Design and Analysis of CMOS Photodiode for Gamma Camera Application

Nur Sultan Salahuddin1, Michel Paindavoine2, Johan Harlan1 1Gunadarma University, Jakarta, Indonesia

2LE2I Laboratory UMR CNRS 5158, University of Burgundy, Dijon, France

1. Introduction

Nuclear medicine today is an integral part of patient care and is extremely valuable in the diagnosis, treatment, and prevention of lots of medical conditions. In nuclear medicine studies, pharmaceutical radionuclides, e.g. 99mTC-Sestamibi (MIBI) are introduced into the patient's body. The radionuclides will be taken up by the destined organ in the body and emit gamma ray signals which are detected by a series of scintillation crystals of a gamma camera. The crystals convert the gamma rays to photons of light, the photomultiplier tubes (PMT) convert and amplify the photons to electrical signals, and a computer digitized the electrical signals and reconstructed them into an image to be viewed on a computer monitor (Figure 1).

Figure 1. Schematic concept of nuclear medical imaging procedure

Conventional full-sized gamma cameras use NaI (Tl) scintillator block coupled with a bulky array of PMT (figure 2), which are by the nature of their large size have been precluded from use in some clinical situations, hence in the recent years there has been a growing interest to develop compact gamma cameras to improve nuclear medical imaging procedures[1].

Figure 2. Nuclear medical imaging procedure with conventional full-sized gamma camera

The compact gamma camera uses an array of discrete scintillation crystals and a matching array of photodiodes to detect the resulted scintillation light when a gamma-ray is absorbed[2]. This scheme replaces the bulky PMT photodetectors used in conventional gamma cameras with small photodiodes, greatly reducing the camera size (figure 3).

Figure 3. Module of discrete scintillation camera

In this context we have designed a new CMOS image sensor array. We introduce the new CMOS photodiode design in the second section of this article. The model and simulation of our CMOS photodiode in VHDL-AMS are described in the third section. In the fourth section we present the fabrication and test results of the new sensor. 2. Pixel Design Description

In a standard CMOS process, several parasitic junctions can be used as photodiode, either p-well or n-well[3]. The two pixel structures (figure 4) use a photodiode formed partly hollowed n+ diffusion in n-well (vertical photodiode) or partly hollowed n+ and p+ diffusion in n-well (lateral photodiode). The photodiode is forward biased, and when incoming photons are absorbed, a photocurrent proportional to the intensity of light flows through the photodiode. This current is converted to output voltage value using current mirroring integration readout circuits[4].

Shielding

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Metal

Substrat P

Shielding

DiffP+

Metal

DiffDiffN+N+ NTUB

Contact Contact Contact Contact

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Substrate pCaisson N

Diff

N+

DiffP+

DiffN+

DiffP+

DiffN+

Diff

P+

ContactContact Contact ContactContactContactContact

Shielding

(b)

Figure 4. Two photodiode structures on p-type substrate. (a) Vertical photodiode (partly hollowed n+ diffusion in n-well);

(b) Lateral photodiode (partly hollowed n+ and p+ diffusion in n-well) 3. CMOS Photodiode model and simulation in VHDL-AMS

Photocurrent happens because incident light incites electron-hole pairs. When a potential applied on two sides of a photodiode separates the electron-hole pairs, the photocurrent is generated. Two major sources of photocurrent are: (1) from the diffusion current due to an imbalance of carrier concentration outside of the depletion region; (2) from the drift current due to the separation of electron-hole pairs within the electric field of the depletion region. The

vertical[5] and lateral[6] photodiode models are developed by analyzing the steady-state response (in one dimension) of the n-p junction diode (figure 5) to optically induced excess charge.

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Figure 5. One dimension n-p junction photodiode: (a) Vertical, (b) Lateral This photodiode model has been implemented in VHDL-AMS with the Advanced-MS simulation tool of Mentor Graphics. We defined the entity and architecture of photodiodes to simulate the response to a monochromatic light. To validate our model, we simulated the spectral sensitivities of vertical and lateral photodiodes (figure 6).

Figure 6. Spectral sensitivities of vertical and lateral photodiodes.

These curves are obtained by scanning the wavelengths. The photodiodes has to be reversed biased and a direct voltage source is therefore needed. Its entity and architecture have also been defined. These elements, i.e. the photodiode and voltage source, were implemented in the test-bench in order to simulate this simple test circuits. To obtain a dynamic response, we have defined an electrical model associated to the component (figure 7)[5]. A photodiode is a PN junction, it can thus be represented by a diode with its junction capacitance Cjunction.

Figure 7. Electrical model of photodiode.

And to validate the model of the capacitance, dynamic simulation was performed. A generator of pulses was applied across the photodiode and resistance was associated with the circuits. The periode of the signal was chosen to let the time constant of the circuits appeared, with the results shown in figure 8. The parameters used in the simulation are those given by AMS foundry for the CMOS 0.6 µm technology.

(a)

(b)

Figure 8. Dynamic characteristics of the photodiode: (a) Lateral (b) Vertical. 4. Fabrication and Test Results Figure 9 shows a layout and photograph of small test pixels implemented in a standard 0.6 µm CMOS process from AMS. The chip consists mainly of current-mirror amplifiers circuit and active pixel sensors (APS) with different sizes and structures: (a) 1 × 1 mm2 (pixels: vertical, lateral and vertical-lateral combination), (b) 400 × 400 µm2 (the four pixels: vertical).

(a)

(b)

Figure 9. (a) Layout of the detector pixel with a 2mm × 2mm area in a 0.6 µm CMOS process, (b) photograph of photodiode sensors.

