The advantages of using additive micromanufacturing in the fabrication of MEMS wafers and sensors

January 23, 2020

11:01 AM

Doug Sparks, president, 3D Printed Wafers

After nearly 40 years of development, additive manufacturing (AM), also known as 3D printing, is finding increasing applications in virtually all fields. A wide variety of materials, reduced system part counts and faster prototyping times are the prime incentives for employing AM.

Aluminium, cobalt, copper, nickel, titanium and stainless steel alloys, tungsten, and low thermal coefficient of expansion (TCE) metals such as Kovar, molybdenum (Mo) and 17-4 stainless steel, which are a closer match to silicon, are used in AM. There are various established metal AM methods, such as: electron-beam melting (EBM) and direct energy deposition (DED), which use powder or wire; binder jetting (BJ) and selective laser melting (SLM), also known as direct metal laser sintering (DMLS), which use powder; and rapid plasma deposition (RPD), which uses wire. Newer AM methods include controlled electroplating and the printing of metal powder in a liquid resin suspension.

AM was used to produce the corrosion-resistant, stainless steel and titanium industrial sensor elements shown in figure 1. Sensor elements such as the diaphragm for a pressure sensor in figure 1a and resonant tubes for a Coriolis mass flowmeter sensor in figure 1b can be integrated with a significant percentage of the housing components, including threaded or flange fluidic interfaces^{1, 2}. Novel structures that cannot be machined, such as the buried, curved fluid channel shown in figure 1c, can also be additively manufactured. Furthermore, electrical elements such as dielectric and metal traces can be added onto these sensors using the screen printing and inkjet printing processes.

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Figure 1a

Additively manufactured industrial sensors in stainless steel and titanium: (a) a flanged flow sensor (top) and a diaphragm for a threaded pressure sensor (bottom); (b) resonant tubes for a Coriolis mass flowmeter sensor; and (c) a buried, curved fluid channel.

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Figure 1b

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Figure 1c

AM+MEMS

3D Printed Wafers, a subsidiary of M2N Technologies, has developed a patent-pending technology called AM+MEMS, which combines additively micromanufactured wafers and microelectromechanical system (MEMS) sensing circuits. The wafer is polished flat and thin-film layers are deposited on its surface to build the MEMS sensing circuits.

Additive micromanufacturing of single metal sensors is still a relatively expensive option and, in many cases, only cost competitive with CNC, casting and welded assembly for low-volume prototyping. To overcome this cost disadvantage, multiple sensors can be produced during the same print operation. This is the same commercialisation path that was taken for MEMS sensors decades ago. Additive micromanufacturing allows the engineer to move from a CAD file to a structured wafer without the cost and time of photomask fabrication. The AM+MEMS fabricated, 100 mm diameter titanium wafer shown in figure 2 can produce multiple small sensors and leverage wafer fabrication lithography tooling for building up surface circuitry layers. The wafer was printed using the DMLS method.

Figure 2

An AM+MEMS fabricated 100 mm diameter titanium MEMS wafer.

Post-processing of additively micromanufactured MEMS wafers

Most metal AM processes do not stop at printing. Challenges associated with AM processes such as cracks and warpage due to stress have to be overcome. For applications demanding high quality and reliability, including the micromachining of wafers, post-processing is required. Post-processing of additively manufactured metal parts can begin immediately after printing.

With both laser and electron-beam AM processes, the resulting part is very similar in state to a welded part, namely significant stress is found in the metal structure. As in the case of welding, annealing can reduce some of this built in stress, thus reducing the likelihood of warpage and cracking. Annealing, which often takes place in an inert atmosphere or vacuum, can be carried out before or after the part is removed from the build plate. Hot isostatic pressure is also applied to some additively manufactured aerospace parts to improve fatigue life. Trapped metal powder from a DMLS or SLM print may need to be shaken out of cavities.

Next, the relatively as-printed matte surface finish can be improved via machining or bead blasting. A metal MEMS wafer requires a polished planar surface. After the annealing and micromachining or bead blasting steps, the part is cleaned to remove any powder or residue.

Finally, metrology is important to ensure that the part meets quality specifications and is free of cracks and voids. Only once the relevant checks have been conducted can patterned films be applied to the surface of a metal MEMS wafer.

