White vibration filtering for equipment and facilities
06/01/1998
White vibration filtering for equipment and facilities
Paul Attaway, Zoltan Kemeny, Vistek Inc., Tempe, Arizona
Virtually every facet of the semiconductor industry pays a price (in investment and revenue) for vibrations, which increase the cost of wafer fab construction and impede improvements in lithography, metrology, and microscopy. In addition, vibrations inherent in all wafer and disc polishing/grinding can degrade processes and damage wafers. This article introduces a new technology to solve these problems - white vibration filtering (WVF).
WVF technology allows passive, mechanical vibration isolation and suppression at levels barely attainable using active vibration cancellation systems or air isolation bearing systems. The technology is called white vibration filtering because it filters out (or cuts off) a broad band of input acceleration frequencies at a constant level (Fig. 1).
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Figure 1. White vibration filtering
The maximum level of vibrations that pass through the filters is predominantly constant, or "white," regardless of the frequency of the input vibrations. A vibration suppression device (VSD) and a vibration isolation bearing (VIB) currently embody WVF technology.
VSDs enhance the performance of tools for chemical mechanical polishing (CMP), silicon slicing, and silicon wafer and disk polishing/grinding by functioning as passive, mechanical broadband, white-tuned mass dampers (TMDs). Tests conducted on CMP tools demonstrate promising results.
VIBs are 1/6th Hz isolators, i.e., isolation begins for vibrations with frequencies above 1/6 Hz. They filter out 95% of all vibrations at 1 Hz, and more than 99% of vibrations in 6 degrees of freedom at >2 Hz. A VIB may be sized for payloads ranging from 12 to more than 5000 lbs. This technology is applicable to lithography, metrology, microscopy, and wafer fab construction; a vibration isolation platform incorporating the VIB is now available for tabletop microscopes and other sensitive inspection and measuring equipment.
Equipment design improvements with VSDs
Many semiconductor process tools create undesirable and unavoidable vibrations. The vibration sources within a tool include fans, vacuum pumps, linear and multiaxis motion control and drive systems, and the friction and occasional hydrodynamic chattering created by CMP processes.
Vibrations have at least three side effects. First, tool frame vibrational instability (or worse, resonance) can hinder the entire process, either shutting down or slowing down the tool. Second, if the natural frequency of the vibration source or the tool frame is close to the natural frequency of the wafer, then the wafer becomes unstable and may begin rattling or moving [1]. Finally, vibrations lead to a hidden cost: tools operate at less than peak efficiency to avoid dangerous vibrations.
Traditional tool design approaches. Manufacturers spend valuable time and resources to solve vibration problems. Current techniques include finite element analysis, "trial and error," or more typically, a combination of both techniques, to design a tool with a natural frequency outside the range of expected natural vibration frequencies. Vibration-free tool design is difficult, since many tools, especially CMP and polishing/grinding tools, have an almost infinite number of potential vibration frequencies. Thus, it is very difficult to predict whether a tool`s natural frequency will fall outside the range of potential vibration source frequencies. The natural frequency of the tool may also be different in the plant than at the factory, especially with CMP tools where the process itself influences the natural frequency. Similarly, the frequency of the vibration source may interact with an external vibration source at the plant, creating a unique, unanticipated, beating phenomenon.
Simplifying tool design. The VSD is a mechanical, passive, broadband TMD that "tunes" the natural vibration frequency of a structure (a semiconductor tool in this instance) away from the vibrating frequency of the vibration source. A TMD consists of a mass and a spring. When located strategically in a tool (which may also be defined as a "mass-spring" system), the vibrational energy of the tool is immediately transferred to the TMD at startup. The mass portion of the TMD then oscillates and serves as an energy accumulator, i.e., a temporary energy sink. The energy is conserved in the VSD and is not dissipated throughout the tool frame. When a process is complete and the vibration source is shut off, the energy stored in the TMD is then transferred back to the tool frame. This energy transfer has a negligible effect on the tool since the mass portion of the TMD is only 0.5-5.0% of the weight of the vibrating portion of the tool, where Energy = (mass ? acceleration) ? displacement. Since the mass of the TMD is small, then the total energy transferred back to the tool frame is small as well.
A conventional passive TMD can "tune off" a structure from only a single modal frequency, because a TMD spring has a natural frequency due to its linear elasticity. Therefore, unless one knows a vibration source`s natural frequency, and knows that it will not change, this type of TMD is of little use.
A WVF VSD is the ideal TMD for semiconductor tools because the spring portion of the device has no natural frequency; rather, it has an infinite number of frequencies in a broad frequency band and none are natural to the system. A natural frequency is one at which a device or body, due to its material properties and its geometry, tends to oscillate once forced. Without a natural frequency, the mass portion of the TMD will always oscillate off-phase from the frequency of the vibration source, canceling the source vibrations.
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Figure 2. The VSD mechanism.
