976 ieee sensors journal, vol. 5, no. 5, …...976 ieee sensors journal, vol. 5, no. 5, october 2005...

976 IEEE SENSORS JOURNAL, VOL. 5, NO. 5, OCTOBER 2005

Fast Hartmann–Shack Wavefront SensorsManufactured in Standard CMOS Technology

Davies William de Lima Monteiro, Member, IEEE, Thomas Nirmaier, Gleb V. Vdovin, andAlbert J. P. Theuwissen, Fellow, IEEE

Abstract—This paper discusses the implementation of fast wave-front sensors based on the Hartmann–Shack method in standardcomplementary metal–oxide semiconductor (CMOS) technologyand evaluates the impact of the capabilities and limitations of thistechnology on the wavefront sensor performance. Aiming at fastoperation ( 1 kHz), we compare the applicability of either con-ventional or dedicated image sensors and investigate how currentcustom concepts can complement each other. To date, three dif-ferent custom CMOS-sensor layouts have been implemented. Be-sides being able to operate at faster rates than conventional sen-sors, these devices demonstrated the ability to achieve high wave-front-detection accuracy and the potential for use in low-light ap-plications (e.g., ophthalmic diagnostics). The goal is to identify themost important practical issues related to using standard CMOStechnology in wavefront sensing.

Index Terms—Adaptive optics, complementary metal–oxidesemiconductor (CMOS) technology, image sensor, optical pro-filing, very large scale integration (VLSI), wavefront sensor.

I. INTRODUCTION

WAVEFRONT SENSORS are used for the dynamic es-timation of wavefront distortions, both in stand-alone

setups for optical diagnostics and in adaptive optical systems forthe compensation of optical aberrations [4]. These distortionscan be induced either by a component profile or by variations inthe light propagation media. The application fields range fromastronomical and scientific to medical and commercial, withan increasing demand for affordable and compact devices. Thetarget accuracy of wavefront reconstruction is indicated with re-spect to the wavelength ( ), which usually lies within the visibleand near infrared spectra. The demanded accuracy varies acrossapplications and must sometimes be smaller than for thediagnostic of prime optical profiles or as large as , for coarse

Manuscript received April 29, 2004; revised August 25, 2004. This work wassupported in part by the Dutch Technical Foundation (STW project DOE.5375),in part by Delft University of Technology, and in part by the University of Hei-delberg. The associate editor coordinating the review of this paper and approvingit for publication was Prof. Eugenii Katz.

D. W. de Lima Monteiro was with the Delft University of Technology, Delft,The Netherlands. He is now with the OptMA_lab, Electrical EngineeringDepartment/DEE, Universidade Federal de Minas Gerais (UFMG), Pampulha,31270-010, Belo Horizonte, Brazil (e-mail: [email protected]).

T. Nirmaier was with the University of Heidelberg, Heidelberg, Germany. Heis now with Infineon Technologies AG, Munich, Germany (e-mail: [email protected]).

G. V. Vdovin is with the Electronic Instrumentation Laboratory, EEMCS,Delft University of Technology, Delft, The Netherlands, and also with FlexibleOptical BV, Delft, The Netherlands (e-mail: [email protected]).

A. J. P. Theuwissen is with DALSA, Inc., Eindhoven, The Netherlands, andalso with the Electronic Instrumentation Laboratory, EEMCS, Delft Universityof Technology, Delft, The Netherlands (e-mail: [email protected]).

Digital Object Identifier 10.1109/JSEN.2005.854487

Fig. 1. H–S method, in which the spot displacements on the projection screenrepresent the incoming arbitrary wavefront.

estimations of aberrations with large amplitudes ( ) and lowspatial frequencies, as often encountered in human eyes. Fastwavefront sensing is demanded where the optical aberrationsfluctuate at a rapid pace because of, for instance, atmosphericturbulance during sky observation or optical data transmission,eye movements during in vivo retinal imaging, and axial spin-ning during the quality inspection of industrial parts.

