Active control of phase shift mask etching process

Details Active control of phase shift mask etching process Active control of phase shift mask etching process

2001-03-26

2003-05-13

Dang, Thi (Department: 1763)

Etching a substrate: processes

Forming or treating mask used for its nonetching function

C156S345240

Reexamination Certificate

active

06562248

ABSTRACT:

TECHNICAL FIELD
The present invention generally relates to semiconductor processing, and in particular to a system for monitoring and controlling the etching of openings in a phase shift mask.
BACKGROUND
In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there have been, and continue to be, efforts toward scaling down device dimensions (e.g., at sub-micron levels) on semiconductor wafers. In order to accomplish such high device packing densities, smaller features sizes and more precise feature shapes are required. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry, such as corners and edges, of various features. When feature sizes become so small that they approach the wavelength of the exposure light used in semiconductor manufacturing, complex exposure techniques including complimentary phase shift masking may be employed. In complimentary phase shift masking, light passing through one or more masks may be phase shifted to facilitate selective interference and cancellation of light waves. The ability to control the phase shift of the light passing through a mask is important to achieving the desired critical dimensions on the chip.
The masks employed in semiconductor fabrication that utilize complimentary phase shift masking may include a quartz layer coated with a chrome layer. The quartz layer allows light waves to pass through, while the chrome layer prevents light waves from passing through the mask. Thus, either a positive or negative of the pattern to be projected onto a chip being fabricated is processed into the chrome layer on a complimentary phase shift mask. The depth and/or width of the openings (apertures) in the complimentary phase shift mask enable light passing through the apertures to be phase shifted.
The process of manufacturing masks may consist of hundreds of steps. One such step is depositing a chrome layer on a clean quartz layer (substrate). Once deposited, openings (apertures) are etched into the chrome layer. Controlling the width and depth of the openings etched into the chrome layer and controlling the width and depth of trenches carved into the substrate is required to enable controlled phase shifting of light that will pass through the mask. Conventional mask fabrication methods may not provide fine enough control of the aperture etching process and thus desired phase shifting may not be achieved. Thus, a system and method for controlling the aperture etching process is required.
The process of manufacturing semiconductors, (integrated circuits, ICs, chips), employing complimentary phase shift masks typically consists of more than a hundred steps, during which hundreds of copies of an integrated circuit may be formed on a single wafer. Generally, the process involves creating several patterned layers on and into the substrate that ultimately forms the complete integrated circuit. The patterned layers are created, in part, by the light that passes through complimentary phase shift masks. Thus, processing the positive or negative of the pattern into the mask is important in fabricating the chips.
The requirement of small features with close spacing between adjacent features requires sophisticated manufacturing techniques, including high-resolution photolithographic processes such as complimentary phase shift masking. Fabricating a semiconductor using such sophisticated techniques may involve a series of steps including exposing the photo resist one or more times to one or more light sources (where the phase of the light may be shifted). In conventional lithography, an exposure is performed using a single mask where the photo resist is exposed by a single radiation source. The resolution, which is typically defined as the smallest distance two features can be spaced apart while removing all photo resist between the features, is equal to:
D=k
1
*(lambda/
NA
)
where d is the resolution, lambda is the wavelength of the exposing radiation, NA is the numerical aperture of the lens, and k
1
is a process dependent constant typically having a value of approximately 0.7. While resolution may be improved by decreasing the wavelength or by using a lens with a larger NA, decreasing the wavelength and increasing the numerical aperture decreases the depth of focus (since depth of focus is proportional to lambda/NA), which creates additional problems. Thus, several techniques have been developed to enhance the resolution of conventional lithography to enable formation of patterned resist layers with smaller dimensions than those achievable with conventional methods. For example, phase-shifted masks (PSM) have been developed. In a PSM mask, features are surrounded by light transmitting regions that shift the phase of the transmitted light compared to the feature. Masks may be constructed to shift the phase of the light varying amounts, including, but not limited to, 30 degrees, 60 degrees, 90 degrees, and 180 degrees. In this way, the diffraction fringes at the edges of the features can be effectively cancelled, resulting in a better image contrast.
The resolution of both conventional and enhanced resolution lithographic processes is better for periodic features, such as those found in memory devices (e.g. DRAMs) because a greater percentage of the exposing radiation is contained in the diffraction nodes of the periodic structures compared to that contained in the diffraction nodes of isolated features. For example, prior art
FIG. 15
illustrates an aerial plot of intensity under a mask
800
having an isolated feature
802
and periodic features
810
,
812
, and
814
having a dimension near the resolution limit of the process. The contrast (difference in intensity) between masked and unmasked regions is much greater for the periodic features
810
,
812
and
814
(curve
806
) than for the isolated feature
802
(curve
808
). Thus, for a given combination of exposing conditions, at some dimension, isolated feature
802
cannot be resolved simultaneously with the periodic features
810
,
812
and
814
that are within the resolution limit of the process.
To alleviate the problems associated with isolated features in complimentary phase shift masking, complementary features are added around the isolated device features on a first mask to produce a periodic structure that allows for improved resolution of the lithographic process. The effects created by the complementary features may require the light passing through the features to have its phase shifted. Such a shift may be accomplished by varying the width and/or the depth of the opening through which the light passes.
In a positive photo resist method, the complementary features are then obliterated by exposure to light passing through a second mask prior to forming the patterned resist layer. The second mask also provides for improved contrast that enables more precise feature shapes. To take advantage of complimentary phase shift masking, and removal of unwanted complimentary structures, precise control of the depth and/or width of the openings in the complimentary phase shift masks is required. If the depth and/or width of the opening is not precisely controlled, then the phase shifting, diffraction and cancellation processes employed in complimentary phase shift masking will not lead to a desired cancellation of light and the isolated features will not benefit from the improved contrast and resulting improved quality.
The complimentary phase shift masking discussed above is possible because light passing through one or more apertures (apertures) on a mask employed in chip manufacturing is diffracted. Diffraction is a property of wave motion, in which waves spread and bend when passed through small apertures or around barriers. A mask may have many such apertures and barriers. The bending and/or spreading of the light waves is more pronounced when the size of the aperture or the barrier approximates or is smaller than the wavelength of the incomin

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