Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2001-03-13
2003-05-27
Rosasco, Stephen D (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
Reexamination Certificate
active
06569580
ABSTRACT:
The present invention relates to the physics, material science, optics, lithography and semiconductor chip manufacture. In particular, it relates to photomasks for use in semiconductor chip manufacture.
BACKGROUND OF THE INVENTION
One of the driving forces of technology today is the desire to produce smaller and smaller devices, which, while being smaller, have the same or even greater operating characteristics as their larger version. No place is this more evident than in the area of semiconductor manufacture. Devices on semiconductors are constantly being reduced in size to the point that sub-micron architecture is becoming commonplace and circuit densities in the millions of transistors per die are the norm. To accomplish this, smaller and smaller feature sizes, a feature being an element of the device such as a lines, holes and corners and edges of surface structures, are required. While numerous techniques for the manufacture of these infinitesimal devices are being tested in the laboratory and even more are being proposed, the mainstay of the semiconductor manufacturing industry remains lithography, primarily optical photolithography.
Optical photolithography requires four basic components, an illumination device, which modernly can provide light of a very narrow range of, even essentially a single, wavelength, a photomask on which an image of the device to be created on a wafer is projected several times larger than the eventual device on the wafer, an optical system which reduces the size of the image and focuses it on the wafer surface, and the wafer itself. The optical resolution obtainable in a photolithography system is constrained by each of the first three parameters. That is, wavelength of the light used, the physical condition of the mask, i.e., whether it contains any defects and the ability of the mask to direct light to the lens with minimum diffraction and the ability of the lens to focus the image on the wafer. Presently, the wavelength of light is selectable and controllable at almost any wavelength from that of visible light (400-700 nanometers (nm)) to that of the extreme uv region of the spectrum (approximately 5 to 254 nm). The capabilities of the lens is characterized by its numerical aperture (NA), which correlates with the ability of a lens to collect and use diffracted light from a source (the more diffraction orders that can be collected, the more information available to form an image and, thus, the greater the resolving power of the lens) has been greatly improved and may be approaching a practical maximum. As control over light sources and lenses has advanced, advances in photomasks have not entirely kept pace. Even the newest generation of photomasks still retain several characteristics that contribute heavily to reduced optical resolution and for which optimal control or correction means are still being sought.
One problem with photomasks is their physical integrity. Pinholes in the material forming the dark areas of the mask (usually sputtered metallic chromium, although such materials as aluminum and molybdenum silicide are also being used) can result in the printing of errant features on a wafer. And, while the materials forming the dark areas of the mask are generally quite hard, they are also very thin and subject to physical damage during use, especially when used in contact mode. Another problem with photomasks is diffraction of light passing though the mask at the boundaries between opaque and transparent regions of the mask which ultimately causes broadening of line widths and blurring of other structural features resulting in reduced resolution on a wafer.
A technique devised for controlling light diffraction at the boundary between opaque and transparent portions of a photomask is phase-shift lithography. Phase-shift masks (PSMs) make use of the phenomenon of wave interference. That is, the phase of the light used to expose a substrate through a PSM is controlled such that light passing through adjacent light-transmitting regions of the mask are out of phase with one another, most often by 180°, although other phase differentials may be used for certain purposes. The result of a 180° phase differential is the creation of a dark line between the adjacent light-transmitting regions due to destructive interference between the out-of-phase light waves. The PSMs currently receiving the most attention are alternating PSMs, rim PSMs and attenuated PSMs. The alternate PSM (FIG.
1
A), is most useful for closely spaced densely packed patterns. The rim PSM (
FIG. 1B
) and attenuated PSM (
FIG. 1C
) are more effective with random patterns of lines and holes and other structural features. The utility of the rim PSM suffers somewhat from the fragility of the overhang portion of the mask and both the rim and attenuated PSMs are limited by the requirement that the phase shifting material be of a certain thickness based on its refractive index and the wavelength of the light being shifted in order to achieve a desired degree of phase shift (Eq. 1, below).
A further problem faced with present masks is the mask error factor (MEF). The MEF is defined as ratio of the actual error in a critical feature size printed on a wafer to the error in size of the feature predicted by the feature size error on the mask and the reduction factor. For example, assuming a critical feature that is designed to be 1.0 micron on the wafer and a 4× reduction from mask to wafer and that the critical feature on the photomask measures 4.04 microns (instead of the ideal size of 4.00 microns). The 40 micron error in the critical feature dimension on the mask would be expected to give a 10 micron error (4× reduction) on the wafer, that is, a feature measuring 1.01 microns. However, due to non-linear behaviour of the wafer lithography process, the resulting feature size may in fact be, for example, 1.02 microns, that is, a 20 micron error. The MEF then would be 2 (20÷10), indicating that the critical dimension error that was printed on the wafer was 2× larger than that predicted based on the error in the mask and the reduction factor. The MEF becomes significant in the realm of sub-wavelength lithography where the geometry, that is, the critical features, being imaged are smaller than the wavelength of the light used to expose the pattern on the wafer.
What is needed is a mask that is physically stronger than those presently available, that has better resolution, a reduced MEF and, in the case of PSMs, greater flexibility with regard to phase-shift and transmission.
The present invention provides masks that meet these needs.
SUMMARY OF THE INVENTION
Thus, in one aspect, the present invention relates to a binary mask having energy-transmitting regions and energy-blocking regions, comprising an energy-transparent substrate, an energy-blocking substance adhered to the substrate in the energy-blocking regions and diamond-like carbon (DLC) adhered to the energy-blocking substance.
In another aspect, the present invention relates to a binary mask wherein the energy being used is visible light, uv light or x-ray energy.
In another aspect, the present invention relates to a binary mask wherein the energy being used is accelerated electrons.
In another aspect, the present invention relates to a binary mask wherein the energy is visible or uv light and the energy-transparent substrate comprises a glass.
In another aspect, the present invention relates to a binary mask where the energy is visible or uv light and the glass is fused quartz.
In another aspect, the present invention relates to a binary mask wherein the energy is accelerated electrons and the energy-transparent substrate comprises a silicon membrane.
In another aspect, the present invention relates to a binary mask wherein the energy is visible or uv light and the energy-blocking substance comprises a metal, a metal oxide, a metal nitride or a metal fluoride.
In another aspect, the present invention relates to a binary mask wherein the energy-blocking substance comprises chromium.
In another aspect, t
Campi Jim G.
Van Den Broeke Douglas J.
Diverging Technologies, Inc.
Rosasco Stephen D
Rose, Esq. Bernard F.
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