Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2001-05-10
2004-06-22
O'Shea, Sandra (Department: 2877)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000, C355S077000, C355S060000, C355S055000, C355S067000, C356S399000, C356S400000, C356S401000, C250S548000, C250S492200, C430S030000
Reexamination Certificate
active
06753947
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to lithography, and in particular pertains to a system for and method of rapidly and cost-effectively performing lithographic exposures in the manufacture of devices.
BACKGROUND OF THE INVENTION
The process of manufacturing certain micro-devices such as semiconductor integrated circuits (ICs), liquid crystal displays, micro-electro-mechanical devices (MEMs), digital mirror devices (DMDs), silicon-strip detectors and the like involves the use of high-resolution lithography systems. In such systems, a patterned mask (i.e., a reticle) is illuminated with radiation (e.g., laser radiation or radiation from an arc lamp) that passes through an illumination system that achieves a high-degree of illumination uniformity over the illuminated portion of the mask. The portion of the radiation passing through the mask is collected by a projection lens, which has an image field (also referred to as a “lens field”) of a given size. The projection lens images the mask pattern onto an image-bearing workpiece. The workpiece resides on a workpiece stage that moves the workpiece relative to the projection lens, so that the mask pattern is repeatedly formed on the workpiece over multiple “exposure fields.”
Lithography systems include an alignment system that precisely aligns the workpiece with respect to the projected image of the mask, thereby allowing the mask to be exposed over a select region of the workpiece. In most cases, the mask image needs to be precisely aligned to a pre-existing exposure field on the workpiece to provide the juxtaposed registration necessary to build up layers of the device being fabricated.
Presently, two types of lithography systems are used in manufacturing: step-and-repeat systems, or “steppers,” and step-and-scan systems, or “scanners.” With steppers, each exposure field on the workpiece is exposed in a single static exposure. With scanners, the workpiece is exposed by synchronously scanning the work piece and the mask across the lens image field. An exemplary scanning lithography system and method is described in U.S. Pat. No. 5,281,996. The projection lenses associated with steppers and scanners typically operate at 1× (i.e., unit magnification), or reduction magnifications of 4× or 5× (i.e., magnifications of ±¼ and ±⅕, as is more commonly expressed in optics terminology).
The ability of a lithography system to resolve (or, more accurately, “print”) features of a given size is a function of the exposure wavelength: the shorter the wavelength, the smaller the feature that can be printed or imaged. To keep pace with the continuously shrinking minimum feature size for many micro-devices (particularly for ICs), the exposure wavelength has been made shorter. Also, historically the device size has increased as well, so that the lens field size has steadily grown. The resolution of the lithography system also increases with the numerical aperture (NA) of the projection lens. Thus, in combination with reducing the exposure wavelength, the numerical apertures of projection lenses tend to be as large as can be practically designed, with the constraint that the depth of focus, which decreases as the square of the NA, be within practical limits.
Until fairly recently, semiconductor industry roadmaps predicted that lithography system field sizes would continue to increase to accommodate the increasing overall device size of memory and micro-processor chips. This trend has significant cost implications for manufacturing. In the case of smaller circuits, multiple devices would be fitted into a single exposure field. Although devices have generally grown in size, they have not grown as fast as anticipated by the industry roadmap makers, and the size of the minimum features used in the devices has shrunk faster than originally expected. The field sizes of the current generation of step-and-scan systems is more than adequate for the next few generations of memory, but the rapidly shrinking minimum geometry sizes are making it very difficult to obtain masks.
In order to provide some relief to the mask makers, the latest International Technology Roadmap update shows a minimum lens image field size of 25 mm×32 mm through 2003 changing to 22 mm×26 mm for 2004 through 2013. This decrease in the required field size allows the reduction magnification to be increased from 4× to 5×, thus providing relief to the mask maker.
State-of-the-art lithography systems constitute some of the most complex machinery ever built, and as a consequence, are extremely expensive. Also, a good deal of effort and expense is also required to maintain and service lithography systems in the manufacturing environment. While it is relatively easy to build various kinds of experimental lithography systems in the laboratory for research and development purposes, it is an immense challenge to develop a lithography system for manufacturing purposes that is affordable and that operates in a cost-effective manner as determined by “cost-of-ownership” considerations.
Choices between different types of lithography equipment are generally made on the basis of their relative cost-of-ownership. This cost-based model takes into account the cost of procuring, operating and maintaining a given lithography system in a manufacturing environment. The cost-of ownership is determined by considering various factors that relate directly to the properties of the lithography system and how the system is used in the manufacturing environment. There are a number of sophisticated cost-of-ownership models, an example being the SEMATECH Lithography Cost of Ownership Model, which take into account scores of different factors in performing the cost-of-ownership calculation. However, reliable cost-of-ownership trends can be obtained by examining a few key factors, such as stepper cost, mask cost, field size, system throughput (defined below), and the number of workpieces processed for a given mask.
It is known that the cost of the projection lens and illuminator for a lithographic system increases roughly as the cube of the lens field size. The cost of a state-of-the-art stepper having a lens capable of 0.13 micron resolution and operable at a 193 nm exposure wavelength with a field size of 22 mm×22 mm is roughly divided evenly between the lens and the rest of the system. The latter includes, the mask and wafer handling systems, the mask and workpiece stages, the laser exposure source, alignment systems, etc. Integrating all of these components into a usable system with installation and warranty leads to system prices of between about $10 M and $20 M.
Lithography system “throughput” (i.e., the number of workpieces capable of being processed per hour) is one of the most significant factors in the cost-of-ownership calculation. To first order, the throughput of a conventional lithography system increases as the square of the exposure field size (diameter). Historically, throughput has been limited in part by the brightness of the radiation source, which must deliver a sufficiently high and uniform dose of radiation to the wafer for every exposure field. However present-day radiation sources are typically narrow-band, pulsed excimer lasers operating at 2000-4000 Hz and are far brighter than the arc sources used previously. The pulse-to-pulse uniformity of a typical excimer laser suitable for use in lithography is quite poor, i.e., 8-10% (3&sgr;).
As lithography systems typically require illumination uniformity over the mask of less than 1% (3&sgr;), numerous (e.g., 100) pulses from the radiation source are averaged together to achieve the required uniformity. With laser repetition rates of several thousand hertz, the exposure times of step-and-scan systems have been reduced considerably so that the throughput rate of current lithography systems is primarily limited by stage motion and settling times (for steppers) and acceleration and scanning times (for step-and-scan systems). Reticle stage turn-around time, including
Markle David A.
Meisburger Dan
Brown Khaled
Jones Allston L.
O'Shea Sandra
Ultratech Stepper, Inc.
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