Substrate fabrication

The surface-micro-machined microhotplate structure that is common to the various platforms was designed for fabricating conductometric gas micro-sensor prototypes. Microhotplate elements include functionality for measuring and controlling temperature, and measuring the electrical properties of deposited films. As their name implies, they are of particular interest because of their ability to examine temperature-dependent phenomena on a micro-scale, and their rapid heating/cooling characteristics have led to the development of low power sensors that can be operated in dynamic temperature programmed modes. Tens or hundreds of the microhotplates can be integrated within arrays that serve as platforms for efficiently producing processing/performance correlations for sensor materials. The micro-devices also provide a basis for developing new types of sensing prototypes and can be used to investigate proximity effects and surface transient phenomena. The evolution of micromachining as a fabrication technology for chemical sensing has allowed miniaturization to progress with improved fabrication methods. Micromachining of silicon to produce sensor platforms also

makes it possible to include on-chip circuitry, and it is straightforward to replicate device structures into integrated arrays. Beginning in the early 1990s, the opportunities of silicon micromachining led to the fabrication of new types of microhotplate devices and arrays for gas sensing. These devices have been produced both by surface [168] and bulk [169, 170]

etching of silicon, and have been used to develop gas micro-sensor prototypes [171]. The ability to locally heat miniature elements has been used both to fabricate gas micro-sensor films and operate devices in the rapid temperature-programmed mode [172]. Recently, the advantages of microhotplates as gas sensor platforms have been amply demonstrated [173, 174]. The Process Sensing Group at NIST (National Institute of Standards and Technology) is investigating generic technology issues related to next-generation chemical measurements with solid-state devices. The technology developed by this group is discussed here in considerable detail because of its importance for micro-machined devices. The microhotplate contains a built-in heater, thermometer, and sensing film. The device uses conductance changes in the sensing film to detect the presence of adsorbed gas species. Temperature changes may be used to alter the reaction kinetics between the gas and sensor surface. A single element is illustrated in figure V.I (left) [175].

Figure V.I. Micrograph of single microhotplate with four arrow-shaped electrodes The microhotplate is a multi-layer structure, which has three functional layers: a polysilicon heater, a metal (for example Al or W) thermometer/heat distribution plate, and sensing film electrical contacts (figure V.I, right). These layers are separated by insulating layers of SiO₂. In order to measure the electrical characteristics of the sensing films, the surface electrodes are in direct contact with the sensing film but isolated from the heat distribution layer by a SiO2 layer. Maximum device temperatures are largely governed by the types of metallization available at a given foundry. For Al metallization, the maximum temperature is approximately 500°C, and metal post processing is needed to produce more acceptable sensing film contact

composition. When other metallizations, such as W or Ti-W, are available at the foundry, devices can be heated to as high as 750°C.

V.1.1 Micro-hot-plates fabricated at National Centre of Microelectronics

The integrated micro-hotplates with arrays of four microsensors were fabricated on double- side polished p-type <1 0 0> Si substrates, 300 µm thick (4-40 Ω⋅cm). The structure of the device basically consists of a gas sensing layer, the electrodes, insulating layers and a polysilicon heater. The technological process needed to fabricate the sensors had the following steps [176, 177]: (1) Deposition of the membrane layer. The dielectric membranes consisted of a 0.3 µm thick Si3N4 layer grown by LPCVD. Each chip had 4 membranes, the size of which was 900 × 900 µm². (2) Deposition and patterning of a POCl3-doped polysilicon heating meander with a resistance of 6 Ω/sq. The temperature coefficient of resistivity (TCR) of polysilicon depends on the doping level which, for our devices, was 6.79×10^-4. The heater was also used as a temperature sensor. (3) Deposition of a 0.8 µm-thick SiO₂ layer to insulate the heater from the electrodes and the sensing film. (4) Opening of contacts for the heater bonding pads to be accessible. (5) Deposition of either parallel or interdigited 0.2 µm-thick Pt electrodes, patterned by lift-off. A thin layer (20 nm) of Ti was deposited prior to Pt to promote electrode adhesion. The electrode area was 400 × 400 µm². Figure V.2 shows a planar view of the membrane with heater and electrode configuration (6) Patterning of the backside etch mask. (7) Deposition of the sensing layer onto the electrode area. (8) Backside silicon etching with KOH at 70^oC (40% wt.) to create the thermally-insulated membranes. (9) Wire bonding and packaging. Each chip was mounted on a TO-8 package. Gold wires with a diameter of 25 µm were used for standard ultrasonic wire bonding. To prevent the membranes from breaking due to air expansion in the cavity below the membranes when the device is heated, the chips were not glued directly to the surface of the metallic package but kept elevated by using two lateral silicon spacers.

