Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2001-06-04
2002-11-12
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S027000, C355S040000, C355S038000, C355S039000, C355S072000, C358S487000, C358S506000, C250S492200, C250S492220, C430S005000
Reexamination Certificate
active
06480261
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to microfabrication, and more particularly to systems, methods, and techniques for manufacturing integrated circuit chips using photon lithography. Still more particularly, the present invention relates to systems, methods, and techniques in connection with programmable masks for microlithography.
BACKGROUND AND SUMMARY OF THE INVENTION
Lithography is used to transfer a specific pattern onto a surface. Lithography can be used to transfer a variety of patterns including, for example, painting, printing, and the like. More recently, lithographic techniques have become widespread for use in “microfabrication”—a major example of which is the manufacture of integrated circuits such as computer chips.
In a typical microfabrication operation, lithography is used to define patterns for miniature electrical circuits. The lithography defines a pattern specifying the location of metal, insulators, doped regions, and other features of a circuit printed on a silicon wafer or other substrate. The resulting semiconductor circuit can perform any of a number of different functions.
Improvements in lithography have been mainly responsible for the explosive growth of computers in particular and the semiconductor industry in general. The major improvements in lithography can, for the most part, be put into two categories: an increase in chip size, and a decrease in the minimum feature size (improvement in resolution). Both of these improvements allow an increase in the number of transistors on a single chip (and in the speed at which these transistors can operate). For example, the computer circuitry that would have filled an entire room in 1960's technology can now be placed on a silicon “die” the size of your thumbnail. A device the size of a wristwatch can contain more computing power than the largest computers of several decades ago.
One type of lithography that is commonly used in the mass production of computer chips is known as “parallel lithography”. Parallel lithography generally prints an entire pattern at one time. This is usually accomplished by projecting photons through a mask onto a photoresist-coated semiconductor wafer, as shown in
FIG. 1. A
mask (designated by an “M” in
FIG. 1
) provides a template of the desired circuit. A photoresist coat, which may be a thin layer of material coated on the wafer which changes its chemical properties when impinged upon by light, is used to translate or transfer the mask template onto the semiconductor wafer. In more detail, mask M allows photons (incident light, designated by an “I”) to pass through the areas defining the features but not through other areas. An example of a typical mask construction would be deposits of metal on a glass substrate. In a way analogous to the way light coming through a photographic negative exposes photographic paper, light coming through the mask exposes the photoresist. The exposed photoresist bearing the pattern selectively “resists” a further process (e.g., etching with acid, bombardment with various particles, deposition of a metallic or other layer, etc.) Thus, this lithography technique using photoresist can be used to effectively translate the pattern defined by the mask into a structural pattern on the semiconductor wafer. By repeating this technique several times on the same wafer using different masks, it is possible to build multi-layered semiconductor structures (e.g., transistors) and associated interconnecting electrical circuits.
Parallel lithography as described above has the advantage that it is possible to achieve a high throughput since the whole image is formed at once. This makes parallel lithography useful for mass production. However, parallel lithography has the disadvantage that a new mask is required each time one desires to change patterns. Because masks can have very complex patterns, masks are quite costly and susceptible to damage.
For mass production, parallel lithography is usually done using a machine known as a “stepper.” As schematically depicted in
FIG. 1
, a stepper consists of a light source (“L”), a place to hold a mask (“W”), an optical system (“lenses”, “L”) for projecting and demagnifying the image of the mask onto a photoresist-coated wafer (“W”), and a stage (“S”) to move the wafer. The process of exposing a wafer using a stepper is summarized in
FIG. 2A
, and is depicted from a side view in
FIGS. 2B-2E
. In each exposure, a stepper only exposes a small part of the wafer, generally the size of one chip. Since there are often many separate chips on each wafer, the wafer must be exposed many times. The stepper exposes the first chip (FIG.
2
B), then moves (“steps”) over (
FIG. 2C
) to expose the next chip (
FIG. 2D
) and repeats this process (
FIG. 2E
) until the entire wafer is exposed. This process is known as “step and repeat” and is the origin of the name “stepper.”
A stepper must also be capable of precisely positioning the wafer relative to the mask. This precise positioning (overlay accuracy) is needed because each lithography step must line up with the previous layer of lithography. A stepper spends a significant portion of its time positioning the stage and the rest exposing the photoresist. Despite the great precision necessary, steppers must be capable of high throughput to be useful for mass production. There are steppers that can process one hundred 8-inch wafers per hour.
One way to increase the usefulness of a chip is to increase its size. In the “step and repeat” example described above, the size of the chip is limited to the exposure size of the stepper. The exposure size is small (roughly 20 mm×40 mm) because of the cost of an optical system that is capable of projecting a high quality image of the mask onto the wafer. It is very expensive to increase the size of a chip by increasing the exposure size of the stepper (for example, this would require a larger lens—which by itself can cost many hundreds of thousands of dollars). Another approach is to modify a stepper so that light only shines on a subsection of the mask at a given time. Then, the mask and wafer can be scanned (moved relative to the fixed light source) simultaneously until the entire mask is imaged onto the wafer, as in
FIGS. 3A-3C
. This modified stepper is known as a “scanner” or “scanner/stepper”.
Scanners offer increased chip size at the expense of increased complexity and mask costs. Because scanner masks are larger, the masks are more fragile and are more likely to contain a defect. The increased size and fragility of the mask mean that the masks for a scanner will be more expensive than the masks for a stepper. Also, because the image is being demagnified, the mask and wafer must be scanned at different speeds, as depicted by the length of the arrows in
FIGS. 3A-3C
. Because of the great precision required, differential scanning increases the cost and complexity of a scanner when compared with a stepper.
Many chip manufacturers are looking toward future improvements in resolution and/or exposure size to help continue the growth that has driven the semiconductor industry for the past thirty years. The improvements in these areas have been partly the result of improvements in the optical systems used to demagnify the mask and of the use of shorter wavelength light. In particular, modem lithography systems used for mass production are “diffraction limited”, meaning that the smallest feature size that it is possible to print is determined by the diffraction of light and not by the size of features on the mask. In order to improve the resolution, one must use either a shorter wavelength of light or another technique such as optical proximity correction or phase shifting.
Another option for improving resolution is to put the mask in contact with the wafer, as in
FIG. 4
; the effects of diffraction can be lessened by not giving the light a chance to “spread out” after it passes through the mask. Unfortunately, contact lithography is not suitable for mass production for at least two reasons. First, the mask must now be the
Cooper Gregory D.
Mohring Richard M.
Brown Khaled
Nixon & Vanderhye P.C.
Pixelligent Technologies LLC
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