SELF-POWERED OPTICAL SENSOR SYSTEMS

H.Wu1*, A. Emadi1, G. de Graaf1, J. Leijtens2 and R.F.Wolffenbuttel1 1Delft University of Technology, Fac. EEMCS, Delft, THE NETHERLANDS

2TNO science and Industry, Delft, THE NETHERLANDS ABSTRACT

A 0.35 μm CMOS process has been used for on-chip integration of a sun sensor composed of a 2×2 photodiode array and a current-to-voltage amplifier. Unlike conventional sun sensors, a shade profile proportional to the angle of incidence of incoming light is projected onto the photodiodes. This concept enables an autonomous self-powered optical system with two the main functions (electrical power generation for the amplifier and the optical position measurement) implemented in the photodiodes by having these operated simultaneously in the photovoltaic and photocurrent mode respectively. The low-power current-to-voltage converter is used to readout the differential photocurrent, while powered from the photodiodes at minimum supply voltage level. Test structures have been designed, fabricated and used for validation of the concept.

KEYWORDS

Energy scavenging, Self-powering, optical system, sun sensor, photodiode INTRODUCTION

Powering of electronic microsystems by batteries has become a bottleneck in the design of highly miniaturized autonomous sensor systems. Limitations are due constraints imposed by volume, weight, maintenance requirements and reliability of power sources in harsh environment [1]. Energy scavenging in combination with low-power circuit and system design has been identified as a promising solution to this problem. Energy scavenging concepts proposed are usually based on extracting electrical energy for vibration [2], temperature gradients using thermo-electric devices [3] or rely on optical energy available and exploited using solar cells.

An application that is served well using optical energy is the sun sensor, which is widely used for attitude control in satellites. With the increased interest in micro-satellites comes the interest in a micro sun-sensor. The sun sensor provides information on the attitude of the satellite with respect to the sun. Since the main interest in micro-satellites originates from the possibility of having multiple satellites operate in a cluster (a swarm), position information is crucial.

The sun sensor is composed of a position sensitive detector (PSD) or an array of photodiodes. Power is typically supplied from the main solar panel or a small dedicated solar cell mounted on the sun sensor [4].

A semiconductor pn-junctions is basically operated either in the photovoltaic mode (solar cell) for electrical power generation or in the photocurrent mode (photodiode) for opto-electrical signal conversion. The concept presented in this paper aims at combining these two modes of operation in order to enable the fabrication of self-powered optical sensor systems. The ability of CMOS processes with a feature size of 0.35 μm and smaller to be operated using a supply voltage at 1V is an essential enabling factor for this concept. The reduced power supply voltage with reduced feature size is usually considered a disadvantage in analog circuit design, as it reduces the dynamic range at a given noise level. However, it does enable direct electrical powering of the circuit from a set of two photodiodes.

The operating principle as compared with the conventional sun sensor is described in detail in the next part. The chip with photodiodes and current-to-voltage converter is designed and implemented in CMOS 0.35 μm process technology. The experimental result shows the differential current between photodiodes can reflect the shadow imposed on photodiode where a needle is on top of chip. Conclusions are drawn in the final section along with an outline of future work.

OPERATING PRINCIPLE The Conventional Sun Sensor

A conventional analog sun sensor measures the position of a satellite relative to the sun by projection of the sunlight through a hole (aperture) in a shield onto a PSD or a 2×2 array of detectors, as shown in Fig. 1. Digital sun sensors have also been investigated using a full CMOS imager and signal processing to calculate the sun position from the image [5]. The position of the projected light spot depends on the angle of incidence and, hence, on the satellite position relative to the sun. This information is used for directing an antenna or the solar panel or by the internal navigation system for attitude control. Since only a light spot is projected, only one photodiode is actually illuminated. The other diodes remain essentially idle. Therefore separate solar cells are required for electrical powering this optical system. Although die area in a small feature size CMOS process should be considered expensive real estate for implementing a solar cell, the benefits of small size and small weight outweigh such concerns when also considering the expenses of launching additional mass into space. Moreover, the loss of expensive die area is minimized when using the same diodes for powering and sun position detection, as is pursued in this project.

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Figure 1: The conventional sun sensor. Sundial Principle Based Sun Sensor

The approach requires a partition wall (pole) in between 4 pn-junctions for projecting a shade profile proportional to the angle of incidence of the sun on the photodiodes. As shown in Fig. 2, all detectors are illuminated, thus can be used for electrical power generation and the position information is simultaneously available in the differential photocurrent [6].

Figure 2: Sundial principle applied to sun sensor. DESIGN AND FABRICATION

CMOS processes with 0.35 μm feature size or less are operated at supply voltage levels as low as 1 V. This is often considered a limitation, because of the achievable dynamic range of the analog integrated circuits, but is an enabling factor in optical self-powering. Two photodiodes in series together with current-to-voltage converter are designed into one unit cell. Two of these are required in the 2×2 detector array configuration as shown in the Fig. 2. The differential photodiode is detected when the shadow is projecting on

either surface of photodiodes while the series equivalent photodiode voltage and current is powering the readout circuit. The system concept is shown in Fig. 3. The AMIS 0.35 μm CMOS process has been used to realize the chip. The photograph of die is shown in Fig. 4. The photodiode area is 550×350 μm2. With this dimension of photodiode, the photocurrent is large enough for electrical powering of the amplifier. IC compatible micromachining for fabrication of the pole has not yet been included in the initial devices. The package has been modified to include a pattern to cast a shade on the diode array in the initial experiments.