The Hartmann–Shack (H–S) method is one of the mostused wavefront sensing techniques; it is structurally simpleand white-light compatible, it prompts straightforward dataanalysis and it has no -ambiguity limitation, as is the casewith most interferometers. In the H–S technique, a microlensarray samples an arbitrary wavefront and the light spots at theprojection screen deviate from a reference grid according to thelocal slopes of the wavefront. Essentially, this method enablesone to estimate the wavefront shape from the displacements oflight spots. Fig. 1 depicts the principle.

In optical shop testing, variations on this principle include, forexample, laser-ray tracing (LRT), in which a single laser beamscans an object under test at particular grid points and the inverseHartmann test, in which small optical fiber heads project a fewbeams on the object surface, whose deviations upon reflectionindicate the object quality.

The projection screen in a H–S sensor is conventionally acamera, which represents a bottleneck to fast operation. Inex-pensive off-the-shelf cameras offer limited frame rates usuallyreaching up to only 150 Hz. High-speed units, operating at framerates of several hundred Hz or more, are mostly used for scientificapplications and require a number of extra fabrication steps andarchitectural changes, which altogether push the costs up. In theH–S method, one is ultimately interested in the positions of lightspots, and the grabbed camera image is solely an intermediate

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DE LIMA MONTEIRO et al.: FAST HARTMANN–SHACK WAVEFRONT SENSORS 977

step. Therefore, the relatively slow image-processing step to per-form the “centroiding” calculations combined with a restrictedcamera frame rate limit the overall sensor speed. This suggeststhe development of a custom layout in order to ensure operationat high repetition rates. As an alternative to the conventionalcamera, one can implement fast H–S wavefront sensors based onan array of optical position-sensitive detectors (PSD) integratedon a single chip, where each light-spot centroid is sensed by aparticular PSD.

For custom PSD designs with their appended circuit blocks,charge-coupled device (CCD) technology does not qualify asthe first choice. Despite being robust for imaging applicationsand suitable for very low light levels, this technology is oftenproprietary and has restricted digital and analog functionality.Complementary metal–oxide semiconductor (CMOS) is amore enticing technology, as flexibility and manufacturingcosts are concerned, offering the possibility to integrate pho-todetectors and reliable analog-digital circuitry together. Thesteadily growing interest in CMOS devices promotes contin-uous research and process improvements, rendering a goodmatch between simulated and implemented circuitry. StandardCMOS is a generic and largely accessible process which favorsthe integration of multiple electronic functions with a highyield and intrinsically enables the implementation of severalphotosensitive structures. CMOS imaging is a growing nichethat complements that of CCD imaging by offering extendedembedded functionality on chip.

II. STANDARD CMOS TECHNOLOGY

The ongoing trend in CMOS technology is the increase incircuit density and speed, and consequently, the reduction of theminimum transistor gate length. Feature sizes from 0.8 down to0.13 m are widely established industry standards today andthe state-of-the-art manufacturing for logic circuits is 90 nm;offline processes reaching features as small as 65 nm have beeninvestigated.

CMOS is not optimal for photodetection, especially in astandard flow chart. There are no pinned and buried layers tominimize dark current and substrate interference, respectively;multiple charge-packet transfer is not efficient due to the fixeddoping profiles and also due to the absence of overlappingpolysilicon layers in some processes. Also, in a standard fab-rication line, even 10-nm fluctuations, from batch to batch, inthe thickness of some dielectric layers on top of silicon leadto unpredicatable antireflective characteristics, considering thenumber of thin films stacked.

As the trend migrates to submicron features, the disadvan-tages to photodetection increase, because in such processesthe p-n junctions that form photodiodes become shallowerand the doping becomes higher, resulting in thinner depletionlayers. These two factors lead to a lower quantum efficiencyfor longer wavelengths ( nm). The introduction of sili-cided drains/sources, to increase speed, as well as silicidedpolysilicon layers impairs the penetration of incident light.Moreover, the reduction of the supply voltages—and thedisproportional scaling of the threshold voltages—constrainthe signal swing, which, in the analog-signal domain, can be

detrimental. Besides, the fabrication costs for state-of-the-artprocesses increase drastically making them less convenient forsmall-scale custom devices and the behavior of novel circuitrybecomes less predictable because of insufficiently accuratemodel parameters due to additional physical phenomena af-fecting transistor operation.

