CMOS Image Sensor

1. CMOS Image Sensor Prepared By: Brad Crook ECE 5320 Mechatronics Assignment #1

2. Sensor Outline • References • Major Applications • Basic Working Principle • Sample Configuration • Major specifications • Limitations • Best Choice For applications • Cost values

3. References • Marc J. Loinaz, Kanwar Jit Singh, Andrew J. Blanksby, David A. Inglis, Kamran Azadet, and Bryan D. Ackland, “A 200-mW, 3.3-V, CMOS Color Camera IC Producing 352x288 24-b Video at 30 Frames/s,” IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 33, NO. 12, DECEMBER 1998. • Andrew J. Blanksby, and Marc J. Loinaz, “Performance Analysis of a Color CMOS Photogate Image Sensor,” IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 47, NO. 1, JANUARY 2000. • Stuart Kleinfelder, SukHwan Lim, Xinqiao Liu, and Abbas El Gamal, “A 10 000 Frames/s CMOS Digital Pixel Sensor,” IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 36, NO. 12, DECEMBER 2001. • Meynants, Guy; Dierickx, Bart; and Scheffer, Danny, “CMOS active pixel image sensor with CCD performance,” SPIE/EUROPTO AFPAEC conference Hotel Movenpick, Zurich Switzerland, 18-19 May 1998.

4. References • Stuart Kleinfelder1, Fred Bieser, Yandong Chen, Robin Gareus, Howard S. Matis, Markus Oldenburg, Fabrice Retiere, Hans Georg Ritter, Howard H. Wieman, Eugene Yamamoto, “Novel integrated CMOS pixel structures for vertex detectors,” Lawrence Berkeley National Laboratory, 2003. • Stuart Kleinfelder, “High-speed, high-sensitivity, low-noise CMOS scientific image sensors,” University of California, Irvine • http://rocky.digikey.com/WebLib/Kodak/Web%20Data/KAC-9630LongSpec.pdf • http://www.fillfactory.com

5. Major Applications • The first and foremost application that comes to mind is the digital camera. “Courtesy www.superwarehouse.com”

6. Major Applications (cont.) • These cameras have become extremely popular in both the CMOS type and in the Charge-Coupled Device, CCD, type. • Due to the compactness of CMOS circuits digital cameras have recently been growing smaller with more resolution.

7. Major Applications (cont.) • They have grown so small that they are able to fit digital cameras into button holes. “Courtesy www.business-home-security.com”

8. Major Applications (cont.) • CMOS digital cameras have also found popularity in the easily accessible, and readily available web camera. “Courtesy of www.yopi.de” “Courtesy of Greg Falcon”

9. Major Applications (cont.) • These devices are also becoming fast enough to store live video into a small, palm sized, digital camcorder . “Courtesy of www.amazon.com”

10. Basic Working Principle • Now we know where CMOS Image sensors can be found, next we must find out how they work. • I will do this by • Explaining the photodiode • Then the MOSFET transistor • Next would be the photogate • Then the Image Sensor array • And finally the image processing

11. - N-Type + P-Type Basic Working Principle (cont.) • A photodiode works in much the same way as a solar cell. • It uses two differently charged elements to chemically dope silicon wafers to create a N-Type and P-Type semiconductor.

12. - N-Type + P-Type Basic Working Principle (cont.) • When light hits the semiconductor it allows free electrons to move through the negatively doped side, N-Type, to the positively doped side, P-Type, creating a current. - Light - -

13. - N-Type + P-Type Basic Working Principle (cont.) • The strength of the light determines how much electricity is produced. • This is called photo-voltaic. - Light - - - - - -

14. Basic Working Principle (cont.) • The Amplitude of the current can be converted into a digital representation. • This Digital number represents the brightness of the light, and it is easily recorded. • A second way to open a path for current is with a MOSFET device.

15. Basic Working Principle (cont.) • The Complementary Metal Oxide Semiconductor Field Effect Transistor (MOSFET) works like a switch made from diodes. • Instead of applying light to produce current we use a electric field to produce a roadway, or channel for electricity to move.

16. P-Type N-Type N-Type Basic Working Principle (cont.) • In a N-Type MOSFET, two negatively doped regions are separated by a positively doped region.

17. P-Type N-Type N-Type Basic Working Principle (cont.) • When an electric field is placed on a gate between the two N-Type regions a channel for current to move through is produced and electricity moves through it, turning it ON. Current in Electric Field Gate Current out

18. P-Type N-Type N-Type N-Type P-Type P-Type Basic Working Principle (cont.) • At this stage the transistor is considered on. • To make it a Complementary MOSFET, or CMOS, a design with two P-Type wells separated by the N-Type well is added to the first design.

