Sensor Business Digest: ISSYS Crafts MEMS Devices to Advance Real-World System Applications

April 1, 2000 By: Peter Adrian

SBD underscores that sensor technology development should be highly focused on specific applications, which have significant growth potential in the marketplace. Sensor technology, when developed for its own sake and "in search of an application," is essentially doomed to languish in the laboratory; and such ethereal sensing technology will not likely provide sufficient payback for the resources required for its development. We also note that sensors with system-level features, such as self-diagnostics and calibration, can be particularly beneficial for enhancing key applications.

Integrated Sensing Systems, Inc. (ISSYS, Ypsilanti, MI, 734-547-9896) is a young company that is developing distinctive MEMS (microelectromechanical systems)-based pressure and flow sensing solutions that can economically offer clear performance benefits for specific industrial and biomedical applications. ISSYS is distinct from other MEMS companies in several key areas, including microfabrication technology, vacuum encapsulation technology, assembly and packaging techniques, and corrosion-resistance techniques.

Founded in 1995 by Dr. Nader Najafi, and Professor Ken Wise and Prof. Khalil Najafi of the University of Michigan, ISSYS focuses on developing and producing high-performance MEMS pressure and flow sensors/systems that provide extremely high accuracy, very compact size, and excellent corrosion resistance.

Nadar Najafi, president and CEO, explained that ISSYS focuses on providing MEMS sensors with value-added system-level performance, in addition to developing sensors with enhanced features. ISSYS is able to synthesize the components of its technology portfolio to provide a sensing solution at the system level for a customer's application. ISSYS strategy, moreover, is to make products that incorporate their sensors, or work with partners to make such products.

Key products under development by ISSYS include: ultra-high sensitivity capacitive pressure sensors for semiconductor applications; ultra-wide range pressure sensors incorporating multiple sensors on a chip; ultra-miniature pressure sensors, which can be as small as two human hairs in width, or sufficiently small to fit two of them in the eye of a needle; and micromachined mass flow sensors that are suitable for low flow applications.

In addition, ISSYS--which operates two class 100 clean rooms, two independent chemical labs, and characterization labs--provides fabrication (e.g., anodic bonding, deep reactive ion etching, dicing, EDP etching, photolithography, sputtering) and characterization (e.g., surface profiling (WYKO), SEM) solutions, turnkey solutions (for applications requiring a high-performance sensor); and consulting solutions (e.g., sensor design, process development, packaging).

In conjunction with Millipore Corporation (Allen, TX), ISSYS has developed an ultra-sensitive capacitive pressure sensor that offers a resolution of better than 1 µTorr (1 x 10-6 Torr), built-in temperature compensation, and vacuum references. The sensor also can provide advanced features (such as self-testing and self-calibration) that considerably reduce end-user maintenance expenses.

The structure, performance, and economical characteristics of the ultra-sensitive pressure sensor illustrate ISSYS' MEMS capabilities. The large diaphragm that forms the bulk of the sensor is fabricated with a thickness from 2 µm to 5 µm, or about 1/40th of a human hair. The diaphragm is suspended above its base at approximately the same distance. Using their dissolved wafer process for processing silicon and creating microstructures, ISSYS has routinely reproduced the membrane's and height to within 0.1 µm or only 1/1,000th of a human hair.

The ultra-precise level of control achieved becomes even more vivid when compared to the diameter of the diaphragm. In the most sensitive products, the diaphragm is fabricated with a diameter of 5 mm or greater. To obtain comparable senstivity on a macro scale, the diaphragm would have to be 1' thick, 1' above the ground, and cover an area equivalent to 70 football fields, with a height or thickness variation of less than 1".

ISSYS notes that its ultra-high senstivity MEMS sensor provides over 300 times the sensitivity of the capacitive gauge diaphragm (which is currently preferred for vacuum measurements in semiconductor manufacturing), while requiring 1,000 times less volume than the conventional capacitive gauge diaphragm. Moreover, ISSYS' ultra-sensitive pressure sensors have demonstrated their robustness under harsh cycling conditions. Pre-production prototypes have undergone over one million cycles at three times their designated operating range, with very minimal change in performance.

Production of the ultra-high sensitivity pressure (vacuum) sensors is expected to begin around Q3 or Q4 of 2000; and the product's balance sheet is expected to be as vibrant as the device's physical performance. ISSYS notes that their ultra-sensitive pressure sensor addresses the needs of end-users in the semiconductor equipment manufacturing market, who seek higher levels of performance, while becoming more cost sensitive.

Randy Grimes, director of operations and business development at ISSYS, noted that the primary target market for the ultra-sensitive pressure sensor is semiconductor manufacturing equipment and that additional potential markets include food & Beverage (e.g., freeze drying).