Recently, CMOS imagers with claimed high image qualityhave been reported [5], [6]. They often use either four-transistoractive pixels or customized steps with well-defined local dopingprofiles and highly doped layers pinned to the substrate, or acombination of those.

III. CMOS H–S WAVEFRONT SENSOR

For a H–S wavefront sensor, two different approaches usingCMOS technology can be considered: a camera or a matrix ofPSDs. The basic difference between them is that a camera hasa limited frame rate and requires image processing, whereas amatrix of PSDs (in which each PSD is associated with a lightspot) enables almost direct information about the spot positionand circumvents any image processing.

A. CMOS Imagers

A two-dimensional (2-D) imager consists of a densely packedarray of adjoining pixels, whose main purpose is to capture animage by mapping a scene into a discrete distribution of light in-tensities. Obviously, a higher pixel density yields a better imagereproduction. In a H–S wavefront sensor, the camera is used toregister a picture of the light spots projected with the microlensarray. Each microlens is associated with a region on the chip,i.e., with a certain number of pixels , over which thespot is free to move without overlapping with the neighboringones. The effective focal spot illuminates yet a smaller numberof pixels, say , within that delimited region. One calculatesthe baricenter of the focal spot by using a simple centroiding(center of mass) formula or a precalibrated curve-fitting algo-rithm, which minimizes the discretization error. In both casesone can achieve subpixel resolution, which increases with thenumber of illuminated pixels and with the array fill factor.The fill factor is the ratio between the effective photocollectingarea and the total exposed area, including gaps between pixels,circuitry, and interconnections.

In the development of a Hartmann sensor, one should find anoptimal compromise between the ratio of the spot size to thepixel size and the ratio between the microlens diameter and thespot size. The first ratio affects the resolution of the centroid cal-culation and the second ratio dictates the lateral range the spotis able to move (i.e., the maximum local wavefront tilt). Thelargest possible ratios are desirable, but practical limits are setby the choices of imaging-chip dimensions, pixel size, and pa-rameters of the microlens array (lateral size, pitch, fill factor,and focal length). Conventional chip dimensions extend up to15 mm and physical pixel sizes range from 5–20 m, typically.The maximum spot displacement depends on the focal lengthof the microlenses and on the magnitude of the wavefront aber-rations. For a H–S wavefront sensor, color cameras are not rec-ommended because many chips for color imaging have a dis-


tribution of color filters on top of their pixels, which introducesnonuniformities to the intensity reponse.

The wavefront accuracy of the H–S sensor is directly asso-ciated with spot centroiding resolution, which is determinedby several noise sources, including intrinsic pixel noise, spa-tial discretization, readout noise, signal-conversion quantiza-tion, imaging-chip architecture, and temporal noise.

Intrisic Pixel Noise: It basically comprises dark-currentshot noise, thermal noise, pixel response nonuniformity due toprocess variations over the array, transistor threshold voltagemismatch, and signal-transfer or reset noise (often referred toas noise). The response nonuniformity and the thresholdvoltage mismatach add together as fixed-pattern noise (FPN),which is observed as a difference in response between pixels inan array which have been submitted to fabrication with identicallayouts. If active pixels are used—where a buffer MOSFETcopies the photodiode signal to the data line—FPN andcan be supressed by double-sampling readout. The dominatingintrinsic noise source depends on the choices of photodetectionelement and on the pixel circuitry.

Spatial Discretization: It derives from the pixalated natureof an imaging chip and its effects can be minimized by havingas many possible pixels illuminated by the spot. In practice, atleast 5 5 pixels should be illuminated.

Readout Noise: It derives from the current-to-voltage con-version and amplification circuitry and consists frequently of

noise (flicker noise), thermal noise, and signal offset. Forphotodiode pixels, the transimpedance amplifier with a com-pensation shunt capacitor usually offers the best performancein terms of noise and bandwidth.

Signal Quantization: It occurs when the analog pixel signalis converted to a digital output and its rms value is given by thevoltage associated with the least-significant bit divided by .

