Circular microhotplate

The first device is a circular microhotplate (Sect. 4.1). One important guideline was to implement the microhotplate in CMOS technology with a minimum of post-CMOS micromachining steps. Additionally the hotplate had to be optimized for drop-coating with nano crystalline tin-oxide layers. This microhotplate was cointegrated with circuitry, and the respective monolithic sensor system will be discussed in Sect. 5.1. 

The second microhotplate design is derived from this circular microhotplate, in contrast to the first device, it does not feature a silicon island underneath the heated area, but exhibits a network of temperature sensors in order to assess the temperature distribution and homogeneity (Sect. 4.2). The measured temperature distribution was compared to simulations, and the model described in Chap. 3 was validated. Furthermore, the influence of the tin-oxide droplet on the temperature distribution was studied. A microhotplate without silicon island is much easier to fabricate, though the issue of sufficient temperature homogeneity has to be evaluated. 

Fig. 4.1. (a) Micrograph of a circular microhotplate without sensitive layer, (b) SEM-micrograph of a metal-oxide coated microhotplate... 

Fig. 42. Schematic of the process flow to fabricate the circular microhotplate...

The circular microhotplate was thermally characterized, and the results were compared with simulations carried out according to the approach discussed in Chap. 3. Applying FEM simulations as described in Sect. 3.3 generate a temperature field, and the temperature in the membrane center represents the overall membrane temperature according to Eq. (3.21). The values that have been used for the simulation are summarized in Table 4.2. 

Fig. 4.5. Measurements and simulation of the temperature increase, AT, versus heating power of a circular microhotplate...

In conclusion, simulated and measured values are in good agreement, and the achieved accuracy is sufficient for system-level simulations. The experimental results for the characteristic data of a circular microhotplate design are listed in Table 4.3. 

Table 4.3. Characteristic data of a circular microhotplate design...

The electrochemical etch-stop technology that produces the silicon island is rather complex, so that an etch stop directly on the dielectric layer would simplify the sensor fabrication (Sect. 4.1.2). The second device as presented in Fig. 4.6 was derived from the circular microhotplate design and features the same layout parameters of heaters and electrodes. It does, however, not feature any sihcon island. Due to the missing heat spreader, significant temperature gradients across the heated area are to be expected. Therefore, an array of temperature sensors was integrated on the hotplate to assess the temperature distribution. The temperature sensors (nominal resistance of 1 kfl) were placed in characteristic locations on the microhotplate, which were numbered Ti to T4. 

Fig.4.6. Close-up of the circular microhotplate with temperatiu e sensor array and without silicon island...

The circular microhotplate presented in Sect. 4.1 features an upper sensor operating temperature limit of 350 °C, which is imposed by the CMOS metallization. At higher temperatures, electromigration, especially in the heater structures, will occur. 

This chapter includes two different sensor system architectures for monolithic gas sensing systems. Section 5.1 describes a mixed-signal architecture. This is an improved version of the first analog implementation [81,91], which was used to develop a first sensor array (see Sect. 6.1). Based on the experience with these analog devices, a complete sensor system with advanced control, readout and interface circuit was devised. This system includes the circular microhotplate that has been described and characterized in Sect. 4.1. Additionally to the fabrication process, a prototype packaging concept was developed that will be presented in Sect. 5.1.6. A microhotplate with a Pt-temperature sensor requires a different system architecture as will be described in Sect. 5.2. A fully differential analog architecture will be presented, which enables operating temperatures up to 500 °C. 

The fabrication of the sensor system was described in Sect. 4.1.2, since this microsystem also features a circular microhotplate. A micrograph of the complete microsystem (die size 6.8 x 4.7 mm ) is shown in Fig. 5.2. The microhotplate is located in the upper section of the chip. The analog circuitry and the A/D and D/A converters are clearly separated and shielded from the digital circuitry. The bulk-chip temperature sensor is located close to the analog circuitry in the center of the chip. The distance between microhotplate and circuitry is comparatively large owing to packaging requirements, as will be explained in Sect. 5.1.6. 

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