**4. Spectral sensors**

8 VLSI Design

VD1 = Vdd-|Vthp|-(3/1}1/2(Vset-Vthn) (3.6)

VD2={Vthn+(.2/4)1/2(Vdd-|Vthp|)/[1+.2/4]1/2} (3.7)

Balance is obtained by setting VD1=VD2. Still assuming that M3 is in saturation the value of

 Vset=Vthn+{(1/3})1/2(Vdd-|Vthp|-Vthn)/[1+(.2/4})1/2} (3.8) At this point we can check the condition for M3 to be in saturation, this being that VDS

 Vthn < Vset{sat} Vthn+(Vdd-|Vthp|)/[1+(3/1)1/2 ] (3.9) Substituting the value of Vset at balance, Equation (3.8), shows that the condition for M3 to be in saturation at balance is 2 3; this normally would be satisfied but can be

Several things are added to the sensor itself per Fig. 2.1. Among these is a differential pair for direct current mode readout followed by a current mode pulse coded neural network to do smart preprocessing to insure the integrity of the signals. Finally a built in test circuit is

From the layout of Fig. 3.1 a Spice extraction was obtained. On incorporating the BiCMOSIS transistor models (Sellami & Newcomb, 1999; Moskowtitz et al, 1999) the extracted circuit file was run in PSpice with the result for the output difference voltage versus Vset shown in Fig. 3.4. As can be seen, adjustment can be made over the wide range of -5V<Vset<5V

Thus, it is seen that a sensor that is sensitive to the dielectric constant of a fluid over an 11 to 1 range of dielectric constant most likely can be incorporated into a multi-sensor chip. Using standard analog VLSI-MEMS processing one can use the bridge for anomalies in a fluid by obtaining Vset for the normal situation and then comparing with Vset found for the

Vset needed to obtain balance is obtained from equations (3.6) and (3.7) as

VGS-Vthn; since VDS=VD1 and VGS = Vset, the use of Equation (3.6) gives

while irrespective of the state of M3

Fig. 3.4. Extracted circuit output voltage versus Vset

guaranteed by making M2 large enough.

included to detect any breakdown in the sensor operation.

We take advantage of the developments in MEMS technologies to introduce new and improved methodology and engineering capabilities in the field of chemical and biochemical optical sensors for the analysis of a fluid. The proposed device has the advantages of size reduction and, therefore, increased availability, reduced consumption of chemical/biochemical sample, compatibility with other MEMS technologies, and integrability with computational circuitry on the chip.

Consequently, integrating MEMS and optical devices will give the added advantages of size reduction and integrability with the electrical circuitry. The integration and compatibility of sensors is very much in demand in the field of system on a chip. Here we extend CMOS technology to build an optical filter which can be used in a single chip microspectrometer. The chip contains an array of microspectrometer and photodetectors and the read out of their circuits.

By the nature of matter in the universe, most evident at the atomic and molecular level, it allows so much information to be deduced from its optical spectra. Because molecule and atoms can only emit or absorb photons with energies that correspond to certain allowed transition between quantum states, optical spectroscopy is one of the valuable tools of analytical chemistry (Schmidt, 2005). Optically based chemical and biological sensors are conveniently classified into five groups, according to the way light is modulated (Ellis, 2005). These light modulations are intensity, wavelength, polarization, phase, and time modulation. Here we focus on MEMS based sensors suitable for Intensity, wavelength, and time modulation.

#### **4.1 Intensity modulation**

As light passes through a material, its intensity attenuates as it interacts with the molecules, atoms, and impurities of the host material. The attenuation is an exponential function of the

VLSI Design for Multi-Sensor Smart Systems on a Chip 11

Time modulation is essentially a subclass of intensity modulation. In time domain fluoremetry (TDF), a pulsed light source generates the photoluminescence. The fluorescence decay signal is measured as a function of time, and the decay curve determines the lifetime of the chemical sample. In time modulation base sensors measure the halftime of the sample.

Time

of the Fabry-Perot is a function of its cavity length (Patterson, 1997).