Architectural Noise: If photodetectors in an array are rectan-gular rather than square, they introduce a false contribution ofastigmatism (cylidrical aberration) in the reconstructed wave-front, unless the image-capture hardware or a dedicated soft-ware routine purposedly mitigates this effect.

Temporal Noise: It is often associated with temperature vari-ations and only becomes relevant if its change rate is higher thanthe wavefront change rate.

A number of important remarks need to be considered whenselecting a CMOS imager for a H–S sensor.

Fill Factor: This dictates the amount of impinging light ef-fectively being detected. Wavefront sensing in astronomy andophthalmology can largely benefit from a high fill factor, pro-vided the intensity levels available or allowed there are low. It isimportant to observe whether the CMOS chip has focusing mi-crolenses deposited directly on chip, counteracting a possiblylow fill factor.

Dynamic Range: CMOS cameras with a very high dynamicrange usually make use of logarithmic pixels, which are inconve-nient forspot-centroiddetectionbecauseof thenonlinearityof theresponse. Several commercial cameras, however, offer the possi-bility to switch between the logarithmic and the linear modes.

Multiple Framing (Windowing): In some cameras, multipleframing is possible, where several subareas can be defined on thechip. This canbe convenient to track individual spots, eliminating

Fig. 2. Some photodetector structures available in standard CMOStechnology. (a) Photodiodes. (b) Phototransistors.

the readout of obsolete dark pixels. However, the sequential sub-frame readout and the switch time between frames slow down theoperation. The number and position of spots required to sense agiven wavefront aberration depend on the aberration spatial fre-quency and sometimes the incoming beam does not have a uni-formintensityovertheopticalaperture.Inthesecases,fullrandomaccess to pixels in the spatial and time domains can be useful.This could readily reject dark pixels, promote longer integrationtimes exclusively for faint spots and increase the overal acquisi-tion speed by reducing the number of pixels readout.

Sensitivity: Most commercial CMOS imagers use the/p-substrate photodiode (shallow junction), which enables

a larger array density, but are more prone to substrate noise.Also, the closer the junction is to the surface, the higher isthe contribution of dark-current shot noise and the lower thephotocollection efficiency for longer wavelengths ( m),on which most wavefront sensing application are based.High-speed cameras require additional custom fabrication stepsand are often manufactured in state-of-the-art processes, forwhich the smaller feature sizes and the use of silicide layersresult in shorter delays (larger operational bandwidth), butyields a lower sensitivity.

B. CMOS Chips With Optical PSDs

These chips comprise full custom designs for H–S sensors.Each design consists of a regular array of PSDs, signal-transferand/or signal-processing electronics and digital control units(shift registers or demultiplexers). Photodiodes and phototran-sistors can be implemented in CMOS as depicted in Fig. 2,whereas some possible 2-D PSDs are shown in Fig. 3. The quadcell, alternating, chessboard-like, and spiral structures all yieldthe and coordinates of the light-spot centroid based on therelative photocurrents on rows and columns of photodetectors.The working principle of each of these structures and the per-formance of the respective implemented CMOS structures wasgiven by de Lima Monteiro et al. [7].


Fig. 3. Two-dimensional PSDs. (a) Quad cell. (b) Alternating.(c) Chessboard-like and (d) spiral structures.

The main advantage of using PSDs is the nearly directinformation about the positions of the spots, rendering imageprocessing unnecessary. The three design approaches presentedhere were fabricated and tested, and they differ from each otherin the feature size of the CMOS process used, the type of PSDschosen and the spot-detection scheme (Fig. 4).

An overview of the technology, layout and circuit parametersfor the three implemented CMOS chips is shown in Table I.

The HSSX chip is based on alternating photodiode structures,where the output signals of rows and columns are connected toinput nodes of and circuit chains, respectively. Each chaincontains as many circuit cells as there are rows (or columns).The cells are arranged in a winner-take-all structure (WTA) [8],which generates a high digital output solely to the output nodecorresponding to the input node with the highest photocurrent.After every millisecond the state of all PSDs is reset. The bi-nary data is then transferred, in daisy-chain architecture, to adata compressor unit for subsequent readout. The PSDs, whichdetect the position of the largest photocurrent, are linear overthe whole dynamic range. The chip features no random accessto the individual detectors.

