Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-07-27
2001-07-10
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06258491
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to masks for optical lithography, and in particular to masks for high resolution optical lithography.
BACKGROUND
Semiconductor devices, such as integrated circuits, are manufactured by replicating patterns onto a surface of a device substrate. The replication process typically involves lithographically transferring a pattern that is on a mask onto the device substrate using an illumination source, such as electron beam, x-ray and optical.
Membrane masks are known for use in x-ray lithography. A membrane mask uses a membrane supported by a frame. The membranes are typically made of silicon, doped silicon, silicon carbide, silicon nitride, diamond, or similar material. Because x-ray wavelengths are quickly absorbed in a substrate, thin membranes are necessary so that the x-rays may be transmitted through the substrate. Typically, the membranes are less than 5 &mgr;m (micrometers) thick and are thus typically delicate and expensive to manufacture. Further, x-ray absorber films used on x-ray masks absorb rather than reflect incident radiation.
Membrane masks are not used in optical lithography because conventional optical lithography uses wavelengths that are readily transmitted through thick substrates. Thus, there is no need to incur the expense and trouble of generating a delicate membrane mask for optical wavelengths.
In optical lithography, a conventional mask includes a pattern of opaque material, such as chrome, overlying a relatively thick substrate of glass or quartz, which is transparent to the wavelength of light being used. The incident light is absorbed and reflected by the opaque material and transmitted through the substrate to expose the device substrate (or an overlying photoresist layer) with the mask's pattern.
The glass or quartz substrate of conventional optical masks is free standing, i.e., without a supporting frame, and is typically several millimeters thick. Thick silicon or quartz substrates are adequately transparent for relatively long wavelengths of light, e.g., 193 or 248 nm (nanometers). However, for high resolution optical lithography shorter wavelength light, e.g., 157 nm, may be used. Thick glass or quartz substrates lack the desired transmissiveness for short wavelength light. Present efforts to develop thick transparent materials for 157 nm focus on material modification (doping) or OH removal of fused silica to increase the optical transmission.
A material that is at least partially transparent at short wavelengths is calcium fluoride (CaF
2
). Unfortunately, CaF
2
has a high thermal expansion coefficient, approximately 40 times that of conventional glass or quartz. During production of the overlying pattern, for example using e-beam writing, a large amount of heat is typically transferred to the substrate. Thus, a substrate with a high thermal expansion coefficient will distort during production of the overlying patterns. Consequently, if the thick glass or quartz substrate in a conventional optical mask is replaced with a CaF
2
substrate, e-beam writing will heat the CaF
2
substrate causing the CaF
2
substrate to distort resulting in distortion of the overlying pattern. This distortion may be difficult to correct for the ground rules of future device generations.
Thus, there is a need for masks that may be used for high resolution, i.e., short wavelength, optical lithography that are not distorted when the overlying pattern is generated.
SUMMARY
An optical mask structure includes a membrane that is at least partially transmissive to light of a desired wavelength, such as 157 nm. The optical mask may be used in a lithography system to generate a device, such as a semiconductor device. The membrane is supported on a substrate that holds it under tensile stress. The lithography pattern lies over the membrane. Advantageously, the membrane is sufficiently thin and is mounted on a support frame so as to reduce distortion of the membrane caused by heating during the generation of the overlying pattern. This is particularly advantageous where CaF
2
is used as the membrane, which has a high thermal expansion coefficient. In addition, because the membrane is thin, when generating the overlying pattern, for example with e-beam writing, the membrane will cause little back scattering of the electrons. Further, a thin layer of absorbing material, such as palladium, is used as the material for the pattern overlying the membrane. Because the absorber is thin, little back scattering of the electrons occurs. Consequently, there are little or no proximity effect corrections necessary during generation of the overlying patterns.
The optical mask is produced by providing a substrate of, e.g., silicon or fused silica, upon which is formed a layer of the membrane material, e.g., CaF
2
or doped OH free fused silica. The silicon or fused silica is etched back to the membrane material to define a membrane area. A layer of absorptive or reflective material is deposited over the membrane material and patterned and etched to from the desired lithography pattern over the membrane area. The layer of membrane material is deposited sufficiently thin that heating and distortion of the membrane area during patterning of the overlying layer is reduced.
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Marumoto et al., Fabrication of Diamond Membranes for X-ray Masks by Hot-Filament Method, Jpn. J.Appl.Phys. (Dec. 1992).
Bloomstein et al., Critical Issues in 157 nm Lithography, J.Vac.Sci.Tech.B. (Nov./Dec. 1998).
Bloomstein et al., Optical Materials and Coatings at 157 nm SPIE Mar. 1999.
Etec Systems, Inc.
Leitich Greg
Rosasco S.
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