SPECTRACAMTM – Random Access Charge Injection DeviceCameras for Spectroscopy

Suraj Bhaskaran, Herb Ziegler, Claudia Borman, John Swab, Carey Beam, Tony Chapman, JohnCapogreco, Steve VanGorden, Mark Greco, Matt Pace, Zulfiqar Alam, Jon Miles, Mike Pilon,

Joe CarboneSpectra-Physics, Cameras and Imagers, Liverpool NY 13088

andHelen Jung, Rindy Finney, Linda Lee, Victor Tsai, and Arnold London

Supertex inc, Sunnyvale CA 94089and

Bruce MeyersOneida Research Services, Whitesboro NY 13492

ABSTRACT

We designed, built, and tested the SpectraCAM84/86, a family of scientific Random Access Charge InjectionDevice (RACID) cameras for analytical spectroscopy and quantitative imaging.

The cameras feature the RACID84/86 imagers, which are CMOS CID image sensors with true random pixelselection, integration, and readout. We developed CMOS CID technology in order to integrate high transparencyRACID pixel arrays with CMOS on-chip camera circuitry. Pixels are 27-micron x 27-micron square. The RACID84contains 1026x1026 pixels. The RACID86 has 540x540 pixels. Pixel data is read out on-chip with low noisepreamplifier-per-column FETs. Image readout speed is selectable: the slow scan runs at 50kHz and the fast scan runs at200kHz.

Dark current is reduced using a three-stage thermoelectric cooler and water recirculating system. Typicalimager operating temperature is -30C to -45C. The SpectraCAM converts integrated pixel charge data into 16-bit digitaloutput. Using nondestructive readout, the linear dynamic range of the camera is 107.

Spectral response extends from 165 nm to 1000 nm. RACID86 UV responsivity is 85-90 ma/watt at 350nmand 57-63 ma/watt at 200nm for illumination with an Oriel 30-watt deuterium lamp. RACID84 UV responsivity is 71-89 ma/watt at 350nm and 40-50 ma/watt at 200nm.

Keywords: CID, CMOS imaging, spectroscopy, UV sensors, scientific cameras

1. INTRODUCTION

Quantitative Imaging is the ability to take pictures and record quantitative optical information in addition to theimage. From astronomy to earth science, from forensic analysis to pharmaceutical biochemistry, modern science andtechnology rely on the methods of quantitative imaging. To serve the needs of scientific and technical markets forquantitative imaging cameras, we designed and built the SpectraCAM84/86.

The SpectraCAM84/86 is a line of scientific cameras with true random pixel selection and integration. At theheart of the SpectraCAM is the RACID84/86 imager. The Random Access Charge Injection Device (RACID) imager isa solid-state imaging device similar to CCD and CMOS image sensors. RACID84/86 imagers are silicon CMOSintegrated circuits with large area RACID pixel arrays and the associated on-chip operating circuitry.

The SpectraCAM controller uses a 16-bit gray scale to present images. The RACID exposure software iswritten for PENTIUMTM CPU/PC104+ computer architecture. Exposure time is automatically varied from pixel to pixelon the basis of real time signal intensity. Weak signals are integrated for long periods while intense signals areintegrated over several short integration cycles. This method, called Random Access Integration (RAI), makes possiblethe extended 107 linear dynamic range of the SpectraCAM. By using the RAI technique with selective region of interestclearing, weak optical signals can be captured even in the presence of very intense light sources.

2. DESIGN AND OPERATION

The RACID84/86 imagers contain X/Y addressable pixel arrays. Each pixel (picture element) contains two MOScapacitors, as shown in figure 1. Each MOS capacitor is formed between the polysilicon top electrode and the singlecrystal epitaxial silicon bottom electrode, separated by the SiO2 gate dielectric. One MOS capacitor is the row chargestorage capacitor. It is addressed by the row circuitry. The other MOS capacitor is the column charge sense capacitor.

The sense electrode is addressed by the columncircuitry. A metal column bus connects thepolysilicon sense electrodes of all pixels in onecolumn. Another metal bus connects thepolysilicon storage electrodes of all pixels in onerow. The full level, 4-pixel layout of the RACID84is shown in figure 2.

Pixel layouts use minimum allowedgeometries as much as possible in order to keep thepixel transparency high. The column bus isconnected to the polysilicon sense gate of a dual-gate readout FET. The sense gate of the dual-gateFET is strapped with metal to reduce the resistanceof the sense gate and the associated Johnson(thermal) noise. There is one readout FET at theend of each column to integrate and amplify thecharge collected in the pixels. This is called thepreamplifier-per-column architecture. It was firstdescribed and implemented on PMOS CID imagers

Figure 1: RACID84 Pixel MOS capacitors by Gerald Michon at General Electric (1-3).

