**5. MEMS based photo-sensors**

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

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 the Fabry-Perot cavity.

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

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

VLSI Design for Multi-Sensor Smart Systems on a Chip 13

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

. . . Series of fabry-perots w ith

p N

p N

Series of p-n photo detectors <sup>P</sup> <sup>P</sup> P N

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

. Flow

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

different cavity sizes

p . N . .

p . N .

Glasses to form flow channel

light sourse

analyte concentrations.

Sample flow s in the channel

detectors

FPPDM

<sup>N</sup> <sup>N</sup>

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).

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

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