In the highly fragmented pressure sensor market, a number of technologies compete with respect to various pressure sensing applications. Each technology has particular strengths and limitations. ISSYS notes that their ultra-wide range pressure sensor addresses a prevalent weakness in available pressure sensors--insufficient operating range. Many applications involve a wide range of pressures and, therefore, typically require multiple sensors to accurately gauge pressure.

The ultra-wide range pressure sensor addresses this challenge by incorporating multiple sensing elements on a single chip, with each sensor tuned to a different pressure region. ISSYS has designed such a sensor measuring 0.4 x 0.4 in2 that incorporates nine pressure sensors and two temperature sensors on a single die. The sensor is vacuum referenced and can incorporate self-testing and self-calibration capabilities that reduce cost of ownership. With each sensor tuned to a different pressure region, multiple devices are no longer needed to accurately characterize the pressure range of interest. The ultra-wide range pressure sensor has a range of 1 x 10-4 to 5 x 103 Torr and a sensitivity of 1/10,000 in each region.

Applications for the ultra-wide range pressure sensor include a variety of industrial processes (such as automotive refrigerant reconditioning and light bulb manufacturing) with process conditions that range from a medium vacuum to moderately high positive pressures. In such applications, end-users and equipment manufacturers often must purchase several varieties of pressure sensors to effectively monitor their process. This approach results in greater equipment expense and increased maintenance cost and systems complexity. The ultra-wide range pressure sensors are anticipated to be available for use around Q2, 2001.

ISSYS' ultra-miniature pressure sensors--which can be as small as two human hairs in width (or sufficiently small to fit two of them in the eye of a needle)--exploit the key benefits of MEMS technology with respect to size. With a total volume of 1/1000 cm3, the ultra-miniature pressure sensors--which have a range of 100-2000 Torr and a response time of <1 msec.--are well-suited for applications with size and weight constraints.

The ultra-miniature sensors provide ample resolution for their intended applications. Even early prototypes provide a resolution of 0.3 Torr, which, purportedly, is several times better than that required for most medical uses. Such applications as blood pressure monitoring, right-heart catheterization, arterial blockage and angioplasty characterization, wound pressure, and intercranial pressure monitoring only require a pressure resolution of 1-2 Torr.

Target applications for the ultra-miniature pressure sensors include disposable products (e.g., specialized catheters) and chronic implantable devices. ISSYS is looking for key partners in the biomedical field to help commercialize the ultra-miniature pressure sensors. ISSYS is working on products that incorporate their ultra-miniature pressure sensors; and products based the sensor's technology are expected to appear as early as Q1, 2001.

A high priority for ISSYS is their micromachined mass flow sensor under development, which is highly suitable for low-flow applications where accuracy is paramount. Target applications for the MEMS Coriolis mass flow sensor include replacing low-flow sensors in such industries as food & beverage and chemical; and new (e.g., micro-fluidic) applications where the micromachined mass flow sensor allows for reduced volume of fluid and for adding very minute amounts of agents very accurately. The micromachined mass flow sensor can, for example, benefit such industries or applications as pharmaceutical, biotechnology, analytical equipment, drug delivery systems, medical products, lab-on-chip, and mixing liquids at the micro level.

ISSYS is seeking partners (e.g., pharmaceutical companies and medical products companies) to help commercialize the micromachined mass flow sensor. Beta versions of the micromachined mass flow sensor are expected to be available circa 2002, and mass-production of such sensors is expected to begin around 2003.

Working prototypes of the micromachined mass flow sensor have been fabricated with a mass flow sensitivity of 2 µg/sec. and a flow density resolution of 0.001%, purportedly surpassing commercially available flow meters and approaching ISSYS' targets. Freestanding micro-fluidic tubes have been successfully actuated at the system's resonant frequency.

Key features and benefits of the proprietary, micromachined, Coriolis-based mass flow sensor include: significantly lower cost; direct mass flow measurement; direct fluid density measurement; small size; low power consumption; measurement of low mass flow rates; high sensitivity (estimated at <1 µg/sec.); high accuracy; rapid response time (estimated at <1 msec.); small fluid flow volume; batch fabrication capability; and excellent stability. Moreover, the device is insensitive to fluid conditions (e.g., temperature, pressure, density, viscosity, conductivity), since it measures mass flow directly.

ISSYS is committed to advancing its silicon fabrication processes and to developing the associated technologies required for enhancing real-world pressure and flow sensor products and applications. Such technologies include MEMS fabrication processes, vacuum sealing, fabrication of stationary and vibrating micro tubes (as small as 30 micrometers in diameter, or 1/3rd of a human hair's thickness); assembly and packaging, corrosion resistance, and testing and characterization.