Preamp-per-row (PPR) architecture is used in the PMOS CID SIcamTM, a PC-based scientific instrumentation cameradeveloped at CID Technologies (4). An off-chip summation amplifier with a feedback loop tied into each columnpreamplifier FET can be used to lessen the sensitivity of the output to the individual amplifier gain differences (5).

The RACID84/86 dual gate voltage amplification readout FET is shown in figure 3. Contact between thepolysilicon sense gate and the metal strap is made over the field oxide region and is not shown in figure 3. The currentdesigns use very large preamplifier MOSFETs: 800 microns Wg x 2.0 microns Lg. The preamplifier-per-columnschematic is shown in figure 4.

RACID84/86 pixels are 27-micron x 27-micron squares. The 1M-pixel RACID84 is built using 2.0-micronCMOS CID technology. It contains a 1026x1026 pixel array. The RACID86 imager is built using 1.2-micron CMOSCID technology. It contains a 540x540 pixel array. The CMOS pixel control circuitry is located on two sides of thepixel array, as can be seen in figure 5.

The block diagram of the RACID84/86 is shown in figure 6. On-chip logic is designed to simplify the cameracontrol interface. The chip architecture has six main building blocks: the serial-to-parallel shift registers, the up/downaddress counters, the TTL level translators, the address decoders, the address latches, and the electrode multiplexers.There are two serial ports for loading X-Y pixel address codes. Row and column addressing is accomplished with the11-bit serial-to-parallel shift registers, address decoders, and electrode multiplexers. Vertical and horizontal latches holdthe row and column addresses for all subarray functions, including pixel binning and subarray injection. User-definedregions of interest within the pixel array can be selected with independent horizontal and vertical control. Pixels in anyregion of interest can be binned for collective readout, in groups of pixels up to 4 columns and up to 4 rows in size.Individual pixels or groups of pixels can be chosen and read out in any order. Image readout speed is selectable: theslow scan runs at 50kHz and the fast scan runs at 200kHz.

Read noise and camera conversion gain are determined by the mean variance method (6-7). The light sourceused for the test is a low power 598-nm LED. The LED is positioned 15 mm behind a 500-micron diameter pinhole. Asmall area of 12 pixels x 12 pixels (324microns x 324microns) on the imager is selected for test. The imager isilluminated by flashing the light source for a short time, typically 1-2 msec. The resulting image is recorded and cleared.

Column PolySiSense

Electrode

Row PolySiStorage

Electrode

Epitaxial SiliconContact

GateOxide

The imager is then illuminated again, with a second flash of equal duration. The second image is subtracted from thefirst, to remove the spatial fixed pattern noise due to pixel-to-pixel variation in response. The resulting difference array

represents the temporal noise ofthe system. The differencearray is then averaged to obtainthe mean and the standarddeviation. The process isrepeated for increasingexposure levels, from 1 flash to9 flashes. The mean andstandard deviation arecalculated for each differencearray. The variance, or squareof the standard deviation of thedifference array, is plottedagainst the signal mean, inanalog-to-digital units (ADUs),as shown in figure 7. The best-fit line is calculated for the data.The slope of the resulting plot isthe system gain incarriers/(ADU). The y-interceptis the square of the read noise.

3. IMAGERAND

CAMERACONSTRUCTION

The RACID84/86 imagersFigure 2: RACID84, full layout of 4 pixels were designed at Spectra-Physics, Cameras

and Imagers. The imager designdatabases were sent to Supertex inc,Wafer Foundry Services, for silicon waferfabrication. Photolithographic masks weremade at Photronics, with direction fromSupertex CAD and Spectra-Physics imagerdesign groups. Wafers were fabricated onthe 150-mm diameter silicon line atSupertex inc in San Jose CA.

The RACID84/86 imagers aredesigned for double polysilicon, doublemetal CMOS CID technology. Startingmaterial is 5 ohm-cm, 28-micron thick, n-type epitaxial silicon on p-type substrates.The CMOS CID processes integrate twin-well CMOS logic circuitry with polysiliconphotogate pixels built in the higherresistivity, native n-type epitaxialsilicon. Active MOS photogates are made

Figure 3: Dual Gate Readout FET in both polysilicon layers.

RACID 84 Dual Gate FET

sourcedrain sensegate

selectgate

metalsense gate

strap

n-well

Epitaxial SiliconContact

Column PolySiSense Electrodestrapped with

Metal 1

Row PolySiStorage

Electrodestrapped with

Metal 2

GateOxide

Borophosphosilicate glass (BPSG)reflow planarization and photoresistetchback planarization are used.

