Radiation imagery chemistry: process – composition – or product th – Radiation sensitive product – Antihalation or filter layer containing
Reexamination Certificate
2001-07-17
2002-04-02
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation sensitive product
Antihalation or filter layer containing
C430S512000
Reexamination Certificate
active
06365333
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates generally to integrated circuits, and particularly, but not by way of limitation, to a low temperature anti-reflective coating for integrated circuit lithography.
BACKGROUND OF THE INVENTION
Trends in modem integrated circuit (IC) technology demand increasingly dense ICs, such as for computer systems, portable electronics, and telecommunications products. IC fabrication includes, among other things, photolithography for selective patterning and etching of photoresist layers. The patterned photoresist layer serves as a masking layer such that a subsequent IC processing step is carried out on only those portions of the underlying IC that are uncovered by photoresist, as described below.
A photoresist layer is typically formed on an underlying integrated circuit substrate. The photoresist layer overlays any structures that are already formed on the substrate. Portions of the photoresist are selectively exposed to light through a lithographic mask that includes clear and opaque portions forming a desired pattern. Light is transmitted through the clear portions of the mask, but not through the opaque portions. The incident light changes the chemical structure of the exposed portions of photoresist. A chemical etchant, which is sensitive to only one of the exposed and unexposed portions of the photoresist, is applied to the photoresist to selectively remove those portions of the photoresist to which the chemical etchant is sensitive. As a result, portions of the photoresist which are insensitive to the chemical etchant remain on the IC. The remaining portions of the photoresist protect corresponding underlying portions of the IC from a subsequent IC processing step. After this IC processing step, the remaining portions of the photoresist layer are typically removed from the IC.
High density ICs require sharply defined photoresist patterns, because these patterns are used to define the locations (and density) of structures formed on the IC. However, light reflects from the surface of the underlying substrate on which the photoresist is formed. Certain structures that are formed on the underlying substrate are highly reflective such as, for example, aluminum or copper layers for forming circuit interconnections. Reflections from the surface of the substrate underlying the photoresist causes deleterious effects that limit the resolution of photolithographic photoresist patterning, as described below.
First, reflections cause the light to pass through the photoresist at least twice, rather than only once. In other words, light first passes through the photoresist to reach the surface of the underlying substrate. Then, light is reflected from the surface of the underlying substrate and passes back through the photoresist layer a second time. The chemical structure of the photoresist changes differently when light passes through the photoresist more than once, rather than when light passes through the photoresist only once. A portion of the light, already reflected from the surface of the underlying substrate, can also reflect again from the surface of the photoresist, passing back through the photoresist yet again. In fact, standing light waves can result in the photoresist from superpositioning of incident and reflected light rays. This overexposure problem is sometimes referred to as the “swing effect.”
Even more problematic, the reflections of the light are not necessarily perpendicular. Light reflects angularly from features on the surface of the underlying substrate, or if the incident light is not perpendicular to the surface of the substrate. The latter results from the diffractive nature of light (i.e., light bends). Off-angle reflections reduce the sharpness of the resulting photoresist pattern. A portion of the light reflected obliquely from the surface of the underlying substrate can also be again reflected obliquely from the surface of the photoresist. As a result of such angular reflections, the light can travel well outside those photoresist regions underlying the transmissive portions of the photolithographic mask. This potentially causes photoresist exposure well outside those photoresist regions underlying transmissive portions of the photolithographic mask. This problem, which is sometimes referred to as “notching,” results in a less sharply defined photoresist pattern that limits the density of structures formed on the integrated circuit. There is a need to overcome these photolithographic limitations to obtain the benefits of high resolution photolithography and high density integrated circuits.
As discussed above, aluminum and other metallization layers are particularly problematic for high resolution lithography. In addition to being highly reflective, such layers typically have a low thermal budget More particularly, after an aluminum or other metallization layer is formed, only low temperature processing steps can be used, in order to avoid vaporizing the aluminum or metallization layer. Thus, there is a particular need to avoid reflections from metal layers that are both highly reflective and incompatible with subsequent high temperature processes.
SUMMARY OF THE INVENTION
The present invention provides, among other-things, an antireflective coating (ARC), such as for use in integrated circuit (IC) photolithography. In one. embodiment, the invention provides an antireflective coating. The antireflective coating includes a first layer formed on a substrate and having a first optical impedance. The antireflective coating also includes a second layer formed on the first layer and having a second optical impedance. The second layer has a thickness that is determined by a grown oxidation of the first layer.
In one embodiment, the first layer comprises polysilicon germanium, and the second layer comprises an oxide that is grown from the polysilicon germanium material of the first layer. In another embodiment, the first layer comprises polysilicon, and the second layer comprises an oxide that is grown from the polysilicon material of the first layer.
In one embodiment, the second layer is substantially transmissive of a wavelength of incident light, wherein the wavelength is ultraviolet (UV) or deep ultraviolet (DUV), and the first layer is substantially absorptive of the wavelength of incident light. According to one aspect of the invention, a thickness of the second layer is less than or equal to approximately ¼ of the wavelength of the incident light. According to another aspect of the invention, a thickness of the first layer is greater than or equal to a thickness that absorbs substantially all of the incident light received by the first layer.
The second layer receives a photoresist layer formed thereupon. The photoresist layer has a photoresist optical impedance. The second optical impedance is approximately equal to the photoresist optical impedance at an interface between the second and photoresist layers.
Another aspect of the invention provides, among other things, a method. A first layer, having a first optical impedance, is formed on an integrated circuit substrate. A second layer, having a second optical impedance, is grown on the first layer. A photoresist layer, having a photoresist optical impedance, is formed on the second layer. The photoresist layer is exposed to incident light having a wavelength. Substantially all of any of the incident light that is received by the first layer is absorbed therein.
In one embodiment, the invention provides a method in which a first layer is deposited on an integrated circuit substrate using chemical vapor deposition. The first layer has a first optical impedance and is substantially absorptive to incident light having a wavelength in one of the ultraviolet (UV) and deep ultraviolet (DUV) ranges. A second layer is formed by oxidizing the first layer at a temperature of less than approximately 300 degrees Celsius. The second layer has a second optical impedance that depends on its thickness. The second layer is substantially transmissive of incident ligh
Ahn Kie Y.
Forbes Leonard
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