Metal additive micromanufacturing versus traditional silicon MEMS fabrication methods

A significant advantage of metal additive micromanufacturing of complex MEMS and microfluidic structures is that it can reduce the number of wafer processing steps required by traditional silicon MEMS fabrication methods. The most time-consuming of these methods are typically deep reactive ion etching (DRIE) and wafer-to-wafer bonding, which only allow one or two wafers to be processed at a time and can each take more than an hour. Furthermore, DRIE is somewhat limited in etching direction, specifically straight down or at a slightly fixed angle into the silicon or glass wafer; and wafer-to-wafer bonding to form channels can result in problems with hermeticity at the bond interface and be prone to burst failure³. In general, silicon has low-fracture toughness and is prone to rupturing under pressure or shock compared with metals such as stainless steel and titanium.

Additive micromanufacturing, on the other hand, can simultaneously produce vertical, horizontal and multi-directional channels, diaphragms and other structures sans a bonding interface and using fracture- and corrosion-resistant materials, as shown by the cross-section illustration of additively micromanufactured structures on a wafer in figure 3. Not only can additive micromanufacturing reduce the step counts of traditional MEMS fabrication methods by hundreds, but it can form wafer features that are not possible using traditional silicon-based micromachining.

Figure 3

A cross-section illustration of the types of structures that can be simultaneously additively micromanufactured onto a wafer.

Several examples of AM+MEMS fabricated sensing and actuation elements in metal and plastic are shown in figure 4. The technology’s versatility is evident in the optically transparent plastic chips for microfluidics and optical sensing applications in figure 4a as well as the corrosion-resistant metal microtube chip in figure 4b.

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Figure 4a

AM+MEMS fabricated metal and plastic sensing and actuation elements: (a) optically transparent plastic and wafer chips; (b) a corrosion-resistant metal microtube chip; and (c) a through-wafer via on titanium WLP.

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Figure 4b

In the past, MEMS wafer-to-wafer bonding has been required to begin the sensor packaging process by enclosing fragile sensing elements into a particle-free vacuum or inert gas-filled chip-scale package (CSP)⁴. Now, optically transparent plastic and metal wafers with cavities and bond pad openings can be additively micromanufactured using Kovar and titanium for wafer-level packaging (WLP). Additive micromanufacturing is capable of producing vias on thick wafers. The through-wafer via (TWV) on titanium WLP in figure 4c has an aspect ratio of 10:1, which is comparable to silicon DRIE. Via formation through 5 mm-thick wafers, roughly 6 to 10 times thicker than a typical 100–200 mm diameter silicon wafer, has been demonstrated.

Figure 4c

Both the male tubing interfaces and female tube inserts shown in figure 4b can be printed as part of a chip-scale fluidic package (CSFP). This overcomes a reliability problem found in silicon and glass microfluidics, where the chip must be epoxied or soldered to the system package. This weak link in the silicon packaging step has resulted in chip attachment failure when the epoxy fails under high temperature or pressure or after chemical exposure over time. A CSFP that can be laser welded for metal or solvent welded for plastic to the package affords a significant improvement in reliability for microfluidics in aggressive applications.

The AM+MEMS fabricated fluidic U-shaped titanium tube in figure 4b is the analogue of a silicon microtube developed in other studies^{5, 6}. The complex microfluidic silicon tube resonator used to produce a Coriolis mass flowmeter or density sensor involves four or five wafers, more than 100 wafer processing steps, four DRIE etches and three wafer-to-wafer bonding steps to form both a horizontal silicon tube and vertical through silicon via. Four of the silicon wafers, the four DRIE etches and the three wafer-to-wafer bonding steps can be replaced by a single additively micromanufactured wafer. The resulting metal microfluidic device can be made faster and at a lower cost. The additively micromanufactured microfluidic chip is more corrosion resistant than its silicon equivalent and can be welded to the system package for higher reliability.

With sensors, there is often a need for an electrical interface, generally on the sensor wafer. Screen printing and inkjet printing are typically limited to minimum feature dimensions for metal traces of 50 to 150 μm across. Traditional MEMS integrate the electrical interface for sensors using wafer fabrication processes, leveraging integrated circuit (IC) fabrication technology. By using wafer fabrication photolithography tools, the minimum feature dimensions for metal traces can go down from 5 to 2 μm for proximity tools and from 2 to less than 0.09 μm for stepper lithography tools. To use lithography fabrication tools on additively micromanufactured wafers requires a round-shaped wafer and a polished planar surface, as per figure 2. Using additive micromanufacturing for the long, single-wafer micromachining steps and retaining IC fabrication processing to form thin-film patterned layers is a more cost-effective solution for many MEMS applications.