The CeHSSA chip is an enhancement of the previous struc-ture. It uses n-well/p-epilayer photodiodes in a chessboard-likearrangement and a novel topology for the WTA circuit. Thelower-junction photodiodes feature capacitance values oneorder of magnitude lower than the upper-junction photodiodesand the novel WTA architecture is based on several independentand interleaved WTA chains, yielding the possibility of pseudo-centroiding because of the availability of several maxima. Afteraddressing a detector, its data is stored in an internal tristate busfor subsequent parallel or 16-bit multiplexed readout.

The QC-WFS chip features 8 8 quad cells with double-junction photodiodes (p+/nwell and nwell/p-epilayer). An inte-grated demultiplexer enables random access to the quad cells.The analog output yieds true centroiding, and although the po-sition response is nonlinear, one can characterize it with a sig-moidal function in a precalibration step. The quad-cell signals

Fig. 4. Photographs of the three implemented chips. (a) HSSX. (b) CeHSSA.(c) QC-WFS.

are multiplexed and delivered to an outup data bus. The geo-metrical arrangement of a quad cell allows both spot-centroiddetection and spot centering. The first is convenient when thesensor is used in diagnostic applications and the latter when it isused to drive a deformable mirror in an adaptive optical system.

IV. PERFORMANCE OF THE CUSTOM CMOS CHIPS

Operational speed, wavefront-sensing accuracy (the min-imum rms wavefront variation the sensor is able to detect), andlight sensitivity are the most important parameters. They all


TABLE ILAYOUT AND CIRCUIT PARAMETERS OF THE IMPLEMENTED CMOS WAVEFRONT SENSORS

TABLE IIOPERATIONAL PARAMETERS OF THE THREE IMPLEMENTED CMOS WAVEFRONT SENSORS

depend on a number of other core factors related either to thetechnology used or to the design choices.

The operational speed depends on the pixel capacitance, typeof PSD and readout scheme. The wavefront accuracy is asso-ciated with the PSD position resolution, which depends on thenoise of the photodetector and on the PSD layout. The sensi-tivity, which can be tagged as the overall quantum efficiencyof the photodetector, depends on many factors, such as, for in-stance, the fill factor of each PSD, the thickness of the oxide andnitride layers on the chip, the doping profile, and the junctiondepth. Table II shows some performace parameters, followedby a discussion of each chip.

A. HSSX Chip

The position resolution is basically limited by the discretenature of the PSD (alternating structure), where the pixel sizeis 17.6 m, and by FPN, which depends on the spot size andon the frame rate. The WTA circuit is reset at the beginning ofevery frame. The WTA circuit needs a certain time to establishthe output state and that time depends on the input photocur-rents. At its maximum frequency, 1 kHz, the WTA circuit maynot reach its final state for low light intensities, which resultsin a larger FPN. For very low intensities, a feedback mode hasbeen implemented with increased bandwidth at the expense ofincreased position noise. The quantum efficiency of this chip

was low (10%) due to a shallow-junction photodiode, especiallyfor m, and misfortune in the combination of the thick-ness of the oxide layers, which were not optimized for opticaltransmission.

B. CeHSSA Chip

The influence of FPN was cancelled out to a large extent inthis chip by using a modified topology for the WTA circuit. Thelarger quantum efficiency and the reduced parasitic capacitancealso increased the bandwidth and the sensitivity. The positionresolution above pW per spot is mostly limited by thepixel size (17.0 m). The sensor can, however, operate up to4 kHz, and the wavefront accuracy can be as good as (

m) for 300-nW spots.

C. QC-WFS Chip

The position resolution depends on the process/pixel ar-chitecture through the signal-to-noise ratio. The quad cellsare based on passive pixels, which consist of a photodiodeconnected to an analog switch. This architecture couples thelarge pixel capacitance (50 pF), due to the large photodiodesize, to the capacitance of the data bus. The direct transfer ofthe photocurrent from this capacitor to the data line bringsalong a large contribuiton of noise, which limits operationat low light levels. The spot position resolution can be as good


as 1 m, for a 10- W spot, resulting in a wavefront accuracyof ( m). Reduction of this noise contribution isa promissing solution for operation at lower light levels. Thischip is able to operate up to 3 kHz.