The VSD mechanism (Fig. 2) is mathematically described as follows:
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where
m0= the rolling mass of the polishing head
m1= the first degree of freedom-tuned mass
m2= the second degree of freedom-tuned mass
G1= the two-way horizontal guide (rotation allowed)
G2= the two-way horizontal guide (rotation prevented)
k1= the white filtering gravity spring (US Pat. No. 5,599,106)
k2= similar to k1 (pat. pend.)
k3, k4= the linear elastic springs
P3= prestressing force
P4= prestressing force
x= the transversal direction
y= the longitudinal direction = the rolling direction
z= the vertical direction = the rocking direction
The positive and the negative branches of the white segments represented by signum (sgn) functions are separated by linear, elastic, short segments that are not shown. P3 and P4 represent calibrated, preset forces.
While sound design principles should not be abandoned, adding a VSD assures the engineer that tool vibrational energy is swept
into the VSD. The tool will not vibrate excessively or reach a state of resonance, and it is not necessary to add mass and/or rigidity continually to combat vibrations.
Controlling wafer process with VSDs. Wafer instability occurs when the vibration frequency of the tool is close (or equal) to the natural frequency of the wafer. When a wafer becomes unstable, it can move or rattle [1]. Rattling wafers are more easily damaged or dropped by automatic transfer subsystems. High-speed, precision processes are more difficult to perform on a vibrating wafer, even if the wafer has not reached a point of instability or resonance.
Not all 200-mm tools will move or rattle a wafer. Those experiencing wafer instability can be easily retrofitted with VSDs bolted to the tool frame. The occurrence of wafer instability in 300-mm tools, however, will probably be higher, since the natural frequency of 300-mm wafers is closer to that of most semiconductor tools [1]. Including a VSD in the early stages of tool design allows a designer to locate the VSD closer to the vibration source to maximize its effectiveness and minimize its size.
Enhancing the CMP process. Our studies characterized the vibrations on a CMP tool performing several different processes, and analyzed the effect on the CMP process of VSDs located on the tool frame. Complete details of the CMP process and the nature of the yield enhancements are not available, since the work is proprietary for the company(s) providing the CMP tool(s). Therefore, specific details are not presented. We recorded tool vibrations with and without four VSDs installed on the tool. In every instance, the VSDs reduced vibrations and gave tool operators greater process control. The study was carried out in two phases. (A third phase is currently in progress.)
During Phase 1, our goals were:
to gain an understanding of the vibrations created by the CMP process, and
to verify that the VSDs would cancel out the vibrations in the predicted manner (see "Specifics of VSD vibration cancellation").
The vibrations we detected were either plain or composites of one or more of the following vibration types: a) random, b) quasiharmonic, c) pulse, d) pulse train, and e) wavelet. The sources of type b were the gear box, drives, pump, and transformer, i.e., the sources were all machine related. The sources of type d were hydrodynamic chattering due to wafer (geometry, stiffness), polishing pad (softness, thickness), and slurry properties, i.e., the sources were all process related.
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Figure 3. A typical process in which a type a (random) vibration is suddenly replaced by a type d (pulse train) vibration. Grinding with pulse train kick-in and sustained pulse train (high imbalance with wafer losses).
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Figure 4. An expanded view of a type d (pulse train) vibration.
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Figure 5. Pulse train cancellations with VSDs by conversion to wavelet. Zoom into the rare (large) wavelet (impulse of negligible energy content). Damping ratio = 37%.
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Figure 6. The VSD degrades high-amplitude random vibration to background noise
Figure 3 illustrates a typical process in which a type a vibration is suddenly replaced (not superimposed) by a type d vibration. Figure 4 shows an expanded view of a few pulses of a type d vibration. The VSD converted an entire segment of a long pulse train with high energy to a wavelet with negligible energy and the same amplitude (Fig. 5), while it degraded an otherwise high-amplitude random vibration to background level (Fig. 6). Note that the amplitude scale of Figs. 3-6 remains the same. The equivalent damping ratio (37% in Fig. 5) was calculated based on the logarithmic amplitude decrement.
We concluded that it may not be possible or practical to design a CMP tool with a natural frequency outside the potential frequency range of the CMP-induced vibrations. However, the VSDs located on the tool produced excellent and promising results.
During Phase 2, our goal was to determine if a more aggressive CMP process could be used without placing the tool into vibrational resonance. We tested a CMP process that delivered a high rate of removal with excellent wafer qualities. Unfortunately, the process caused tool vibrations at an unacceptable level and, occasionally, at resonance. Vibrations were measured on the tool performing this process, with and without the VSDs. Once again, the VSDs cancelled the vibrations as they had during Phase 1. More importantly, the operator successfully performed the process unimpaired by vibrations, thus improving the wafer quality.
We can conclude at this point that the aggressive CMP process produces tremendous vibrations that limit the window of possible process variations. There are probably high-removal-rate CMP processes that cannot be used because of the vigorous machine vibrations involved. VSDs can cancel out these vibrations, and therefore broaden the processing window available to the operator.
