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Adopting semiconductor metrology to meet the challenges of MEMS manufacturing

Roger A. McKay Jr., Susan Redmond, and Ron Weller, Hewlett-Packard; Kuni Yamamoto, Paul Knutrud, and John Podlesny, Nanometrics; and Ganesh Sundaram, formerly of Soluris

MEMS manufacturers face the unique challenge of extending semiconductor manufacturing technologies to the fabrication of 3-D mechanical devices. For example, two of the most commercially important MEMS applications, accelerometers and ink-jet printheads, are typically made using processes derived from the semiconductor manufacturing area. Processing costs for the relatively small MEMS industry would be prohibitively expensive if manufacturers were forced to employ custom toolsets made from scratch. However, although MEMS processing technologies are often based on semiconductor technologies, ICs and MEMS devices bear very little similarity to each other. Hence, a key ingredient in the successful adaptation of semiconductor toolsets to the MEMS area is communication and cooperation between IC equipment vendors and MEMS tool owners.

This article describes how an optical metrology tool that was originally designed to measure critical dimensions (CDs) and pattern alignment in semiconductor devices was adapted to the unique requirements of ink-jet MEMS manufacturing at the Hewlett-Packard facility in Corvallis, OR. The article addresses several challenges facing ink-jet MEMS manufacturers, including varying wafer thicknesses, wafer deformation during fabrication, the etching of channels and holes, and the measurement of vertical CDs and custom feature shapes.

Ink-Jets and the Ink-Jet Manufacturing Process

All ink-jet printers use one of three ink-jet technologies—continuous, piezoelectric, or thermal. Invented in the 1960s, continuous-jet technology is the oldest. In continuous ink-jets, a pump feeds ink to the printhead, which generates a continuous train of droplets. The printhead selectively charges droplets as they pass though the nozzle and imposes an electric field across the droplets’ path to the paper. The field then deflects the charged droplets into a return collector, and uncharged droplets continue undeflected to the paper. Continuous ink-jets resist clogging, since the jet is continuously in use. However, they require complex ink-supply and return mechanisms. Hence, they are used primarily in high-end commercial printers.

Both piezoelectric and thermal ink-jets (TIJs) are known as drop-on-demand technologies. In piezoelectric ink-jets, a piezoelectric actuator deforms under an applied electric field to propel a droplet from the nozzle. Piezoelectric ink-jets control droplet size precisely but are somewhat expensive to manufacture.

In TIJs, a small resistor (heating element) rapidly heats the water-based ink, causing the formation of a tiny steam bubble. The bubble’s expansion propels the ink droplet from the nozzle. The subsequent collapse of the bubble draws new ink into the nozzle chamber from the supply system. The TIJ mechanism is shown in Figures 1 and 2. TIJs, which are used in most desktop printers, are reliable and inexpensive. Like chip manufacturing, the production of TIJ printheads is a high-volume process that must be controlled carefully to yield the largest number of functional devices possible.

Figure 1: Diagrams of (a) a printhead containing an array of ink-jets, and (b) a closeup of an ink-jet. Each ink-jet includes underlying control circuitry, a heating element, an ink chamber, and a nozzle.

The TIJ manufacturing process incorporates ICs on a silicon substrate with an overlying thermal printhead. The manufacturing process must integrate conventional CMOS IC processes with other process steps that are unique to the ink-jet device.

Figure 2: Optical image showing the ink chamber, supply channels, and heating element before the formation of the nozzle structure.

The initial steps create the circuit on the silicon substrate. Like all IC processes, this phase builds the circuit as a layered structure, with each layer requiring several iterations of material deposition, photolithography, etch, implant, and other process steps. Each step must be controlled carefully to ensure the reliability and performance of the completed circuit.

Once the circuitry is complete, processing continues with the fabrication of the resistors and directional channels that feed ink through the substrate to the firing chamber and ultimately out through openings in the front side of the wafer. Finally, the finished components are attached to a pen body. The full process includes the manufacture of several protective layers that safeguard the components from unwanted interactions and the damaging effects of the chemicals to which they will be exposed.

Adapting IC Metrology to the MEMS Area

The complexity of the TIJ manufacturing process requires continuous control and feedback at every step. Although metrology systems are readily available for the IC portions of the process, later steps use materials and create structures that often challenge the capabilities of even the most flexible tools. Yet manufacturers have significant incentives to extend the use of IC manufacturing tools to all phases of the TIJ process: Doing so would eliminate the expense of developing or acquiring new tools, save space on the fab floor, and leverage existing operator skill sets and support capabilities.

Optical tools have long been the principal metrology technology for performing CD measurements and overlay registration in the semiconductor industry. Since the IVS series of optical metrology tools from Nanometrics (formerly Soluris) in Concord, MA, had demonstrated excellent precision and reliability in IC applications at HP, the facility was strongly motivated to extend their applicability throughout the TIJ process. However, several hurdles had to be overcome.

