Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
2002-09-30
2004-11-16
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C430S030000, C356S237100, C356S237500, C382S144000, C382S149000
Reexamination Certificate
active
06818360
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to semiconductor processing, and in particular to a system and methodology for monitoring and/or controlling the fabrication of a phase shift mask.
BACKGROUND
In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities, there has been and continues to be efforts toward scaling down device dimensions (e.g., at submicron levels) on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller feature sizes are required in integrated circuits (ICs) fabricated on small rectangular portions of the wafer, commonly known as dies. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, the surface geometry such as corners and edges of various features as well as the surface geometry of other features. To scale down device dimensions, more precise control of fabrication processes are required. The dimensions of and between features can be referred to as critical dimensions (CDs). Reducing CDs, and reproducing more accurate CDs facilitates achieving higher device densities through scaled down device dimensions and increased packing densities.
The process of manufacturing semiconductors or ICs typically includes numerous steps (e.g., exposing, baking, developing), during which hundreds of copies of an integrated circuit may be formed on a single wafer, and more particularly on each die of a wafer. In many of these steps, material is overlayed or removed from existing layers at specific locations to form desired elements of the integrated circuit. Generally, the manufacturing process involves creating several patterned layers on and into a substrate that ultimately forms the complete integrated circuit. This layering process creates electrically active regions in and on the semiconductor wafer surface. The layer to layer alignment and isolation of such electrically active regions depends, at least in part, on the precision with which features can be placed on a wafer. If the layers are not aligned properly, overlay errors can occur compromising critical dimensions and the performance of the electrically active regions and adversely affecting chip quality and reliability.
The requirement of small features with close spacing between adjacent features requires the implementation of high-resolution lithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the photoresist, and an exposing source (such as light, x-rays, or an electron beam) illuminates selected areas of the surface of the film through an intervening master template, mask or reticle for a particular pattern. The lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the photoresist coating.
Light projected onto the photoresist changes properties (e.g. solubility) of the coating such that different portions thereof (e.g. the illuminated or un-illuminated portions, depending upon the type of photoresist) can be manipulated in subsequent processing steps. For example, regions of a negative photoresist become insoluble when illuminated by an exposure source such that the application of a solvent to the photoresist during a subsequent development stage removes only non-illuminated regions of the photoresist. The pattern formed in the negative photoresist layer is, thus, the negative of the pattern defined by opaque regions of the template. By contrast, in a positive photoresist, illuminated regions of the photoresist become soluble and are removed via application of a solvent during development. The pattern formed in the positive photoresist is, thus, a positive image of opaque regions on the template. Less soluble portions of the photoresist are removed in subsequent processing stages after the image has been transferred onto the wafer. The accuracy with which patterns are transferred onto the wafer is thus important to the success of the semiconductor fabrication process.
As feature sized are continually reduced, however, limitations due to the wavelength of the light utilized in semiconductor processing can adversely affect the accuracy of pattern transfers. More particularly, as feature sizes approach the wavelength of the light utilized in processing, diffraction can occur. Diffraction is a property of wave motion, in which waves spread and bend when passed through small apertures or around barriers. The pattern(s) defined within masks can contain many such small apertures and barriers, and 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 incoming wave. Diffraction can occur for instance where light passes adjacent an edge of a pattern formed in the mask and is scattered in multiple directions by the edge. Diffraction can lead, for example, to rounded features and/or features that do not have a desired size and/or shape. Diffraction can also result in a reduction in intensity where exposure is desired and an increase in intensity in areas where no exposure is desired.
For example, in prior art
FIG. 20
, a light source is directing light waves
2002
at a mask
2004
. Some of the light waves
2002
pass through an aperture
2006
that is close to the size of the wavelength of the light waves
2002
. The mask
2004
has been designed to develop a region
2008
on a photo resist layer
2010
, so that two desired features
2012
and
2014
can be formed. The features
2012
and
2014
are desired to be rectangular, with substantially square edges. The aperture
2006
is small because the desired features
2012
and
2014
are correspondingly small.
With conventional lithography, the light waves
2002
may pass directly through the aperture
2006
, exposing the region
2008
, but the light waves
2002
may also be diffracted as illustrated by light waves
2016
,
2018
and
2020
. The diffracted wave
2016
has exposed a region
2022
and the diffracted wave
2018
has exposed a region
2024
. Neither region
2022
nor region
2024
were intended to be exposed. Further, diffracted wave
2020
has exposed a triangular area
2026
on either side of the region
2008
. Thus the desired feature
2014
may not have a substantially square edge due to the undesired region
2026
being exposed by the diffracted wave
2020
.
Reticles known as phase shift masks can be utilized in photolithographic processing to account for diffraction. Phase shift masks facilitate compensating for the effects of diffraction which limit the precision and size to which imaged features can be reduced. The underlying concept of a phase shift mask is to selectively introduce interference and cancellation of light at portions of an image where diffraction effects may deteriorate the resolution of the image.
In lithography, resolution is typically defined as the smallest distance two features can be spaced apart while removing all photo resist between the features, and 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 0.5. 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
2
). In phase shift masks, features are surrounded by light transmitting regions that shift the phase of transmitted light. Masks may be constructed to shift the phase of transmitted light by varying amounts, such as 30 degrees, 60 degrees, 90 degrees, and 180 degrees. In this
Phan Khoi A.
Rangarajan Bharath
Singh Bhanwar
Advanced Micro Devices , Inc.
Amin & Turocy LLP
Young Christopher G.
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