The RACID84 imager is laidout on a 2.0-micron design rule.Minimum geometries for poly 1 FETgates, contacts and vias are 2.0 microns.The 1-M pixel RACID84 imaging arrayis 27.7mm x 27.7mm square. TheRACID84 imager die size is 32.3mm x31.8mm. Because of the large die size,which exceeds the field size of moststeppers, the RACID84 is built usingthe SVG700 projection aligner.

The RACID86 imager is laidout on a 1.2-micron design rule.Minimum geometries for poly 1 FETgates, contacts, and vias are 1.2microns. The 290-K pixel RACID86imaging array is 14.6mm x 14.6mmsquare. The RACID86 imager die sizeis 19.1mm x 18.5mm. Photolithographyis done using the Canon5X stepper. Tosupport the voltage requirements of thedesign, all MOSFETs on the 1.2-micron

designFigure 4: Preamplifier-per-Column Schematic incorporate lightly doped drain (LDD)

structures. LDD spacers are removed from the pixel polysilicon photogates. SEM cross-sections of the RACID86imager are shown in figure 8.

Imager wafers are probed at Spectra-Physics, Cameras and Imagers. SpectraCAM control electronics are integratedwith custom probe cards and PENTIUMTM CPU/PC104+ computers so that 16-bit gray scale images are produced at thewafer probe workstation. Wafers with defect-free imagers are sawed and imager die are separated on the saw membrane

by stretching the membrane with an underside inflation method.Imagers are attached to gold-plated ceramic substrates and

wire-bonded to the imager specific interface (ISI) electronicsboard. The imager-ceramic unit is mounted on the three-stagethermoelectric cooler.

The SpectraCAM block diagram is shown in figure 9. Infigure 10 is the expanded CAD drawing. There are five basicbuilding blocks for the SpectraCAM camera controller: thecentral processing unit (CPU), the timing signal processing(TSP) board, the analog signal processing (ASP) board, theedge input/output (I/O) board, and the thermoelectric coolingdewar assembly which contains the imager. The SpectraCAMcan be used in either a purged head or a sealed headconfiguration. The purged head SpectraCAM is pictured infigure 11. For the purged head camera, the case is anodizedaluminum. The sealed head camera case is built in KovarTM.

Dark current is reduced using a three-stage bismuth-telluride thermoelectric cooler and water recirculating system.Typical imager operating temperature is -30 to -45C. At thesetemperatures, dark current is 6-9 carriers/pixel/second.

Figure 5: RACID84 imager

Horizontal Scanner

Row Voltage

Column 1 Column 2 Column n

Row 1

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Row m

Preamplifier-per-Column Architecture

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Figure 6: RACID84/86 Block Diagram

4. PERFORMANCE

The SpectraCAM84/86 responds to light from 165 nm to 1000 nm. UV/visible responsivity was measuredusing Oriel deuterium and tungsten light sources through a 2nm diameter aperture in an optical bench monochromator.When illuminated with the Oriel 30-watt deuterium lamp, the SpectraCAM86 UV responsivity was 100 ma/watt at350nm and 80 ma/watt at 200 nm. The SpectraCAM84 UV responsivity was 71-89 ma/watt at 350 nm and 40-50ma/watt at 200 nm. Responsivity and quantum efficiency curves are shown in figures 12 - 15. Quantum efficiency iscalculated from the responsivity using Planck’s constant.

At 50 kHz, the single read noise is typically to be 172-189 carriers for the RACID86. To date the lowestmeasured read noise was 135 carriers. By signal averaging with 256 non-destructive reads, the read noise is reduced to10-11 carriers. For the flat-field 598-nm LED signal at full well amplitude, the 50 kHz single read signal-to-read noiseratio was 4K. For 256 non-destructive reads, the signal-averaged signal-to-read noise ratio was 58K. For the RACID84,the single read noise at 50 kHz was 169–304 carriers at 50 kHz. The read noise is reduced to 13-28 carriers with 256non-destructive reads.

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SpectraCAM linearity is characterized by plotting output signal vs light exposure. Using the linearity curve,the pixel full well capacity is measured from the point at which nonlinearity is 2% of full well. By this method, theRACID86 imager full well capacity is 600k carriers. Injection is the process for clearing charge from the array.Injection efficiency is characterized globally and for small segments of the pixel array. N/N+ RACID86 sample302P060-22-10 (figure 13) showed higher responsivity than did the n/p+ samples. N/N+ global injection was lessefficient than for the n/p+ samples, but adequate to meet current product needs. Pixel segment injection was inefficient.Further optimization of the n/n+ structure is needed to meet the segment inject specification for the current product.

5. APPLICATIONS

The SpectraCAM84/86 are designed for quantitative, lens-less imaging. In many scientific and industrialapplications, it is important to measure photons or charged particles emitted from the subject with true spatial resolutionand intensity. Due to the absorption and reflection of lens materials, particularly in the vacuum UV, it is not alwayspossible or advantageous to use lenses.