ISSYS' micromachined sensors and actuators are fabricated using silicon (whose mechanical properties are beneficial for sensor fabrication). Monocrystalline silicon, ISSYS, notes has a similar Young's modulus and hardness as stainless steel, but has 1/3rd the density and a three-fold yield strength. Silicon conducts heat well and can be electrically insulating or conducting. ISSYS adds that monocrystalline silicon, which will not undergo plastic deformation and stays perfectly elastic until it breaks, is highly beneficial for sensing applications. When free of plastic deformation, silicon-based sensors do not suffer from fatigue and hysteresis problems, thereby providing an advantage over metallic or polysilicon-based products.

There are four basic silicon microstructure processing or microfabrication technologies: surface micromachining; bulk micromachining; LIGA (Lithography, Galvanoformung, Abformung); and ISSYS' exclusive Dissolved Wafer Process (DWP). While ISSYS can use all these techniques, their Dissolved Wafer Process is the common foundation for all their sensing systems and purportedly distinguishes the company from other MEMS entities.

In surface micromachining, structures are created by building up and patterning successive sacrificial and mechanical layers on the front side of a silicon wafer. The sacrificial layer is then removed, leaving freestanding mechanical components. In bulk micromachining, techniques are used to directly etch structures into the silicon wafer. The etching can be performed selectively using such parameters as crystal orientation, doping concentration, and the presence of other materials (e.g., silicon oxide). The LIGA process uses x-ray lithography to pattern thick layers of photoresist in order to create molds. The molds can then be filled by, for example, electroplating metallic films. A freestanding metallic structure is left, after the photoresist mold is stripped away.

ISSYS' Dissolved Wafer Processing technique creates structures inside of a silicon wafer via boron diffusion. The wafer is then attached to a substrate; and the non-boron diffused portions of the wafer are dissolved, leaving the structures that were built into the wafer.

The essential elements of the Dissolved Wafer Process are illustrated in the following simplified fabrication process for a capacitive pressure sensor. The DWP starts with a standard, single-crystal silicon wafer. Using a simple wet chemical etching process, a cavity is formed in the wafer. The etch determines the separation of the device's capacitive elements. A deep boron diffusion process is then used to form the base of the capacitive membrane. In the next step, shallow boron diffusion is used to form the sensor's actual membrane (which will move in response to change in external pressure). The base of the sensor is then attached to a glass substrate via anodic bonding. Finally, the remainder of the wafer is dissolved away, leaving a freestanding microstructure.

The DWP offers a number of advantages over conventional surface or bulk micromachining technologies for fabricating ISSYS' products, such as: a simple, single-sided process that reduces fabrication costs; yields very accurate and reproducible results; creates a boron-doped single-crystal microstructure with superior mechanical properties and chemical resistance; allows for creating thick and thin microstructures on a single chip; and the ability to produce a high density of microstructures.

Vacuum encapsulation is a key, difficult issue for MEMS sensors and actuators, ISSYS notes. Typically, vacuum encapsulation is achieved at the package level, one sensor at a time. This expensive serial approach further obfuscates the complicated packaging process.

ISSYS' patent-pending wafer-level vacuum encapsulation technology allows for encapsulating hundreds of sensors at a time. For the technique to work, a sensor must have a hermetic electrical lead transfer from within the encapsulated space to the outside. As part of their pressure sensor development, ISSYS has developed two patent-pending lead transfer techniques. The use of ISSYS' proprietary technologies for wafer-level vacuum encapsulation and hermetic lead transfer offers key manufacturing advantages to, for example, the micromachined mass flow sensor products, such as the potential for considerably reduced cost, ease of assembly and packaging, and enhanced reliability.

Recognizing that assembly and packaging "are the most difficult and most underdeveloped technologies for the manufacturing and production of MEMS products," ISSYS has focused on the assembly and packaging of their pressure and flow sensors.

Another vital and underdeveloped technology for many MEMS applications is resistance to corrosive, harsh environments. ISSYS notes that the prevailing technology for configuring MEMS pressure or flow sensors to work in harsh environments is problematic. In this conventional technology for media compatibility, the sensing elements are isolated from the environment by enclosing the sensor with a stainless steel case and filling the cavity with an incompressible fluid (e.g., silicone oil).

In contrast, ISSYS' patented corrosion-resistant technology uses thin-film protective coatings and innovative packaging techniques to protect their MEMS sensors. ISSYS' MEMS corrosion-resistant technology, which has yielded very promising results in gaseous and liquid corrosive environments, further enhances the company's advanced microsensor and microsystem capabilities.


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