Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
1998-10-30
2003-04-22
Fuller, Rodney (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S071000
Reexamination Certificate
active
06552776
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to photolithographic systems used for fabricating integrated circuit devices, and more particularly, to photolithographic systems that compensate for lens errors.
2. Description of the Related Art
An insulated-gate field-effect transistor (IGFET), such as a metal-oxide semiconductor field-effect transistor (MOSFET), uses a gate to control an underlying surface channel joining a source and a drain. The channel, source, and drain are located in a semiconductor substrate, with the source and drain being doped oppositely to the substrate. The gate is separated from the semiconductor substrate by a thin insulating layer such as a gate oxide. The operation of the IGFET involves application of an input voltage to the gate, that sets up a transverse electric field in order to modulate the longitudinal conductance of the channel.
Polysilicon (also called polycrystalline silicon, poly-Si or poly) thin films have many important uses in IGFET technology. One of the key innovations is the use of heavily doped polysilicon in place of aluminum as the gate. Since polysilicon has the same high melting point as a silicon substrate, typically a blanket polysilicon layer is deposited prior to source and drain formation, and the polysilicon is anisotropically etched to provide the gate. Thereafter, the gate provides an implant mask during the implantation of source and drain regions, and the implanted dopants are driven-in and activated using a high-temperature anneal that would otherwise melt the aluminum. This self-aligning procedure tends to improve packing density and reduce parasitic overlap capacitances between the gate and the source and drain. As such, the gate length (or “critical dimension”) has a major influence on the channel length.
The performance of an integrated circuit depends not only on the value of the channel lengths, but also upon the uniformity of the channel lengths. In an integrated circuit having some devices with relatively longer channel lengths and other devices with relatively shorter channel lengths, the devices with shorter channel lengths have a higher drain current than the devices with the longer channel lengths. The difference in drain currents can cause problems. For instance, devices with too large a drain current may have a high lateral electric field that causes significant hot carrier effects despite the presence of a lightly doped drain (LDD), whereas devices with too small a drain current may have unacceptably slow switching speeds. Therefore, accurate gate lengths can be extremely important to achieving the required device performance and reliability.
Photolithography is frequently used to create patterns that define where a polysilicon layer is etched to form the gates. Typically, the wafer is cleaned and prebaked to drive off moisture and promote adhesion. An adhesion promoter is deposited on the wafer and a few milliliters of positive photoresist are deposited onto the spinning wafer to provide a uniform layer. The photoresist is soft baked to drive off excess solvents. The photoresist is irradiated with an image pattern that renders selected portions of the photoresist soluble. A developer removes the soluble portions of the photoresist and an optional de-scum removes very small quantities of photoresist in unwanted areas. The photoresist is hard baked to remove residual solvents and improve adhesion and etch resistance. The etch is applied using the photoresist as an etch mask, and the photoresist is stripped. Therefore, the photoresist has the primary functions of replicating the image pattern and protecting the underlying polysilicon when etching occurs.
Photolithographic systems typically use a light source and a lens in conjunction with a mask or reticle to selectively irradiate the photoresist. The light source projects light through the mask or reticle to the lens, and the lens focuses an image of the mask or reticle onto the wafer. A mask transfers a pattern onto the entire wafer (or another mask) in a single exposure step, whereas a reticle transfers a pattern onto only a portion of the wafer. Step and repeat systems transfer multiple images of the reticle pattern over the entire wafer using multiple exposures. The reticle pattern is typically 2× to 10× the size of the image on the wafer, due to reduction by the lens. However, non-reduction (1×) steppers offer a larger field, thereby allowing more than one pattern to be printed at each exposure.
Photolithographic systems often use a mercury-vapor lamp as the illumination source. In mercury-vapor lamps, a discharge arc of high-pressure mercury vapor emits a characteristic spectrum that contains several sharp lines in the ultraviolet region—the I-line (365 nm), the H-line (405 nm) and the G-line (436 nm). Photolithographic systems are designed, for instance, to operate using the G-line, the I-line, a combination of the lines, or at deep ultraviolet light (240 nm). To obtain the proper projection, high power mercury-vapor lamps are used that draw 200 to 1000 watts and provide ultraviolet intensity on the order of 100 milliwatts/cm
2
. In some systems, air jets cool the lamp, and the heated air is removed by an exhaust fan.
The reticle typically includes a chrome pattern on a quartz plate. The chrome pattern has sufficient thickness to completely block ultraviolet light, whereas the quartz has a high transmission of ultraviolet light. Although quartz tends to be expensive, it has become more affordable with the development of high quality synthetic quartz material.
Lens errors in step and repeat systems are highly undesirable since they disrupt the pattern transfer from the reticle to the photoresist, which in turn introduces flaws into the integrated circuit manufacturing process. Lens errors include a variety of optical aberrations, such as astigmatism and distortion. Astigmatism arises when the lens curvature is irregular. Distortion arises when the lens magnification varies with radial distance from the lens center. For instance, with positive or pincushion distortion, each image point is displaced radially outward from the center and the most distant image points are displaced outward the most. With negative or barrel distortion, each image point is displaced radially inward toward the center and the most distant image points are displaced inward the most.
Replacing the lens in a step and repeat system is considered impractical since the lens is a large, heavy, integral part of the system, and is usually extremely expensive. Furthermore, it is unlikely that a substitute lens will render subsequent corrections unnecessary. Accordingly, the lens error can be measured so that corrections or compensations can be made.
A conventional technique for evaluating lens errors includes performing a photoresist exposure and development using specially designed mask patterns to be used for evaluation purposes. After such an imaging process, the wafer is either subjected to an optical inspection or is further processed to form electrically measurable patterns. The use of photosensitive detectors fabricated on silicon to monitor optical systems is also known in the art. For instance, U.S. Pat. No. 4,585,342 discloses a silicon wafer with light sensitive detectors arranged in a matrix, an x-y stage for positioning the wafer so that each one of the detectors is separately disposed in sequence in the same location in the field of projected light, and a computer for recording the output signals of the detectors in order to calibrate the detectors prior to evaluating the performance of an optical lithographic system.
After the lens error is measured, some form of corrective measure is typically employed. For instance, U.S. Pat. No. 5,308,991 describes predistorted reticles that incorporate compensating corrections for known lens distortions. Lens distortion data is obtained which represents the feature displacement on a wafer as a function of the field position of the lens. The lens distortion data is used to
Dawson Robert
Fulford Jr. H. Jim
Gardner Mark I.
Hause Frederick N.
Michael Mark W.
Advanced Micro Devices , Inc.
Fuller Rodney
LandOfFree
Photolithographic system including light filter that... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Photolithographic system including light filter that..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Photolithographic system including light filter that... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3040853