In semiconductor device manufacturing, photolithography is typically used to transfer a pattern for forming semiconductor features onto the semiconductor wafer for the formation of multi-layered structures forming integrated circuits. During a photolithographic process, radiant energy having relatively small wavelengths such as ultraviolet light is passed through a photomask also referred to as a reticle to expose a radiant energy sensitive material such as photoresist formed on the wafer process surface. The mask includes predetermined circuitry patterns having attenuating regions and non-attenuating regions where the radiant energy intensity is modulated. For example, Ultraviolet (UV) light passed through the photomask onto the photoresist causes chemical reactions in the exposed portion of the photoresist altering it properties. Upon development of the photoresist resist exposed portions are removed in the case of a positive photoresist and non-exposed portions are removed in the case of a negative photoresist forming a pattern for subsequent processes such as anisotropic etching.
As semiconductor device feature sizes have decreased to sizes smaller than the wavelength of light used in photolithographic processes, the stray light incident on the exposed photoresist has increasingly become a problem in forming features with small critical dimensions (CDs), for example less than about 0.25 microns. Scattered light from undesired sources can cause a loss of pattern resolution in transferring the reticle pattern to the wafer photoresist. To increase the resolution of a transferred photolithographic pattern, phase shift masks (PSMs) have been developed where the phase of the wavefronts of light passing through alternating portions of the reticle pattern are shifted out of phase with respect to light passing through adjacent portions to produce destructively interfering wavefronts thereby reducing undesired exposure of the wafer photoresist due to diffraction of light at feature edges of the recticle pattern. As a result, the contrast, and therefore transferable resolution of the reticle pattern is improved.
There have been several different types of masks developed to improve resolution for different types of reticle patterns. For example, in an attenuated or halftone phase shift mask, the phase shifting function is typically accomplished by adding an extra layer of transmissive material to the mask with predetermined optical properties. Some PSMs are designed to produce improved resolution while having little improvement in depth of focus, while other PSMs are designed to have relatively modest increases in resolution while producing a greater improvement in depth of focus. For example, attenuated PSMs, also referred to as halftone PSMs, are of the latter type.
In a conventional mask, an opaque layer is formed which is not transmissive to light. Metals such as chromium are frequently used to form the opaque layer. The opaque layer is typically photolithographically patterned and etched, for example using an E-beam, laser or conventional UV light source to pattern a resist layer followed by etching to form a circuitry pattern including lines, pads, and contact holes.
For example, in the exposure process a mask formed on what is referred to as a reticle is repeatedly used to expose the circuit pattern on the reticle onto the photoresist covered process wafer. Typically, the term mask and reticle are now interchangeably used, although a photomask in the past has typically been used to refer to a mask that contains the pattern image for a complete wafer die array. Masks used in modern technology are typically referred to as reticles as well as masks where the reticle includes one or more mask patterns for individually patterning wafer die. In modern photolithography practice, a step and repeat process, such as step and scan, is used to expose the wafer surface with light passed through the mask over multiple wafer die. The mask on the reticle is larger by a factor of about 4 to 5 and is reduced to the appropriate dimensions on the wafer surface by optical reduction methods. For example, in the exposure process, the mask is illuminated by a light source either centered on the centerline of the projection optics or at an angle to the centerline of the projection optics, referred to as off-axis illumination to reduce the resolution limit and increase the depth of focus.
One problem with prior art processes is the presence of scattered light present during the exposure process. Scattered light present in the exposure process can alter a carefully designed exposure of the photoresist. For example, the light dose in an exposure process is critical for appropriate exposure of the photoresist to achieve desired pattern feature resolution, also referred to as critical dimension (CD), in the photoresist development process. Scattered light present in the exposure process can reduce the effectiveness of phase shift masks designed to reduce the effect of light diffraction around feature edges in the mask.
Prior art processes have attempted reduce scattered light by the use of pellicle film, a thin layer of flexible and optically transparent material including anti-reflective properties that is tightly stretched on a frame about 5 mm to about 10 mm above the surface of the reticle. This configuration is not useful in many exposure processes, for example DUV exposure processes where the pellicle film may not be sufficiently transparent or may be degraded after repeated exposure.
Another shortcoming in the prior art relates to the thickness of the photoresist layer typically required to successfully etch PSMs, due to less than desirable selectivity in the etching process to the underlying PSM. As a result, as device sizes decrease it is difficult to achieve the desired CD even in the absence of undesired light reflections, as well as being affected in the exposure process by considerations of insufficient depth of focus.
Thus, there is a need in the semiconductor manufacturing art for an improved method and mask to reduce light scattering in a photolithographic exposure process while increasing an etching selectivity in a PSM etching process to improve critical dimension uniformity.
It is therefore among the objects of the present invention to provide an improved method and mask to reduce light scattering in a photolithographic exposure process while increasing an etching selectivity in a PSM etching process to improve critical dimension uniformity, in addition to overcoming other shortcomings and deficiencies of the prior art.