MEMS foundries that produce silicon complementary metal-oxide-semiconductor (CMOS) and bi-polar CMOS (BiCMOS) wafers do not allow wafers made from transition metals and alloys such as those used in most AM+MEMS fabricated wafers. This is because cross-contamination of these wafers with transition metals can cause high PN junction leakage currents, emitter-collector pipes and degradation of minority carrier lifetimes and gate oxide, plus transition metals are known to diffuse rapidly through silicon wafers.

However, many traditional silicon MEMS foundries that do not produce CMOS or BiCMOS wafers have allowed the processing of sodium-containing wafers such as Borofloat 33 borosilicate glass and titanium. This opens up the possibility for not only AM+MEMS fabricated wafer prototypes but high-volume manufacturing. Some MEMS foundries are processing 100, 150 and 200 mm diameter metal and glass wafers, and it is these that would benefit from the commercialisation of AM+MEMS technology.

Conclusion

AM+MEMS affords a significant improvement in manufacturing scalability for fluidic sensors. Large, >1 m diameter additively manufactured structures are already being produced for aerospace applications⁷; and small, micron-sized copper and gold structures are being achieved via different additive micromanufacturing techniques⁸. Silicon MEMS are often limited to channel dimensions ranging from 1 μm to 1 mm. Traditional metal machining, drawing and casting of channel and tube sizes starts at around 1 mm. AM can span the entire micron to metre dimensional range using a broader scope of materials. AM+MEMS is a method that adds integrated electrical and sensing circuitry on the small end of this manufacturing capability.

In summary, AM+MEMS has been used to fabricate metal and plastic MEMS wafers and sensors. It has proven suitable for the duplication of many traditional MEMS and WLP structures, as well as been found capable of the fabrication of multiple types of structures, channels and tubes in differing orientations in a single step, which is not possible using traditional silicon-based DRIE and wafer-to-wafer bonding processes. AM+MEMS enables chip-scale fluidic packaging in which traditionally weldable male tubing interfaces and female tube inserts are formed as part of the microfluidic chip. Using additive micromanufacturing and post-processing techniques such as wafer polishing, traditional MEMS circuit and sensing layers can be deposited and patterned on wafers, thus expanding their utility. Leveraging the existing MEMS wafer foundry infrastructure will enable fast commercialisation of AM+MEMS

3D Printed Wafers

https://3dprintwafer.com/

References:

¹Sparks, D. (2018). Metal-based wafer level and 3D-printed packaging. Chip-Scale Review, volume 22, issue 4, pp.14–7.

²Sparks, D. (2018). Using 3D metal printing for flow and pressure sensors. Flow Control, volume 24, issue 2, pp.23–5.

³Sparks, D. (2018). Advances in high-reliability, hermetic MEMS CSP. Chip-Scale Review, volume 20, issue 6, pp.36–9.

⁴Schmidt, M. (1998). Wafer-to-wafer bonding for microstructure formation. Proceedings of the IEEE, volume 86, issue 8, pp.1575–80.

⁵Enoksson, P., Stemme, G. and Stemme, E. (1997). A silicon resonant sensor structure for Coriolis mass-flow measurements. Journal of Microelectromechanical Systems, volume 6, issue 2, pp.119–25.

⁶Sparks, D., Smith, R.T., Massoud-Ansari, S. and Najafi, N. (2004). Coriolis mass flow, density and temperature sensing with a single vacuum sealed MEMS chip. Solid-State Sensor, Actuator and Microsystems Workshop, Hilton Head Island, South Carolina, US, June 6–10, pp.75–8.

⁷Vialva, T. (2018). Lockheed Martin produces its largest 3D printed parts for space [press release]. July 13. 3D Printing Industry. Available at: http://bit.ly/32dnAyk

⁸Hirt L., Ihle, S. and Pan, Z. et al. (2016). Template-free 3D microprinting of metals using a force-controlled nanopipette for layer-by-layer electrodeposition. Advanced Materials, volume 28, issue 12, pp. 2311–5.