The sensing films of interest include oxides, metals and polymers that can be produced and tailored by a wide variety of techniques. Typically, the films are deposited after silicon has been etched away to produce the thermally-isolated structures. Etching chemicals are usually too aggressive to properly maintain the integrity of pre-deposited films. The etch pits can be problematic, for example, when spin-on methods or certain types of lift-off processing are used. Unlike previous studies, the technological procedure reported in [178] enables the

sensing layers to be deposited before the membranes have been etched. This prevents themembranes from being damaged during film deposition, which leads to gas sensor microsystems with an excellent fabrication yield. The deposition method overcomes disadvantages such as low porosity and low surface area, generally associated to chemical vapour deposition or sputtering methods, and keeps power consumption low (80 mW for a working temperature of 480^oC).

Figure V.2. Planar view of the micromachined gas sensor membrane; left: interdigited electrodes; right: parallel electrodes

V.1.2 SOI Micro-hot-plates fabricated at Catholic University of Louvain

Another way to further minimize the power consumption is the silicon-on-insulator technology. Figure V.3 shows a SOI solid-state gas-sensor with an original design of a polysilicon loop-shaped microheater fabricated on a thin-stacked dielectric membrane [179].

The microheater ensures high thermal uniformity and very low power consumption (20 mW for heating at 400°C). The use of completely CMOS-compatible TMAH-based bulk micro- machining techniques during the fabrication process means that it is straightforward to integrate smart gas sensors in SOI-CMOS technology.

Figure V.3. Picture of a 600x500 µm² gas sensor in SOI technology

The gas-sensor has high thermal uniformity, low-power consumption, low cost and is compatible with standard IC processes. Because it is fabricated on SOI substrate, it can be used to build a fully integrated smart gas sensor in SOI-CMOS technology by integrating both the sensing device and the control electronics on the same chip. In this case, the 400nm-thick SOI buried oxide can be advantageously used as a part of the membrane and the upper monocrystalline silicon film can be used to integrate reliable electronics close to the sensor (Figure V.4). SOI technology is uniquely suited to such harsh environments as high temperatures or radiation and has unique advantages in microsystem applications.

Figure V.4. Schematic process flow of integrated SOI-CMOS microheater. A SOI transistor is grown (left) in same time as the microheater on membrane (right).

The thin-film membrane is one of the most important parts of the whole integrated structure, since it is the mechanical support for the microheater and the sensing film and it is responsible for good thermal isolation and temperature uniformity, and therefore low-power consumption.

The original loop-shaped design for the phosphorus-doped polysilicon heater was defined using electro-thermal simulations (ANSYS^TM) to optimize the size of the heater and the thermal uniformity on the membrane area.

The membrane consists of three stacked dielectric layers: SiO2 (400nm LPCVD), Si3N4

(300nm LPCVD) and SiO₂ (290nm PECVD). The thickness and deposition parameters are chosen carefully to compensate for the high residual stresses of the deposited films. Strain- free membranes with areas larger than 1mm² were successfully produced. The small total thickness of the membrane (approximately 1 µm) also leads to extremely high thermal insulation (about 15°C/mW) from the bulk and a thermal response as low as 16 ms to reach the temperature of 400°C.

The membrane is released in a single post-processing step, consisting of a backside bulk micromachining with a TMAH-based etching solution. TMAH is used because of its excellent clean room compatibility and excellent selectivity to silicon oxide and nitride. In order to use aluminium as the low-cost metallic layer, aluminium was classically passified with additives in the etching solution since pure TMAH attacks aluminium interconnections. A TMAH 5%

solution was used at a working temperature of 90°C. For aluminium passivation, the etching solution was saturated with Si powder. To reduce surface roughness, ammonium persulfate powder was added. No deterioration of the aluminium interconnections was observed after completely etching the 200µm deep cavity (~3 hours) to release the membrane. The sensitivity and selectivity of the fabricated sensor can be significantly modified by treating surfaces with additives such as Pt, Pd and Ru [180, 181].