Fig 5.1. The Fabry-perot etalon with AI bottom Mirror

Fig. 4.3. (a) Fluorescence decay curve (b) corresponding schematic for measurement

An important part of any spectrometer, aside from the light source, is the optical filter and photo detectors. Recent engineering developments in the field of MEMS and microelectronics have shown that both of these devices can be produced in the micro level using existing technology (Hsu, 2008). Optical spectrometers can be produced using a tunable Fabry-Perot cavity (here simply called Fabry-Perot). The band-pass frequency range

Fabry-Perot can be fabricated in the CMOS technology with photo-detectors integrated underneath it. In other words, Fabry-Perot is fabricated on top of a p-n diode in the CMOS technology. In this configuration, the p-n photo-detector is acting as a transducer that converts optical intensity of light that is passed through the Fabry-Perot to a proportional electrical signal. The existence of the Fabry-Perot in the optical path causes the photodiode to only respond to the light intensity of selected wavelength, which is set by the thickness of

As illustrated in Fig. 5.1 below, the fabrication of Fabry-Perot and photodiode (FPPD), which starts with the fabrication of a p-n photo diode in a CMOS process technology, undergoes a post process in order to integrate a planer Fabry-Perot on top of the p-n photo diode. This process involves four steps. First, a portion of the top oxide layer immediately above the p-n diode is trimmed, by chemical itching, to reduce its effect on light

Ag (silver), 45nm

18

Al (aluminuim), 20nm

SiO , 300nm <sup>2</sup>

(b)

Puls ed light source

X

450 500 550 700 nm

Lamda

Transmission (%)

Sample Filter Detec tor

**4.3 Time modulation** 

Inte nsity

**5. MEMS based photo-sensors** 

the Fabry-Perot cavity.

cavity

distance of its path length, x, traveled in the material. The absorption coefficient, , is defined relative to the concentration, M, and the cross section, S, of the absorbing molecules (Svanberg, 2001).

$$\mathbf{I}\_{k}(\mathbf{x}) = \mathbf{I}\_{k}(\mathbf{0}) \text{ . } \exp \left( \mathbf{u}\_{k} \mathbf{x} \right) = \mathbf{I}\_{k}(\mathbf{0}) \text{ . } \exp \left( \mathbf{-S} \mathbf{M} \mathbf{x} / \mathbf{N} \right) \tag{4.1.1}$$

Where I(x) is the light intensity at distance x, I(0) the incident light intensity at x = 0, and N Avogado's number (6.022 x 1023 mol-1).

Changes of the analyte concentration in the sample can alter the absorption coefficient . An absorption based sensor measures these changes by the transmitted light intensity in terms of absorbance (A) units:

$$\mathbf{A}\_k = \log[\mathbf{I}\_i(\mathbf{0})/\ I\_i(\mathbf{x})] \tag{4.1.2}$$

#### **4.2 Wavelength modulation**

Wavelength modulation can provide us with more information than just the intensity modulation. Several numbers of fixed wavelength sources are used simultaneously and their responses, intensity, are detected using photo detectors. Several sources that are modulated at different electrical frequencies can be used simultaneously in order to use a single photo detector. One of the wavelengths could serve as a reference channel for calibration.

Fluorescence occurs when an atom or a molecule makes a transition from a higher energy state to a lower one and emits lights. Excitation and subsequent emission can occur not only by photoluminescence but also by chemical reaction (chemiluminescence) or biological reaction (Bioluminescence). In resonance fluorescence, absorption and emission take place between the same two energy levels, and therefore the wavelength of the excitation and emission lights are the same. In non-resonant fluorescence, emission occurs either at higher wavelength than excitation wavelength (Stokes Fluorescence), or lower wavelength than excitation wavelength (anti- Stokes Fluorescence). The decay rate d*N* /d*t* of the fluorescence for a two level system is

$$\text{dN}^\*/\text{dt} = \text{ $k\_t$ } \dots \text{N}^\* \tag{4.2.1}$$

where *k*t is the total fluorescence rate, in sec-1, and *N* is proportional to the number of electrons excited due to the fluorescent state in a time t. Hence

$$N^\* = N\_0^{"\prime}. \ \exp(\cdot k\_{\cdot}.t) \tag{4.2.2}$$

Fig. 4.2. (a) Attenuation of the optical intensity as it travels along the x axis throught the matter versus the wavelength. (b) corresponding schematic for measurement.
