Photocopying – Projection printing and copying cameras – Distortion introducing or rectifying
Reexamination Certificate
2000-05-25
2002-06-11
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Distortion introducing or rectifying
C355S030000, C355S053000, C355S067000, C355S072000, C378S034000, C378S035000
Reexamination Certificate
active
06404481
ABSTRACT:
The development of this invention was partially funded by the Government under contract number N00019-98-K-0110 awarded by the Defense Advanced Projects Research Agency. The Government has certain rights in this invention.
This invention pertains to apparatus and methods for compensating distortions in membrane lithography masks, wafers, or both.
The size of circuit elements used in state-of-the-art integrated circuits continues to decrease. New lithography techniques will be needed to continue this reduction to sizes much smaller than those currently in use. Proximity X-ray lithography is a particularly promising technique, as it allows the largest exposure field of any of the contenders for the next generation of integrated circuit lithography, on the order of 5 cm×5 cm in a single exposure. The large exposure field provides a significant throughput advantage, but it also makes image placement more critical, to the point where accurate image placement is widely regarded as a major factor limiting the use of X-ray lithography in very large scale integrated (“VLSI”) circuits. Overlay errors in proximity X-ray lithography may arise from several factors, including for example the following: (1) errors in the pattern writing tool; (2) distortions in the membrane mask caused, for example, by stresses in the absorber; and (3) distortions that are already present in the pattern on the wafer. Much effort has gone into minimizing all three effects, as well as at least partially compensating them by adjusting the magnification.
The industry's response to this critical problem has typically been to design masks that are as rigid as possible, to try to minimize one potential cause of distortion at its source. While other types of masks are inherently more rigid, the inherent rigidity of membrane masks is relatively low. The rigidity of membrane masks has been increased, for example, by the use of diamond substrates. Membrane masks are currently required for X-ray lithography, ion beam lithography, and some types of e-beam lithographies. Although membrane masks may in principle be used in almost all lithography techniques, they have generally been considered less desirable. One approach in a projection electron beam system (the so-called SCALPEL system) has been to reinforce the required membrane masks with “grillage” to increase their rigidity.
The type of “membrane mask” used in X-ray lithography comprises a membrane and an absorber. The membrane is a continuous sheet that is relatively transparent to the radiation used to expose a resist on a wafer. The main function of the membrane is to support the absorber. The absorber, which adheres to the membrane, is relatively opaque to the radiation. The absorber is patterned to correspond with the pattern desired in the exposed and developed resist on the wafer, and need not be continuous since it adheres to the membrane.
In other lithographies, other types of membrane masks have been used. For example, the membrane may be opaque to the radiation, except where holes are placed in the membrane (so-called “stencil” masks). Alternatively, the absorber may be replaced by a patterned layer that scatters incident radiation instead of absorbing it.
Generally, the industry has addressed the problem of distortion by trying to manufacture masks that are as accurate as possible, considering the ideal to be features positioned on orthogonal, perfectly linear axes. Much of the cost of mask-patterning tools lies in the references, metrology, and feedback used to enhance accurate image placement. However, this approach cannot accommodate changes in a mask that occur in processing steps subsequent to resist exposure, nor accommodate changes that occur as a mask ages, nor match a mask to distortions that may exist on the wafer being exposed.
A technique called “pattern-specific emulation” has been used to compensate for distortion in X-ray masks, such as distortion caused in etching the absorber. In this method a “send ahead” mask is first made, and is then used to expose a level on a wafer. Pattern displacements from the desired positions are noted, and are fed back to the mask-writing tool. A new mask is then written incorporating these displacements. Although time consuming and costly, this method did improve overlay. See, e.g., A. Fisher et al., “Pattern transfer on mask membranes,”
J. Vac. Sci. Technol. B
, vol. 16, pp. 3572-3576 (1998).
R. L. Engelstad and F. Cerrina of the University of Wisconsin have considered the displacement of features on a membrane mask by heated gas jets (private communication).
Magnification correction is considered one of the critical issues in lithography. In some lithographic techniques, magnification correction has been accomplished by an adjustment of the exposure tool. For example, in projection optical lithography it is routine to adjust the magnification by axial displacement of the reticle and refocusing in a projection system which is non-telecentric on the reticle side. Similarly, adjusting the gap in point-source X-ray lithography changes the magnification. Adjusting magnification is more difficult in storage-ring X-ray lithography.
It has also been proposed to correct for magnification errors by expanding or contracting either the mask or the wafer. Both mechanical and thermal means have been suggested for correcting magnification errors.
U.S. Pat. No. 5,155,749 discloses expansion of an X-ray membrane mask by heating a support ring to facilitate magnification matching between the mask and the wafer.
A method has been proposed to correct magnification errors by preheating the wafer, and then vacuum-chucking it so that its size is “frozen in” by the chucking force when the wafer is cooled back down. See H. Aoyama et al., “Magnification correction by changing wafer temperature in proximity X-ray lithography,”
J. Vac. Sci. Technol. B
, vol. 17, pp. 3411-3414 (1999).
U.S. Pat. No. 5,504,793 discloses a method for applying torque to an X-ray mask at severallocations around its edge with mechanical actuators, to stretch or compress the mask membrane to provide magnification correction.
It has been proposed to mount the wafer on a spherical vacuum chuck, and to adjust the size of the front surface by changing the radius of the chuck via application of an internal pressure or vacuum. See M. Feldman et al., “Wafer chuck for magnification correction in X-ray lithography,”
J. Vac. Sci. Technol. B
, vol. 16, pp. 3476-3479 (1998).
A novel method has been discovered, called the adaptive membrane mask technique, for locally heating the membrane in a lithographic mask to compensate for distortion in the mask, the wafer, or both. This technique may be used both to shrink and to expand areas of the mask, in order to adjust for varying magnitudes and signs of distortion. The adaptive membrane mask represents a major change in philosophy, from increasingly costly and difficult “dead reckoning” methods to one using feedback.
In one embodiment, the correction method comprises two steps: (1) A send-ahead wafer is exposed and measured by conventional means to determine the overlay errors at several points throughout the field. The errors may be the result of distortion in the mask, in the wafer, or both. (2) During exposure of subsequent wafers, calibrated beams of light are focused on the mask. The source of light may be, for example, a modulated laser beam, a halogen lamp, a capillary lamp, or a cathode ray tube. The light could have wavelengths from infrared to visible to ultraviolet. The light may be scanned across the mask, or projected on the mask directly or through a transparency or liquid crystal array, or produced by another projection system. The heating from the absorbed light produces displacements that compensate for the overlay errors measured with the send-ahead wafer. While heating a portion of a mask causes it to expand, a portion of a mask may also effectively be shrunk by heating the areas surrounding it.
In some circumstances, an alternative embodiment may be preferred. There is a delay inherent in the
Feldman Martin
Lim Michael H.
Murooka Ken-Ichi
Smith Henry I.
Adams Russell
Board of Supervisors of Louisiana State University and Agricultu
Brown Khaled
Runnels John H.
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