Figure 3: Configuration of each subset in the system.

Figure 4: Photograph of the die. EXPERIMENTS Photodiode Characterization

The spectral response of one diode is shown in Fig. 5. The figure shows the different spectral responses of the diode formed by the p+top layer and the n-well as compared to that of the n-well-p-substrate diode illustrated in Fig. 5(a) and confirms the shift of maximum responsivity to longer wavelength of the latter type of photodiode [7]. The measurement is done by the HORIBA Jobin Yvon TRIAX-180 monochromator with a wavelength resolution of 0.3 nm. The 5 μm passivation layer makes the ripple

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which resonant along the spectral curve. Figure 6 shows the photovoltaic performance of diodes for three different illumination intensities. The maximum power point is at 600 mV in case of a 7.6 μA load (115 W/m2 illumination) and drops to 400 mV in case of an 8.3 μA load.

(a)

(b)

Figure 5: (a) Two stacked diodes in the CMOS process and (b) Spectral response of the two stacked diodes.

(b)

Figure 6: The photovoltaic performance of diode at three different illumination intensities.

Measurements on the system

Figure 7 shows the circuit schematic of the self-powered system. The required minimum voltage and current to drive the commercial off-die amplifier is 1V and 9 μA. The requirement is even critical on the amplifier built on the die. Two photodiode in series can just make the demand of power for amplifier. As a result, it indicates that a step-up converter is required, which should also designed for operating the system at the maximum power point in the photovoltaic operating mode of the photodiode [8].

The preliminary measurements confirm proper operation of the self-powered position sensor for one-dimensional position measurement using two diodes, which are connected to the readout circuit for electrical powering. The two diodes are separated by 36 mm. The needle with 0.75 mm diameter was put in between the light source and diodes to form a shadow. The distance between the needle and the diodes is 10 mm. The measurement result is shown in Fig. 8. The change in output voltage with the position of the needle is in reasonable agreement with the expected linear response.

Figure 7: Schematic of the system.

Figure 8: Measurement result of two diodes with moving shadow. CONCLUSION AND FUTURE WORK The new concept of the self-powered sun sensor has been

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presented and the operation has been experimentally validated. The sensor and basic analog readout circuit have been integrated in the AMIS 0.35 μm CMOS process. The first experimental results confirm operation. However, proper operation is achieved only at relatively bright exposure. The power budget is critical and will become more demanding with the implementation of more sophisticated circuits and telemetry.

On-going research is aiming on the investigation of the minimum diode at a realistic power budget. The implementation of a step-up converter with a maximum power tracking circuit are also considered in a next design in order to operate at maximum output power of the photovoltaic operating mode of photodiode.

Moreover, the fabrication of a MEMS pole in between 4 pn-junctions for projecting a shade profile proportional to the angle of incidence of the sun on the photodiodes will be included as a CMOS-compatible post process. ACKNOWLEDGEMENTS

This work has been supported in part by the Dutch technology foundation STW under grant DET.6667 and in part by MicroNed project MISAT 1C2. REFERENCES [1] P. Niu, P.L. Chapman, R. Riemer, and X. Zhang,

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[3] L. M. Goncalves, C. Couto, P. Alpuim and J. H. Correia, “Thermoelectric micro converters for cooling and energy-scavenging systems”, J. Micromech. Microeng., 18, 2008.

[4] C. W. de Boom, J. A. P. Leijtens, L. M. H. V. Duivenbode, and N. van der Heiden, “Micro digital sun sensor: System in a package”, in Proc. ICMEMS, NANO and Smart Syst., Aug. 25-27, 2004, pp. 322-328.

[5] N. Xie, A. J. P. Theuwissen, X. Wang, J. A. P. Leijtens, H. Hakkesteegt and H. Jansen, “A CMOS Image Sensor with Row and Column Profiling Means”, IEEE Sensors Conference, pp. 1356-1359, 2008.

[6] R.F. Wolffenbuttel, G. de Graaf and J.A.P. Leijtens, “Sensor voor standbepaling”, NL patent 2000789/ NL 47.432, 30 July 2007.

[7] M.L. Simpson, M. Nance Ericson, G.E. Jellison, W.D. Dress, A.L. Wintenberg and M. Bobrek, “Application specific spectral response with CMOS compatible photodiodes”, IEEE Transactions on Electron Devices,

46, 905–913, 1999. [8] Y. Ramadass and A. Chandrakasan, “Minimum Energy

Tracking Loop With Embedded DC-DC Converter Enabling Ultra-Low-Voltage Operation Down to 250 mV in 65 nm CMOS Solid-State Circuits”, IEEE Journal of SOLID-STATE CIRCUITS, 43(1), pp.256-265, 2008.

CONTACT * H. Wu, tel: +31-15-27-86285; h.w.wu@tudelft.nl

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