The photo response of test photodiodes on the chip is obtained by measuring the photo current under illumination from 400 nm to 720 nm. Figure 10a shows response of the photodiode sensors to a monochromatic. Figure 10b shows the response of the photodiode sensors under illumination of the blue light (430 nm).

(a)

(b)

Figure 10. The response the photodiodes sensors. Figure 11 shows variations of photo current in dynamic mode. In this mode, light emitting diode is driven with a pulse generator. Results presented in figures 11a and 11b are obtained with a blue LED calibrated for a 0.5 lux. In these figures the upper curves represent the LED voltage input and the lower curves represent the sensor output.

(a)

(b)

Figure 11. Photocurrent in dynamic mode: (a) vertical photodiode 1mm × 1mm area and (b) lateral photodiode 1mm × 1mm area.

In order to measure the capacitance of the photodiodes, we use circuit shown in figure 12. in this mode, voltage source is driven with a pulse generator. The results are presented in figures 13.

Figure 12. Photodiode used in integrator mode.

Figure 13. The measurement of the capacitance photodiodes.

The voltage V discharges with the time-constant: t = R × Cd The obtained values for the photodiode capacitance are 65 pF and 35 pF for 1 mm × 1 mm and 0.4 mm × 0.4 mm respectively. Table 1 summarizes the overall measurement characteristics.

Table 1. Measurement characteristics Parameter Result

Technology

Photodetectors

Pixel pitch

Typical capacity

Fill factor

Spectral response

Power supply

Response in illumination

0.6 µm CMOS, 2-layer metal at 1-layer poly

Diffusion n+ in n-well / p substrat (vertical photodiode), diffusion n+ and p+ in n-well (lateral photodiode)

1 mm × 1 mm

65 pF (1 mm × 1 mm), 35 pF (0.4 mm × 0.4 mm)

98%

430 nm (blue)

5 Volt

Logarithmic

5. Conclusions and Perspective The CMOS active photodiode sensor and current mirror amplifier has been fabricated using a 0.6 µm CMOS process. The experimental results show that this sensor has logarithmic response to illumination and is capable of detecting very low wavelength blue lights emitting

diode. These results allow us to consider using of this technology in new solid state gamma cameras. NaI(Tl) scintillator in a conventional full-sized gamma camera produces light wavelength of 415 nm in response to gamma ray, whereas with CsI(Tl) scintillator to be coupled with CMOS photodiode to obtain a compact gamma camera design, the wavelength of spectral response to gamma ray is 540 nm[7]. In the near future we hope we can conduct further experiment to develop CMOS photodiode with maximal sensitivity to detect the wavelength of spectral response of CsI(Tl) scintillator. References [1] Choong WS, et al. A compact 16-module camera using 64-pixel Csl(TI)/Si PIN photodiode

imaging modules. IEEE Trans Nucl Sci. Vol. 49:2228-35, 2002. [2] Gruber GJ, Moses WW, Wang SE, Beuville E, Ho MH. A discrete scintillation camera module

using silicon imaging. IEEE Trans Nucl Sci. Vol. 45:1063-8, 1998. [3] Jin XL, Chen J, Qiu YL. Sensitivity and photodetector considerations for CMOS imager sensor. J

Nucl Med. Vol. 38:31P, 1997. [4] Salahuddin NS, Paiandavoine M, Ginhac D, Parmentier M, Tamda N. A CMOS image sensor array

dedicated to medical gamma camera application. In: 26th International Congress on High-Speed Photography and Photonics, September 2004, 5580-144u.

[5] Reginald J, Arora K. Using PSPICE to simulate the photoresponse of ideal CMOS integrated circuits. Proc of IEEE Southeast Conf Bringing Together Education, Science and Technology, 374-80, 1996.

[6] Moini A. Centre for High Performance Integrated Technologies and Systems (CHIPTEC), Adelaide, SA 5005, 1997.

[7] Kerek A. Detectors and nuclear electronics in medical imaging. Available from: http:// www.particle.kth.se/htmlnew/research/medicalphysics/5A1414/Jan-2003Detlab.pdf [cited on Oct 12, 2007].

Glosary CMOS (complementary metal-oxide-semiconductor): a kind of technology that is used in chips such as

microprocessors, microcontrollers, and other digital logic circuits; also in analog circuits such as image sensors, data converters, etc.

Collimator: a device that filters a stream of rays so that only those traveling parallel to a specified direction are allowed through

Gamma camera: a device used in nuclear medical imaging (nuclear medicine), to view and analyse images of the human body of the distribution of medically injected, inhaled, or ingested gamma ray emitting radionuclides

Gamma rays: forms of electromagnetic radiation or light emissions of a specific frequency produced from radioactive decay

Nuclear medicine: a branch of medicine and medical imaging that uses the nuclear properties of matter in diagnosis and therapy

Photomultiplier tube (PMT): extremely sensitive detectors of light in the ultraviolet, visible and near infrared. These detectors multiply the signal of electromagnetic radiation by an acceleration of electrons released from a photocathode through a series of dynodes; as each electron strikes a dynode stage, 3 to 4 electrons are liberated and accelerated to the subsequent dynode

Photodiode: a semiconductor diode that functions as a photodetector (diode: a component that restricts the directional flow of charge carriers)

Radionuclide: an isotope of artificial or natural origin that exhibits radioactivity. Radionuclides serve as agents in nuclear medicine and genetic engineering, play a role in computer imaging for diagnosis and experiment, and account for a percentage of background radiation to which humans are exposed

Scintillator: A substance that emits visible light when hit by a subatomic particle or X- or gamma ray