For example, semiconductor metrology has historically focused on measurements in the x-y plane. Although these measurements remain critical in MEMS metrology, the TIJ process places increased emphasis on measuring 3-D structures as well. Other challenges included handling wafers with large ink channels and through-holes and accommodating the different wafer shapes that result from certain process steps.

Overlay and CD Metrology

Although the extreme topography of TIJ wafers requires that users pay special attention to focus optimization, IVS metrology’s focusing, overlay, and CD measurement algorithms work well on TIJ structures. The focus optimization algorithm determines the best focus by scanning a range of focus values while seeking to maximize image contrast and accommodating extended z-range requirements, as illustrated in Figure 3. The system’s overlay measurement routines, as shown in Figure 4, accommodate a range of standard test structures, such as box-in-box and CD structures, ensuring data commonality across both IC and TIJ processes. As depicted in Figure 5, the CD algorithm locates a feature’s edge at the point of maximum gradient in the signal intensity profile, providing good repeatability on high-relief structures.

Figure 3: Images from the metrology tool with three different z-axis degrees of focus. The system focuses automatically by seeking maximum image contrast. The wafer deformation and extreme topography of ink-jet devices require that special attention be paid to focusing routines.
Figure 4: Images showing (a) a box-in-box overlay test structure, (b) another box-in-box overlay test structure, and (c) a CD test structure. The metrology tool’s measurement algorithms are designed to work with standard semiconductor test structures, preserving data-type commonality across IC and TIJ processes.
Figure 5: The CD measurement algorithm locates the edge of the structure at the point of maximum gradient on a transverse signal intensity profile.

CD-Z. Measurements in the z direction are critical in the TIJ process. Figure 6 illustrates how these measurements can be accomplished by determining the difference between best focus values for the top and bottom of the measured structure. Although the focus position may be varied in increments as small as 25 nm, the real resolution of z measurements is determined by the depth of focus of the objective lens. The accuracy of z measurements may be established through calibration. Step height can be calibrated using readily available standards or a “golden” wafer functioning as an internal standard.

Figure 6: Measurements in the z direction are calculated as the difference between the best focus planes of the features.

Nozzle Measurements. Most measurement algorithms that are designed for semiconductor applications presume that feature shapes are rectilinear. However, TIJs often include round or oval features. An example of such a feature is the nozzle opening, whose dimensions are absolutely critical to the TIJ’s performance. In collaboration with the metrology systems’ manufacturer, HP users were able to develop a specialized algorithm to measure round and oval shapes, examples of which are presented in Figure 7. This algorithm reports the major axis, the minor axis, and the ovality of the measured structure.

Figure 7: Images showing (a) an in-circuit top opening and (b) and in-circuit bottom opening. Unlike ICs, ink-jet devices typically contain round or oval features, for which specialized measurement routines are necessary to perform size and shape control.

Automated Wafer Handling

Perhaps the most basic issue facing the use of the metrology system was whether it could handle TIJ wafers reliably. Among other features, these wafers contain relatively large slots on the backside and holes that penetrate through the wafer. Because IC tools’ automated wafer-handling systems use vacuum to secure the wafer to the handler, the presence of slots and holes made it difficult for the metrology system’s handlers to achieve a secure vacuum seal, which affected wafer handling on the prealigner, the robot end-effector, and the measurement chuck. Furthermore, the absence of a tight vacuum seal interfered with the tool’s ability to detect the presence of the wafer, which is usually performed by monitoring the vacuum level. Consequently, the HP team, in collaboration with the vendor, made a range of modifications.

To correct an inadequate seal in the prealigner that caused wafer sensing errors:

• A larger, dedicated vacuum line was installed between the source and the prealigner, increasing capacity.

• The prealigner chuck was redesigned to incorporate smaller holes and a different hole pattern to reduce vacuum leakage and increase holding power.

• The vacuum threshold in the wafer-sensing mechanism was adjusted.

To eliminate leakage in the measurement chuck that caused wafer sensing errors, provided inadequate clamping, and allowed wafer debris to enter into the vacuum system:

• The vacuum sensor was replaced with an optical sensor that interfaced with the wafer sense circuit.

• The chuck was redesigned to apply vacuum only at the wafer periphery, where slots are not present.

• An in-line filter was installed between the chuck and the vacuum control valve.

To repair an inadequate vacuum seal in the robot that increased the risk of the wafer falling off the end-effector:

• High-friction material was installed on the end-effector surface.

• Robot acceleration and velocity parameters were reduced.

Handling Wafers with Unusual Characteristics

Wafer Thickness. TIJ wafers may be significantly thinner than wafers used in standard IC processes, causing the wafer surface to exceed a metrology system’s focusing range. Hence, the IVS system was recalibrated to accommodate thinner wafers, and the new calibration data were saved in a separate file. Start-up batch files were then written to provide users with easy access to the appropriate calibration data depending on the thickness of the wafer being measured.

