Enabling next-generation ICs with photonics tools
02/01/2004
Reduction in feature size drives a reduction in the source wavelength as well as an increase in the required illumination uniformity and overall source stability, tighter overlay conditions, and more critical inspection stations. To keep up with next-generation chip manufacturing, the complexity of photolithography, inspection, and metrology tools is continually increasing, and semiconductor-equipment manufacturers must look to incorporate photonics innovations — including next-generation lasers, motion-control systems, and optical-delivery systems — as they concentrate on system integration.
Because motion-control applications such as mask and wafer alignment, laser beam steering, and active stabilization of pointing instability are "set and hold" (where first the position is optimized and then maintained for a period of time), they call for high resolution and excellent long-term stability or high stiffness. These applications also require exceptional cleanliness to eliminate contamination of the optics, wafer, and mask. A cleanliness benchmark at which the level of outgassing of organics and nonorganics is compatible with the UV-beam environment is vacuum compatibility to 10-9torr, although in many cases vacuum compatibility to 10-6torr is adequate.
The Picomotor actuator [1], a compact "stick-slip" motor, was developed specifically for photonics applications that require high stiffness and high resolution (<30nm). It can also be manufactured to be vacuum-compatible (i.e., UV-compatible) and nonmagnetic, so it can be used in electron-beam (e-beam) environments (see "Stick-slip motor design" for details).
One area in which photonics tools have had tremendous success is in excimer beam delivery for photolithography and best-of-breed wafer metrology applications, including thin-film measurement systems, patterned-wafer inspection systems, and advanced reticle contamination-inspection systems. Automated beam-delivery systems with photonics tools and optomechanical assemblies have been used to counteract laser drift in these metrology applications, eliminating pointing instabilities and resulting in significant improvements in the overall system accuracy.
Figure 1 shows a system used to continuously stabilize a UV laser beam's location and pointing in an advanced reticle contamination-inspection station. In this configuration, the beam leaves the laser and goes through two motorized mirror mounts prior to reaching the worksite. The beam pointing is held constant by adjusting two mirror mounts, M1 and M2. Points P1 and P2 are imaged respectively on quadrant detectors, QD1 and QD2. The detected signals in turn are used to servo the beam's location and pointing. By keeping P1 sufficiently close to M2 and far from P2, the sampled legs 1 and 2 can be treated independently.
The autostabilization system, along with the optical subassemblies in the system, must provide very high-resolution motion for accurate alignment to eliminate any extraneous movements of the UV beam, and high-quality mirror surfaces to ensure the highest beam quality; the optomechanical modules have to be optically very flat. Flatness tolerances as critical as λ/28 (where λ = 633nm, or about 23nm) for 1 in. mirrors and l/20 (about 32nm) over 2.5 in.-dia. mirrors have to be maintained in the assemblies even after mounting. Conventional mounting methods, such as gluing or pressure mounting, introduce unwanted tension within the optics that results in unacceptable distortion. A custom bonding process was developed to maintain these tight flatness tolerances even after mounting. This process satisfies the exceptional clean requirement and does not induce optical distortion after curing.
With feature size ≤100nm, overlay (typically around 10% of feature size) is increasing in difficulty and for some time has relied on nonmechanical means. Today, phase-grating alignment systems with coherent sources are used in lithographic systems because they enable nanometer repeatability, and thus, good overlay for advanced IC processes [2, 3]. Future advances will require even more precise tools. Beam conditioning that affects the geometrical shape as well as the wavelength, phase, polarization, intensity, and linewidth (or coherence) of the source will become critical.
Phase modulation in the radio-frequency (RF) band helps eliminate unwanted interference effects caused by spurious reflections in phase-grating alignment systems. Other methods, such as state-of-the-art antireflection coatings or polarization filtering techniques, do not reduce the interference effects in these high-precision alignment systems, and incoherent sources cannot be used because they require too much output power and have insufficient wavelength stability. Phase modulators reduce the unwanted interference effects in phase-grating alignment systems in a controlled manner, significantly improving the intrinsic accuracy and repeatability of phase-grating alignment systems.
In Fig. 2, phase modulation at carefully selected radio frequencies eliminates interference effects (middle of the graph). The modulating signal consists of only three sinusoidal RF modulation signals. Careful selection of the phase-modulator radio frequencies selectively eliminates unwanted etalons in complex photolithographic optical systems.
As feature-size reduction drives semiconductor-equipment manufacturers to next-generation technology nodes and the cost of new equipment skyrockets, photonics products and their technologies will play important roles in extending the lifetime of the installed equipment base.
Acknowledgment
Picomotor is a trademark of New Focus.
References
1. US patent #5,410,206.
2. S. Wittekoek, M. van den Brink, H. Linders, J. Stoeldraijer, J.W.D. Martens, et al., "Deep UV Wafer Stepper with Through the Lens Alignment," Proc. SPIE, Vol.1264, pp. 534–547, 1990.
3. J.H. Neijzen, R. Morton, P. Dirksen, H. Megens, F. Bornebroek, "Improved Wafer Stepper Alignment Performance Using an Enhanced Phase Grating Alignment System," Proc. SPIE, Vol. 3677, 1999.
Kathy Li Dessau received an AB in physics from Princeton U. and a PhD in applied physics from Stanford U. She is now a product manager at New Focus.
Aaron Van Pelt received a BSc in physics from the U. of Wyoming-Laramie and a MSc in physics from Washington State U.-Pullman. He is now product marketing manager of OEM Tools at New Focus.
Milan Zeman received a BS in chemistry and an MS in imaging sciences from the Rochester Institute of Technology. He is now product marketing manager of Photonics Tools at New Focus Inc., 2584 Junction Ave., San Jose, CA 95134; ph 408/919-6022, fax 408/980-8883, e-mail [email protected].
Stick-slip motor design
The stick-slip motor design uses a piezo element positioned between two jaws to provide the torque to turn the screw just like a person turns a screw (see figure). Two jaws grasp an 80-pitch screw; one jaw is connected to one end of a piezoelectric transducer, and the other jaw is connected to the other end of the transducer. A slowly rising electrical signal applied to the piezo gradually changes its length, causing the two jaws to move in directions opposite and tangential to the screw, just as a thumb and forefinger do as a person turns a screw. This slow motion makes the screw turn by static friction.
 Schematic of the action of a stick-slip motor that uses piezos to turn a screw. |
At the end of the transducer motion, a fast-rising electrical signal quickly returns the jaws to their starting positions. Because of the screw's inertia and low dynamic friction, it remains motionless, holding its position. Simply reversing the order of the fast and slow pulses reverses the direction of the motor. Because the piezo is used only to turn the screw and not to hold the adjusted position, the mechanical stability of the motor is identical to a nonmotorized screw and nut set.