MEMS at SEMICON West: Smaller consumer die creates need for new etch, clean processes

by Dr. Paula Doe, Contributing Editor, Solid-State Technology

As the growing demand for MEMS systems in consumer gear brings consumer market volumes and price pressures to the MEMS sector, device makers are looking to smaller geometry die to cut costs. The finer processes needed on those smaller die may need new kinds of etching and cleaning processes — and the higher volumes mean pushing the tools a lot harder, bringing new interest in the kinds of uptimes big semiconductor equipment vendors are used to supplying.

Yole Développement projects MEMS unit growth of more than 25% a year (see Figure 1), but expects pressure for 6%-7% overall cost reductions each year. “There’s room for a lot of innovation,” says managing director Jean-Christophe Eloy. “A lot of new companies are coming in with different solutions for etching and cleaning the smaller geometry die.” Among the crowd of new entrants in Europe eying the MEMS business are PVA TePla, with a lower-temperature microwave plasma etch and ash process, and memsstar, bringing an integrated approach to MEMS release etch and coat. Big semiconductor tool suppliers like Lam Research are also making inroads by applying their workhorse semiconductor etch technology to MEMS depths.

Figure 1. MEMS market forecast per application, US $M. (Source: Yole Développement)

PVA TePla (Feldkirchen, Germany) aims to extend its semiconductor plasma organics removal business to MEMS single-wafer and small-batch ashing and etching of tough SU-8 resist and sacrificial layers at low temperatures. Gerhard Liebel, VP of semiconductor equipment sales and marketing, says the company gets uniform removal across the wafer by starting with a large area source (see Figure 2), one downsized from the company’s flat panel display equipment. And it uses a microwave source instead of RF, which it claims allows faster removal rates at lower temperatures.

The company claims that the 0.5-1.0 Torr running rate of the microwave source allows a simpler design without turbo pumps. And the much lower electron potential of the microwaves (~5eV) is less likely than RF to cause damage from ion bombardment acceleration, allowing the source to be closer to the wafer for more efficient chemical attack. “We can run many MEMS removal processes at 100°C or less,” says Liebel. “At pretty good rates for MEMS.” He says the tool is in commercial use for SU-8 removal, getting in the range of 1-3μm /min using O2 and CF4, while softer materials can be removed with just O2. One customer is reportedly removing SU-8 layers of up to 1000μm, though it does take several hours. He says the highly isotropic process can also remove hidden sacrificial layers through tiny 2-3μm holes more efficiently at lower temperatures than straight downstream plasmas. The tool runs four 150mm wafers or one 200mm or 300mm wafer.

Figure 2. MEMS wafer during processing with large-area microwave plasma discharge. (Source: PVA TePla)

Scottish startup memsstar (Livingston, UK) says it gets higher-throughput vapor etching with an integrated process that targets highly selective release etch and coat steps. Senior technologist Alan Atherton reports the equipment can do isotropic release etching of silicon or transition metals at 10μm/min with xenon difluoride (XeF2) with 20,000:1 selectivity, or etch oxides with vapor hydrogen fluoride (VHF) with almost infinite selectivity at >0.5μm/min.

Key to the process is delivering the etchant in a carrier gas in a continuous controllable stream, and precisely monitoring the reaction process with a feedback loop to maintain tight control. Different film layers can be removed by switching between VHF and XeF2 for precise control, then the wafer moved to another chamber without breaking the vacuum to coat the surface with a self assembled monolayer. “If the surface is never exposed to water vapor in the atmosphere after sacrificial etch you get better adherence,” says Atherton. “The antistiction and hydrophobic coatings perform much better.” The tool has three chambers with a common load lock.

Atherton says a number of companies are using the product in mass production in Europe, making RF products and oscillators.

Meanwhile, Lam Research (Fremont, CA) has applied its workhorse IC etching technology to the deep etch throughputs required of MEMS, bringing IC-style in situ cleaning between wafers for longer uptimes needed for 24/7 volume MEMS production. Lam also reports its IC source technology provides very high uniformity across the wafer. “For MEMS gyros that require a very uniform plasma to get good yields we’re getting 3×-4× higher yields,” reports Steve Lassig, senior program manager, MEMS and advanced packaging etch group. He notes that Lam has tweaked the tool to handle the 20-100μm etch depths required for MEMS, and says one customer is even using it to etch completely through a 400μm wafer.

These developments will be among the new technologies for volume consumer MEMS production to be discussed at the Emerging Markets TechXpots at SEMICON West. P.D.


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