97.577 L6: Surface Micromachining, Foundries 1/5 01/22/02

6.1 Surface Micromachining

Surface micromachined sensors are formed by stacked and etched thin films. Surface micromachined devices typically can make use of much smaller features than bulk micromachined devices, which can be an advantage or a limitation. Quite complicated structures can be constructed using stacks of thin films and sacrificial spacer layers. Bulk micromachined structures usually have few layers, and only one layer that is machined. Smaller dimensions make surface micromachined structures more sensitive to environmental factors and contamination. Often a combination of bulk and surface techniques are employed to span a range of length scales. Thin film materials have more variable electrical and mechanical characteristics, resulting in variation in device characteristics. The thin film layers often have significant internal stresses. 6.1.1 Surface micromachining techniques Materials are generally variations on standard materials used for silicon electronics, as discussed last lecture. Sacrificial layers A thin film must be deposited on a solid surface. To make a suspended structure it is necessary to deposit a structural layer on top of a sacrificial layer which can later be removed to leave the suspended structure. Multiple sacrificial layers can be used. As one of the final processing steps the structures are wet released (wet chemicals) or dry released (plasma etch). The sacrificial material must be feasible to deposit with well controlled thickness and uniformity. Particularly with small thicknesses the uniformity and repeatibility will dramatically influence the performance variation in devices. In the release, the material must be completely removed, and the etch rate must be high due to finite selectivity between the sacrificial layer and the other layers. In some 97.577 L6: Surface Micromachining, Foundries 2/5 01/22/02 structures access of etchant to the sacrificial layer is through long narrow channels. (slow) Low Temperature Oxide (LTO) or phospho-silica- glass (PSG) are candidates as they can be deposited conformally and removed with concentrated HF, without significantly affecting silicon or aluminum. Selectivity for LTO/poly S~105. Organic materials like photoresist or polyimide are also used because they are easily removed in oxygen plasmas. However, processing temperatures after deposition of the sacrificial layers are limited and multiple layers can be difficult to apply. Design considerations: Sacrificial layers are removed by opening a hole or channel from the layer to the surface. The etchant penetrates through these holes to remove the layer. The access holes have to be designed to allow removal of the sacrificial layer in a reasonable time period. Rate can be diffusion limited. Access holes should be as large and numerous as possible! For wet release, capillary or surface tension has to be considered. Liquid surface tension can pull a flexible structure towards an adjacent surface during the drying process. Once in contact the surfaces often stick together (pinning or 'stiction'). The exact mechanism is not clear but may simply be Van der Waals attraction between two smooth surfaces. Dry release avoids the pinning problem. This has to be an isotropic dry etch, and is often very slow. Processes such as vapor phase HF etching are being used! Wet release pinning can be avoided by freeze drying or supercritical drying. Freeze drying actually freezes the liquid and sublimates it, for example using t-butyl alcohol which freezes at 26C. Critical point drying controls the pressure and temperature to maintain the liquid at its critical point where surface tension is zero. Typically the steps would be: i)release using sacrificial oxide by immersion in aqueous HF ii)passivation in sulfuric or hydrogen peroxide, resulting in hydrophilic surfaces iii)deionized water rinse, followed by a methanol rinse iv)methanol soaked samples into supercritical drying chamber v)methanol displaced by CO2 at 20C, 1200 psi (liquid). vi)temperature increased above the critical point (1073 psi, 31.1C - no latent heat for transition, no distinction between liquid and gas) vii)pressure dropped until CO2 is in gas phase. Pinning is generally reduced by reducing contact surface area, for example by using dimples (bumps) or roughening surfaces. Hydrophobic self assembled monolayers (SAMs) are often used as post-release treatments to reduce stiction in use. Typically SAMs are long chain hydrocarbons with a chlorinated silicon head group which bonds to the surface. Coating is usually achieved 97.577 L6: Surface Micromachining, Foundries 3/5 01/22/02 by a sequence of wet chemical rinses. (hydrophobic surfaces don't have the surface tension problem!). Sealed cavities To use a sacrificial layer an opening to the surface of the wafer is required. In some applications a sealed pressure or vacuum cavity is required. Usually release holes are kept small, and deposition of a thin film over the released structure seals the openings. Often the material used to plug the hole is a bit porous, and an overcoat of impermeable material is used. Often made by wafer bonding, with corresponding complications. Planarization For multi-level IC metallization, planarization is a problem as the many layers of metal and isulator are used, each with some topography. As the layers are deposited and patterned, surface irregularities can multiply until the depth of field is exceeded for the photolithography tool. The step heights produced in micromechanical systems is even more severe than this. As a result, techniques are sometimes used for multiple resist coatings, very thick resists, and spray or vacuum deposited photoresists. IC planarization techniques such as spin-on glasses or CMP are also used to try and control this problem. 5.4 High aspect ratio microstructures