Wafer Bow. Advanced semiconductor manufacturing processes use very flat wafers and include planarization processes to reduce surface topography to the very low levels required by advanced lithography. These practices permit the use of optical metrology tools that have high-resolution objectives with short working distances and very shallow depths of field.
In contrast, the TIJ process includes a die-attach step in which a heated metal plate is applied to the top surface of a slotted wafer under high pressure, which can induce wafer bow greater than several hundred microns from center to edge. This effect can cause the height of the wafer surface to exceed the focusing range of the optical system. The problem was solved by increasing focusing activity at the center and edge of the wafer to ensure adequate localized focus. Although resolution remains important in TIJ applications, the excessive bow induced by the process requires the use of objectives with longer working distances to ensure adequate clearance.

Conclusion

Two of the most important goals of HP’s measurement and process control activities are to improve yields in high-volume production and reduce the length of the new-process development cycle. Each of these goals, in turn, is supported by the addition of new measurement capability, allowing the control of process parameters that were previously uncontrolled, or by a reduction in feedback time, providing quicker response to yield excursions and faster process ramps. Of course, the value of these benefits must always be weighed against the cost of achieving them. Extending the capabilities of existing tools to realize these benefits is a compelling strategy, since it leverages both sides of the cost-benefit equation.

In the case described in this article, HP was able to use a system to perform TIJ processes that had originally been designed for IC metrology processes, adding new measurement capability and shortening feedback cycles. The investigators transitioned from manual wafer handling to a significantly faster and more reliable automated process. They implemented linked recipe templates to achieve more than a threefold improvement in cycle time, developed the ability to perform measurements in the z-axis, and succeeded in measuring multiple features on both thin and standard wafers with minimal operator intervention.

The system modifications discussed here extended the metrology tool’s capabilities without compromising its performance in other areas. Because the company avoided having to acquire, qualify, and train new personnel to support the tool, gains were achieved at a relatively low cost.


Roger A. McKay Jr. works in metrology engineering with the imaging and printing group of Hewlett-Packard in Corvallis, OR, where he has been since 1979. He received an AS engineering transfer degree from Linn-Benton Community College in Albany, OR. (McKay can be reached at 541/715-8437 or roger.mckay@hp.com.)

Susan Redmond focuses on metrology engineering in the imaging and printing group at Hewlett-Packard. She began her tenure with the company in 1979 in the x-ray lithography department and has been in the imaging and printing group for the last seven years. (Redmond can be reached at 541/715-1779, e-mail susan.redmond@hp.com.)

Ron Weller is a metrology technician at Hewlett-Packard. Since 1984, he has held positions at the company as a technician in the areas of dry etch, implant, and deposited films. He received a BS in electronics technology from Walla Walla College in College Place, WA. (Weller can be reached at 541/715-8845 or ron.weller@hp.com.)

Kuni Yamamoto is a senior field service engineer for Nanometrics (formerly Soluris). He services optical and CD-SEM systems. Previously, he was an engineering specialist at Agilent, where he operated an E-beam lithography system in the area of microwave devices. For 19 years, he was a research assistant in a nuclear energy and a high-energy laser laboratory. He was also involved in electronic/engineering technology for large vacuum coating and aircraft navigation systems. (Yamamoto can be reached at 541/513-3456 or kyamamoto@soluris.com.)

Paul Knutrud is technical marketing manager at Nanometrics. He began his career as a sales engineer at IVS, where he became an applications engineer and manager for the optical CD, overlay, and CD-SEM product lines. Subsequently, he was director of marketing at Schlumberger ATE. Knutrud holds a patent on multilayer overlay metrology and received a BSBA degree from Babson College in Babson Park, MA. (Knutrud can be reached at 978/318-4041 or pknutrud@soluris.com.)

John Podlesny is applications group leader at Nanometrics. In 1996, he began working at IVS, where he was responsible for worldwide applications technical support and customer training. He has also worked at Motorola as a senior metrology engineer and at Veeco as an applications engineer. He received a BS in chemical engineering from the University of New Mexico in Albuquerque and an MS in materials science and engineering from the University of Arizona in Tucson. (Podlesny can be reached at 480/755-9876 or jpodlesny@soluris.com.)

Ganesh Sundaram, PhD, was a CD-SEM product manager at Soluris. Before joining the company, he was applications manager at Micrion, where he specialized in focused ion beam technology. Before that, Sundaram held positions at the central research laboratories of Texas Instruments, the UK Science and Engineering Councils, and the Rutherford-Appleton Laboratory. The recipient of the Lise Meitner prize in physics from the Austrian Science Foundation, he received a PhD in solid-state physics from Oxford University in the UK. (Sundaram can be reached at gsundaram12000@yahoo.com.)


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