Improvements in inspection, lithography, and wafer fab construction
Vibration is a dirty word in the semiconductor industry. Critical process tools are approximately 100 ? more sensitive to vibrations than people are [2]. Yield improvements in steppers, microscopes (SEM, FIB, AFM, etc.), and inspection tools (in general) are limited, in part, by the presence of vibrations. Vibration sources include foot traffic, process equipment on the fab floor, parts within a tool such as pumps, fans, reciprocating motion control systems, as well as other pumps and mechanical systems on the fab floor and within the building. VIBs may be used to isolate each of these vibration sources and/or to isolate the sensitive operations of a tool.
A VIB uses two filters. Vibrations in the horizontal plane are cut off with a nonlinear spring such as a Ball-N-Cone (BNC) filter (see "Control of seismic activity, vibration, and contamination: Are these cleanroom design goals mutually exclusive?"), which physically limits vibration transmissions regardless of the input frequency. Relative displacements between the bottom half and the top half of the BNC filter occur, with the top half remaining quasistationary. A nonlinear spring ensures a constant restoring force to counterbalance the displacements.
A near-zero stiffness, nonlinear-elastic spring mechanism filters vibrations in the vertical plane. This mechanism, while supporting the vertical load with high secant stiffness, effectively levitates the isolated structure by providing small positive tangent stiffness balanced with equal and opposite negative stiffness. A structure has the property of negative stiffness if its deformation increases while its loading decreases, and it has the property of positive stiffness if its deformation increases as the load increases.
Structures with negative stiffness are unstable, whereas positive stiffness structures are stable. If a structure or mechanism has zero-stiffness characteristics, then the positive and negative properties are balanced, and the structure is stable.
VIB isolation begins at approximately 0.2 Hz and in 6 degrees of freedom. The device is mechanical, passive, and outperforms air isolation and active vibration cancellation systems. Figure 7 shows that the VIB filters out a higher percentage of vibrations at every frequency.
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Figure 7. Transmission curve for the Vistek 1/6th Hz VIB; a) air table, b) active, c) Nano-K, and d) Vistek VIB
Studies showed that a lightweight, tabletop platform <1 in. tall was as effective as current air isolation tables to solve vibration problems for microscope users [3]. A simple VIP can provide the high resolution and fast settling time required for state-of-the-art microscopes and other optical-based inspection systems.
A VIB may also be used to simplify and enhance stepper, metrology tool, and other tool designs. Historically, a tool designer could
1. design a rigid, massive tool, 2. use air isolation bearings with supporting pneumatic controls, or 3. use expensive piezoelectric, active vibration cancellation systems.
Since the VIB is a passive, mechanical device requiring no pneumatic or electronic supporting hardware, it simplifies the design significantly. Furthermore, the VIB is fully metallic, maintenance free, smaller, and more effective than all other isolation solutions. For example, a VIB with a vertical load-carrying capacity of 250 lbs is approximately 1.75 in. in dia. ? 1.75 in. tall; a 5000-lb VIB is approximately 4 in. in dia. ? 2 in. tall.
The final sector of the semiconductor industry that can realize savings with WVF technology is the wafer fab construction industry. Vibrations are taken into consideration at every step of fab construction. Buildings are designed as rigid, massive structures, so they will not transfer vibrations readily throughout. In addition, tools sensitive to vibrations and tools that create vibrations are often placed on large steel and concrete pedestals to isolate them from each other.
VIBs can be located directly under a tool, either as the platform or as the bearing feet of the tool. The VIBs will filter the passage of vibrations either from the floor into the tool or from the tool into the floor. Consequently, the large concrete or steel pedestals are no longer necessary. The tools may be located directly on a concrete floor slab or a raised access floor, and freely located within the fab facility without concern for vibration problems.
Summary
WVF technology addresses two major issues in the semiconductor industry today - yield and cost. A tool equipped with a VSD tunes the tool off from the process vibrations and minimizes vibration-dependent yield loss. Wafer fab construction costs decrease when tools are individually isolated with tool stands or tool legs, without the requirement of 30-40 in. concrete floor slabs to reduce the vibration transmissibility throughout the fab. Since the technology is passive and mechanical, there are no reliability concerns, such as those commonly associated with air isolation or active electronic vibration cancellation systems.n
Acknowledgment
Ball-N-Cone is a registered trademark of Tekton Inc. Vistek is a trademark of Vistek Inc.
References
1. E. Marsh et al., "Identification and Abatement of Silicon Wafer Vibration in Semiconductor Process Tools," Future Fab Intl., Vol. 1, No. 3, pp. 255-259.
2. C. Gordon, "Vibration Control in Microelectronics Cleanrooms," CleanRooms `96 West Proceedings.
3. Personal communication, R. Bender, president/founder of Bender Associates Inc., Tempe, AZ.
PAUL ATTAWAY received his BSBA degree from Georgetown University School of Business Administration, and his JD from the University of Georgia Law School. He is the president of Vistek Inc. 1565 W. University Drive, Suite 101, Tempe, AZ 85281; ph 602/303-9888, fax 602/303-9096.
ZOLTAN KEMENY received his BS degree in construction engineering from M.P. Construction College, Hungary, in 1968; and his MS degrees in structural engineering and architecture from the Technical University of Budapest, Hungary, in 1970 and 1974, respectively. He is the VP and chief engineer of Vistek Inc.