High aspect ratio microstructures are more three dimensional structures than the planar devices generally fabricated with other techniques. The most visible applications are micro gears and engines. These structures require very specialized processes (LIGA, lithographie, galvanoformung, abformung, or deep RIE). LIGA Three steps, lithography, plating, and molding. 97.577 L6: Surface Micromachining, Foundries 4/5 01/22/02

Thick photoresist (PMMA poly methyl methacrylate) from 300 to 500 um thick on a conductive substrate. To expose this resist with well defined sidewalls (good depth of focus) requires highly collimated x-ray radiation from a synchrotron through an x-ray mask. After developing the resist, the features are filled with electroplated metal. The resist is removed, and the metal is used as a mold for injection molding plastic parts. The molded parts can be used to electroplate many metal parts. The synchrotron needs to produce 1GeV energies, wavelengths < 7A. The exposure time is several hours.

The mask is usually a high mass metal on a suspended Si3N4 membrane, or a Ti foil. PMMA cannot be spun on at these thicknesses. Tricky control of thick plating to avoid bubbles and stress. Expensive to make the initial mold, but the manufacture of many parts is cheap.

Similar but lower quality structures can be realized with thick resists designed for conventional lithography equipment (up to ~40um thick with 10um resolution).

Combinations of these processes (bulk + bonding, bulk + surface ) are often required for complicated structures. 6.2 Foundry services To make a micromechanical device there are a limited number of options, depending if you want to try a few parts or are committed to developing a product. 1) Low cost, research/prototype: i) Develop a custom process You need an expensive lab and a lot of time ii) Use standard CMOS multi-project services and post process Very popular about 10 years ago. Still a good route for an easily path to production. Normally openings are left to the Si using the CMOS back end layers. When chips or wafers are received from the foundry they can be etched using one of the techniques listed above. Only front side machining is generally possible (no diaphragms). Design Rule Violations preclude fabrication in many modern processes. Older processes are generally suitable. No control over layers, properties, or thickness. Capabilities are limited. You need a lab, but a lot of the work is done for you. The same result might be more easily obtained with a hybrid device (IC+MEMS)

CMC Support Mitel 1.5 µm CMOS multi-project runs, primarily for micromachining. 97.577 L6: Surface Micromachining, Foundries 5/5 01/22/02

*mitel layers, example?

MOSIS US CMOS foundry service, They currently seem to encourage 'do it yourself' micromachining by providing some guidelines from NIST. http://www.mosis.org

CMP Like MOSIS service, based in Europe. Not much information available, but CMOS, BiCMOS, GaAs seem to be available http://cmp.imag.fr iii) Find a MEMs multi-project service There aren't many. Some bulk machining services seem to have vanished. Surface machining services are still available, but options seem to be shrinking

Cronos MUMPs (available through the sources above) Surface micromachining service available for academic or commercial access. Design kits for different environments and some models are available. Decent library of basic parts, parameterized or not.

NO electronics. Flip chip circuits were available?? http://www.memsrus.com

Sandia National Lab Basic surface machining process seems to be available, but at fairly high cost (dedicated runs). http://www.sandia.gov 2) High cost product development partnerships. Any of the services listed above could be used. In addition there are several industry oriented foundries: Cronos Sandia I-MEMs is available this way Intellisense CAD tool vendor and foundry service Standard MEMs