**6. Optical micro-chemical and biochemical sensors**

Optical sensors can be fabricated as shown in Fig. 6.1. A series of Fabry-Perot of different wavelength is fabricated in series, each having its own p-n photo-detectors, immediately underneath. These photodiodes are optically and electrically isolated from each other to reduce cross interference. A micro channel is fabricated on top of the series of Fabry-Perot photodetectors (FPFD) modules. Of course, FPFD modules can appear in any efficient

transmission. Second, a thin Aluminum layer is deposited, to form the lower mirror. Third, a layer of Silicon dioxide is added then etched to different sizes, using several masks. This way, each photodiode will have a different size of SiO2 layer on top of it. Fourth, a thin layer of silver (Ag) is deposited on top of all oxide to form the top mirror layer (Tyree et al, 1994).

p-n diode

p-n diode

p-n diode

Aluminum Silver

Metal 1, 2

etch top oxide layer

N-well

Fig. 5.2. Schematic structure for fabrication of a CMOS p-n photo diodes

N-well

P

N-well

\_

P-diffusion

P epilayer P substrate +

respectively, on top of p-n diodes to form Fabry-perot cavity filter

**6. Optical micro-chemical and biochemical sensors** 

\_

P epilayer P substrate +

Fig. 5.3. Post CMOS process, 1st step, trimming the top oxide layer above the diode

Fig. 5.4. Post CMOS process, Step 2nd, 3rd, and 4th. Depositing AL, PECVD oxide, and Silver,

Optical sensors can be fabricated as shown in Fig. 6.1. A series of Fabry-Perot of different wavelength is fabricated in series, each having its own p-n photo-detectors, immediately underneath. These photodiodes are optically and electrically isolated from each other to reduce cross interference. A micro channel is fabricated on top of the series of Fabry-Perot photodetectors (FPFD) modules. Of course, FPFD modules can appear in any efficient

PECVD oxide

P

P epilayer P substrate + \_

Oxide

configuration, such as a matrix format, under the flow channel. The entire structure of micro-channel and their FPFD modules can be fabricated in a twin parallel configuration, as shown in Fig. 6.2. In time modulation, this configuration can be used when one channel is empty and one channel is filled with chemical sample. In this situation, there are two received signals for each wavelength. One is the attenuated signal due to the sample, and the other one is a signal for cross-reference and evaluation of the intensity attenuation due to the chemical sample. This configuration can be also used in measurement of fluorescence. Two different dyes can be introduced in two channels in order to evaluate two different analyte concentrations.

Fig. 6.1. Schematic structure of optical micro sensors using fabry-perot and p-n photo detectors

Fig. 6.2. Two parallel micro flow channels, each with its own FPFD module underneath

VLSI Design for Multi-Sensor Smart Systems on a Chip 15

works best for small embedded memories. Some recommend providing embedded

For BIST to be effective, there must be a means for on-chip test response measurement, onchip test control for digital and analog test, and I/O isolation. There are three categories of measurements that can be distinguished: DC static measurements, AC dynamic measurements, and time domain measurements. The first of these, DC static measurements, includes the determination of the DC operating points, bias and DC offset voltages and DC gain. DC faults can be detected by a single set of steady state inputs. AC dynamic measurements measure the frequency response of the system under test. The input stimulus is usually a sine-wave form with variable frequency. Digital signal processing (DSP) techniques can be employed to perform harmonic spectral analysis. Time domain measurements derive slew rate, rise and delay times using pulse signals, ramps or

This stage consists of a mixed and intelligent DSP system that allows for the following

Analog-to-digital conversion: provides a signal interface between the sensor outputs

 Determine fluid properties (physical and chemical): Neural and DSP algorithms as well as circuits can be used to carry out computations of fluid parameters such as dielectric constant, resistivity, spectrum, and chemical composition from the digitized sensor

 Detection and identification: The information obtained in step 2 above is fed to a microprocessor that can identify the chemical composition of the fluid and makes an intelligent decision in relation to the condition that is being monitored (water safe or not for drinking, dialysis needed or not, etc.). This can be readily programmed using

 Parameter selection and adjustment: These will be for various situations so as to include function selection to tell the sensor what to measure. In addition, the system must have the capability to compensate for deviations, detected by the built-in self test unit, of parameters such as amplifier gain, and micro-processor and neural circuit weight

In this chapter we developed a general framework for the design and fabrication of a multi-sensor system on a chip, which includes intelligent signal processing, as well as a built-in self test and parameter adjustment units. Further, we outlined its architecture, and examined various types of sensors (fluid biosensors for measuring resistivity and dielectric constant, spectral sensors, MEMS based photo-sensors, and optical microchemical and biochemical sensors), and fabrication techniques, as well as develop a transistorized bridge fluid biosensor for monitoring changes in the dielectric constant of a fluid, which could be of use for in-home monitoring of kidney function of patients with

memories with their own BIST circuitry.

**9. Smart signal processing** 

functions to be performed.

outputs.

constants.

**10. Summary** 

renal failure.

triangular waveforms as the input stimuli of the circuit.

(analog) and the signal processor inputs (digital).

look-up tables and threshold levels.

An array of FPPD is made of many individual FPPD that have different cavity thickness and therefore different range of pass band frequencies. The thickness of these oxide cavities is changed gradually in order to cover some desired range of the light spectrum. The array of FPPD can be formed in one or several columns, all entirely under the microchannel. Any light source that is transmitted through the micro channels will eventually reach these FPPD array under the channel. Each individual FPPD will react only to a small spectrum band of the light that is passed through its Fabry-Perot. Each individual FPPD is connected to the electronic circuit on the chip that will perform the signal conditioning and final post data processing.