With true random pixel access and inherent resistance to blooming, the SpectraCAM84/86 are well-suited foruse as detectors in emission spectroscopy. The ability to quantify weak analyte emission lines in the presence of strongmatrix emission wavelengths is critical in atomic emission spectroscopy (8). Thermo Electron (Waltham MA) has builtseveral types of emission spectrometers using the SpectraCAM as a general purpose detector, including the IRISInductively Coupled Plasma – Optical Emission Spectrometer (ICP-OES) (9).

Other applications for the SpectraCAM include direct proton imaging, x-ray crystallography, and astronomy. ThePMOS photogates are naturally resistant to radiation damage. By coating the RACID84/86 with lumogen, thewavelength response can be extended further into the vacuum UV with additional protection from UV radiation damage.GdO2S coatings can be used to enhance SpectraCAM response to higher energy x-rays.

11

Figure 7: SpectraCAM84 Mean Variance Plot at 50 kHz

RACID VoltageSettingsVrowINJ: 4.95VcolINJ: 4.95VrowLO: -3.50VrowHI: 4.60VcolLO: 0.50Vskim: 2.00VcolHI: 4.70

Figure 8a: SEM image of RACID86 pixel array, 1800X

Row

ColumnEpi Strap

Figure 8b: SEM image of RACID86 pixel array, 3000X

Figure 9: SpectraCAM Block Diagram

Figure 8c: SEM image of poly 2 – poly 1 overlap, RACID86 pixel, 30,000X

metal

poly 2

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(optional)

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(optional)

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(optional embedded

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per S Van Gorden

Figure 10: SpectraCAM Expanded

Figure 11: SpectraCAM86, purged camera head

CID84 Imager Responsivity Comparison

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C. Beam 11-4-2003

Figure 12: SpectraCAM84 Responsivity

CID86 Imager Responsivity Comparison

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Figure 13: SpectraCAM86 Responsivity

CID84 Imager QE Comparison

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C. Beam 11-4-2003

Figure 14: SpectraCAM84 Quantum Efficiency

CID86 Imager QE Comparison

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C. Beam 7-16-2003

Figure 15: SpectraCAM86 Quantum Efficiency

6. ACKNOWLEDGEMENTS

This work was supported by the Thermo Electron Corporation. We thank Steve Herschbein and Larry Fischer (IBMAnalytical Services) for ion beam modification of imager circuits, which aided in product development. We thank SkipTobey, Bob Schleicher, Mike Wassall, Huw Prytherch, and the Thermo Electron SpectraCAM86 product developmentteam for careful evaluation of RACID84/86 imagers and candid feedback. We thank Alice Everett and Sheila Escobar(Spectra-Physics) for technical support. We thank Jeff Pazak (Spectra-Physics) for support and guidance. We thankJim Cook and Gary Sims (Spectral Instruments) for coating RACID imagers with lumogen.

7. REFERENCES

1. G. Michon, “ CID Image Sensor With Improved Sensitivity,” SPSE Conference on Electronic Imaging, October 13-17, 1986.2. G. Michon, “CID Image Sensor with a Preamplifier for Each Sensing Array Row,” US Patent # 4689688, 8/25/87.3. G. Michon, “CID Image Sensor with Parallel Reading of All Cells in Each Sensing Array Row,” US Patent #4807038, 2/21/89.4. J. Carbone, J. Hutton, F. Arnold, J. Zarnowski, S. VanGorden, M. Pilon, and M. Wadsworth, “Application of lownoise CID imagers in scientific instrumentation cameras,” SPIE 1447 (1991) 229-242.5. Joseph Carbone, M. Bonner Denton, Stephen W. Czebiniak, Jeffrey J. Zarnowski, Steven N. VanGorden, Michael J.Pilon, “Collective Charge Reading and Injection in Random Access Charge Transfer Devices,” US Patent # 5,717,199,2/10/1998.6. L. Mortara and A. Fowler, “Evaluations of charge-coupled device (CCD) performances for astronomical use,” SPIE290 Solid-State Imagers for Astronomy (1981) 28-33.7. James Janesick, Kenneth Klaasen, and Tom Elliott, “CCD charge collection efficiency and the photon transfertechnique,” SPIE 570 Solid State Imaging Arrays (1985) 7-19.8. M. Bonner Denton, Michael J. Pilon, and Jeffery S. Babis, “Vacuum Ultraviolet inductively Coupled plasmaSpectroscopy for Element-Selective detection of Nonmetals,” Applied Spectroscopy, 44 (1990) 975-978.9. Michael J. Pilon, Ronald L. Stux, and Robert W. Foster, “Achieving improved optical emission spectroscopyperformance through advances in charge injection device detector technology,” American Laboratory, April 2000, 32-34.