19. Basic Working Principle (cont.) • This additional P-Type transistor works in the exact opposite of the N-Type. • If there is an electric field between the P-Type wells then the transistor turns OFF otherwise it is ON.

20. Basic Working Principle (cont.) • With an understanding of a photo diode and a transistor we now look at a photogate. • A photogate is a transistor that works like a photodiode. • When the photogate of the transistor detects light then the transistor turns on.

21. Basic Working Principle (cont.) • This photogate, with its accompanying CMOS logic is called a pixel • A pixel can be made of either a photodiode or a photogate

22. Basic Working Principle (cont.) • After the pixel emits the proper reading it is up to peripheral logic to drain the column capacitors, and convert the stored energy from an analog to digital signal. • Some image sensors treat the output as a digital signal directly.

23. Basic Working Principle (cont.) • The computative hardware and software then converts the digital signal into a picture and compresses it into a format like JPEG. • To explore further a look at the configuration of the pixel is needed.

24. VDD Reset Pout Photogate Output P-substrate N-Well Row Select Basic Configuration • A pixel looks something like this:

25. Basic Configuration (cont.) • A typical fabrication layout of a pixel Andrew J Blanksby

26. Basic configuration (cont.) • This configuration is called an active image sensor. • It is considered active because when the row select is high and the reset is low then the output is a value close to VDD. • This value then gets stored into a capacitor on the column select line.

27. Basic Configuration (cont.) • To create a color photograph a grid of primary color filters are set on top of designated pixels. • The voltage value of each pixel will then designate the color of the image.

28. Basic Configuration (cont.) • Most of the configurations are in matrices of close to 1 million pixels to 8 million pixels. • Each row and column can be selected to store the data using external hardware. • With so many pixels specifications are needed to determine the quality of the sensor.

29. Major Specifications • Other major specifications used on a CMOS image sensor are: • Resolution – How many pixels are in a matrix. This can be from just a few to 10 million. • Pixel Size – Tells how large the pixel is, usually measured by microns.

30. Major Specifications (cont.) • Array format – How many vertical lines there are compare to horizontal lines. It is useful for display properties. • Sensitivity – Declares how many output volts the photodiode, or photogate will produce when struck by so many lumens of light. • Read Time – How much time it takes to read all of the pixels.

31. Major Specifications (cont.) • Noise level – How many bits may get messed up due to noise in the system. • Power Consumption – How much power the matrix will use during sleep mode or low power mode. • Outputs – How many I/O lines there are to communicate with the processor.

32. Major Specifications (cont.) • Other specifications are self explanatory like shutter speed, frame rate, and input voltage.

33. Limitation (cont.) • A problem now arises with all of these pixels squished into an array. • As the resolution increases, the photosensitive pixel area decreases. • This gives rise to the fill factor, or how much light is hitting the photogate when exposed.

34. Limitation (cont.) • Fill factor is a problem because an under lit pixel will not put out enough voltage to open the transistor gate onto the output. • This causes many CMOS image sensors to work poorly under low light. • Smaller transistor sizes and better layouts have helped elevate this problem.

35. Limitation (cont.) • Another problem with CMOS image sensors is the signal to noise ratio. • Due to such low levels of voltage that the sensor is working with, it is hard to discern if light has hit the photogate, or if it has not. • On the other hand it keeps the power consumption down to a minimum.

36. Best Choice For Applications • The best use for a CMOS image sensor would be for low power, compact devices. • These are devices that cannot use the power or room that a CCD uses.

37. Best Choice For Applications (cont.) • Some of these compact applications are as follows: • Cell phones with cameras • Web cameras • Motion detectors • Button cameras • IR detectors • And more

38. Cost Value • Though the CMOS is as cheap as an integrated circuit the application is what really drives the cost. • High priced applications are those that need color, high resolution, low noise, fast pictures, and lots of peripheral doodads. • Low cost applications are those that are grainy, monotone, and slow.

39. Cost Value • Some of the higher end CMOS imagers, with over 8Mega pixels cost as follows: • Canon EOS 1D 17.2 Mega Pixel • $5,900 – $8,497 • Nikon D2X 12.84 Mega Pixel • $5,000 – $5,270 pricing from www.shopping.com

40. Cost Value • Some of the Lower end CMOS imagers, with under 1 Mega pixels cost as follows: • Digital Peripheral SolutionsQS513K 0.35 Mega Pixel • $11 • Videoman DPC-320 0.1 Mega Pixel • $25 pricing from www.shopping.com