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Nikon vs. ASML, Part I| thinfilmmfg.com
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Nikon vs. ASML, Part I |
The US lithography market is one of the industry's most important battlegrounds, accounting for about a third of world equipment sales. Moreover, the US is home to acknowledged technology leaders like IBM and Intel. These companies tend to be among the first to adopt each successive technology generation, giving their suppliers an important head start as new technologies spread through the semiconductor industry. Since last year's acquisition of Silicon Valley Group by ASM Lithography, the US is also a market without a domestic supplier of leading edge exposure systems. None of the major competitors has a significant home field advantage.
Because most exposure systems are imported to the US, patent infringement suits like the one that erupted between Nikon and ASML in December, often result in trade litigation as well. Nikon simultaneously sued ASML for patent infringement in the Federal District Court of California, and filed an infringement complaint with the International Trade Commission. The ITC complaint seeks to prevent ASML from importing equipment which infringes Nikon's patents. The judicial suit seeks damages and an injunction against further infringement. The two suits also involve different patents. Part I of this article deals with the seven patents referenced in the ITC complaint. Part II will review the five patents at issue in the infringement suit. According to Frank Masciocchi, VP of sales and marketing at Nikon Precision, rulings on both complaints are likely to take 12-18 months.
The seven patents mentioned in the ITC complaint appear to cover three critical elements of advanced lithography system design: off-axis illumination, stage positioning, and system engineering. The first group, which includes US Patents #5,638,211 and #6,233,041, deals with illumination systems. As feature sizes drop below the exposure wavelength, off-axis illumination is a critical weapon in the lithographer's arsenal. By illuminating mask features from multiple directions, a stepper can reduce diffraction effects, improving resolution without optical proximity correction, phase shifts, or other mask-based resolution enhancements.
US Patent #5,638,211 is titled "Method and apparatus for increasing the resolution power of a projection lithography exposure system." It describes a method for illuminating the features of a mask with a pair of light beams. The two beams, representing the ±first order and the zeroth order diffractions, arise when the exposure pulse strikes an aperture with regions of high and low transmission.
The second illumination patent involved in the ITC complaint, U.S. Patent #6,233,041, describes "An exposure method utilizing diffracted light having different orders of diffraction." Like the '211 patent, it describes a method of illuminating a photomask with multiple diffracted beams in order to improve resolution and depth of focus.
The second group of allegedly infringed patents deals with stage positioning and transport. To achieve high throughput, the stage, a massive object, must accelerate and decelerate quickly while still achieving accurate position control.
US Patent #6,008,500 describes an "Exposure apparatus having dynamically isolated reaction frame." As the patent explains, a reticle stage for step-and-scan exposure systems must allow smooth and precise scanning, while controlling small displacements perpendicular to the scan direction. While a guide structure can be placed directly under the wafer stage, the reticle stage must allow light to pass through.
The stage described in the '500 patent uses a window frame guide structure. The window frame moves along a set of parallel rails to scan in the X direction. In the Y direction, the stage moves within the window frame. Spring-mounted air bearings support the stage and the sides of the window frame.
The whole assembly rests on its own support frame, independent of the frame supporting the projection lens. Reaction forces due to the stage drive motors are transmitted to the earth's surface without inducing vibrations elsewhere in the system. These reaction forces are transmitted through the stage's center of gravity, reducing unwanted torque moments.
The second stage positioning patent, US Patent #6,271,640, describes a wafer positioning system with support arms resting on isolation blocks. The blocks absorb vibrations transmitted from the ground, while isolating the reaction forces due to the stage itself.
A third stage-related patent, US Patent #6,323,935, describes an "Electromagnetic alignment and scanning apparatus." As the patent explains, step-and-repeat exposure systems typically fix the projection optics and the wafer drive to the same structural frame. This design transmits vibrations from the stage drive to the lens, potentially distorting the pattern on the wafer.
Typical designs also move the wafer stage over a relatively long distance from the wafer carrier arm, which removes the wafer from the input cassette and places it on the stage, to the exposure position. Because of the distance, the laser interferometer used to detect stage position requires an extremely long mirror. The mirror increases the weight of the stage, requiring larger drive motors. The motors, in turn, generate waste heat that can distort interferometer measurements.
An ideal stage would use electromagnetic forces for positioning, eliminating vibrations due to the drive motors. A light structure with no moving parts would improve control bandwidth. The '935 patent claims to achieve those goals with a completely non-contact electromagnetic positioning system.
The last two patents pull the illuminator, stages, and optics together into a complete system. US Patent #5,473,410, for a "Projection exposure apparatus," addresses many of the issues inherent in step-and-scan exposure systems. It defines a mask stage for moving the mask relative to the optical axis of the projection lens. Next, it describes a substrate stage that can synchronize the wafer and mask scan velocities. Finally, it introduces an improved illuminator which can control the size, shape, and position of the exposure aperture as required by the movement of the mask stage. All three pieces are necessary for high resolution step-and-scan exposures.
Lastly, US Patent #6,255,796 describes a support system for the projection lens. The structure allows the lens to rotate within predetermined limits, for example to correct for aberrations. At the same time, the support structure is flexible enough to attenuate microvibrations without loss of optical resolution.
As a group, the seven patents at issue before the ITC appear to cover most aspects of step-and-scan system design, particularly the critical issue of vibration control. ASML's Twinscan product line appears to be a particular target of the litigation. (ASML declined to comment on the suit, and Nikon would not discuss its legal strategy.)
To read the full text of these patents, visit Delphion's patent database.
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