Technology and Nanofabrication Conference, Chicago, Ill. (1998) discloses a process in which graphite sheets are attached to a silicon wafer using PMMA as a bonding layer, and a metal layer consisting of a thin copper layer on top of a chromium layer is sputtered onto the graphite. Then a PMMA resist layer is spin-coated onto the sample. The sample is then exposed with x-rays through an intermediate x-ray mask, and developed to remove exposed resist and to etch the thin copper layer. Then the sample is electroplated with gold, the remaining PMMA is dissolved away, and the remaining copper is etched away.
X-ray lithography masks formed of graphite have been made by micromilling techniques, and lithographically through the use of an intermediate X-ray mask. See P. Coane et al., “Fabrication of composite X-ray masks by micromilling,”
SPIE Proceedings
, vol. 2880, pp. 130-141 (1996); C. Friedrich et al., “Micromilling development and applications for microfabrication,”
Microelectronic Engineering
, vol. 35, pp. 367-372 (1997); C. Friedrich et al., “Metrology and quantification of micromilled x-ray masks and exposures,”
SPIE Proceedings
, vol. 3048, pp. 193-197 (1997); and P. Coane et al., “Graphite-based x-ray masks for deep and ultradeep x-ray lithography,”
J. Vac. Sci. Technol. B
., vol. 16, pp. 3618-3624 (1998).
P. Coane et al., “Fabrication of HARM structures by deep-X-ray lithography using graphite mask technology,”
Microsystem Technologies
, vol. 6, pp. 94-98 (2000) (paper presented at Third International Workshop on High Aspect Ratio Microstructure Technology (June 1999)) discloses the manufacture of graphite masks in which a single graphite wafer accommodates, on opposite faces, both an intermediate mask and a working mask.
It is an object of this invention to provide a method to fabricate deep X-ray lithography and ultra-deep X-ray lithography masks reliably and inexpensively, facilitating the cost-effective production of high aspect ratio microstructures using either “hard” or soft X-ray sources.
We have discovered an improved method for producing x-ray or deep x-ray masks on graphite membranes (or thicker graphite substrates) inexpensively (under $1000) and rapidly (within about one day once an optical mask is available). The novel method eliminates the need for an intermediate x-ray mask, instead using a less expensive intermediate UV lithography step (either exposure through a UV mask, or UV direct writing). The absorber structures are electroplated directly onto the graphite. The capabi
Coane Philip J.
Göttert Jost S.
Kelly Kevin W.
Board of Supervisors of Louisiana State University and Agricultu
Rosasco S.
Runnels John H.
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