V. DISCUSSION

A. Choice Between a Conventional Imager and a DedicatedSensor

A generic H–S wavefront sensor should feature a large inten-sity dynamic range, markedly operable at low-light levels (pico-or nanowatts per spot), while suitable to aberration detection atfrequencies larger than 1 kHz. Besides, in order to become wide-spread in medical and commercial fields, the unit costs should belimited tofewthousanddollars, includingsensor, samplingplane,and generic software. Also, the portability of the device and theability to change the number of sampling points, focal plane dis-tance and grid geometry are desirable. None of the devices im-plemented so far, either with a conventional or with a dedicatedimaging chip, fulfill all those wishful requirements at once.

CCD-based sensors still offer the best performance in regardto low-light levels, and scientific CCDs, often cooled, have beendeployed in astronomical applications. However, long integra-tion times, sometimes of several seconds, are not uncommon.In terms of speed, fast CCD and CMOS cameras with framerates in excess of 1 kHz have been developed, some of whichare already on the market. The speed, nonetheless, comes at theexpense of a reduced number of pixels, which limits the res-olution in the computation of the centroid of light spots, or atthe expense of a lower sensitivity. Data reduction of the imagegrabbed from a conventional camera represents an additionaldelay. CMOS cameras that offer the user full access to pixelscan be used to emulate arrays of PSDs, by the division of theimaging array in subregions, skipping the readout of obsoletepixels and increasing speed. One of the great advantages ofan imaging chip is the flexibility to use different layouts forthe sampling-plane array (geometry and density of microlenses)and the absence of strict alignment requirements. For wave-front detection below 150 Hz, inexpensive and, yet, sensitive,off-the-shelf cameras are available.

Dedicated CMOS sensors based on PSDs are primarily aimedat high operational speed ( kHz), without necessarily com-promising the number of operative light spots or the sensitivity.Random access enables the reduction of the number of PSDsto be readout to further increase speed. Some disadvantagescompared to a conventional camera are the requirement of pre-cise alignment of the microlenses on top of the chip and thematching of the microlens grid to that of the PSDs. The compat-ibility with CMOS enables future integration of smart functionson chip without changing the core layout, offering the possi-bility of fully customized solutions at reasonable low-volumecosts and the feasibility of compact sensors with integrated re-construction algorithms and mirror drivers. Low-light level op-eration (nanowatt input beams) and good wavefront accuracy( ) at high speeds have been proven in independent chipsand a combination of these quality parameters on a single chiphave been investigated, as suggested in the following section.

Dedicated chips based on position-sentitive detectors couldbe fabricated in hybrid CCD/CMOS processes, benefiting fromthe efficiency of CCD and the circuitry functonality of CMOS,with the due extra costs for this additional process, however.

B. Directions for Future Improvement of the Dedicated CMOSChips

The dedicated chip HSSX features high operational speed butdoes not offer random access to the PSDs. It has a very goodlight sensitivity (nanowatt range) but the position detection isbased solely on the peak intensity of the spot, which limits theposition resolution to the pixel pitch. The second chip (CeHSSA)represents an enhanced version of the previous one. It enables fullrandom access, enhances the light sensitivity to the sub-nanowattrange, and improves the position resolution (wavefront accuracy)by using pseudocentroiding with multiple peak detections. It isalso able to operate at 4 kHz in the centroiding mode. This sensoris suitable for low-light operation but the position resolutionstill limits its use to low-order aberrations with relatively largemagnitudes. The third presented sensor (QC-WFS) offers fullrandom access, very good position resolution (wavefront accu-racy), but requires light levels in the microwatt range due to thehigh capacitive noise associatewith the large photodiodes and thepassive-pixel architecture. Both the CeHSSA and the QC-WFSchipsstillofferroomforimprovement,thefirst intermsofpositionresolution and the latter in terms of low-light operation.

The basic limitation of wavefront sensors based on the WTAcircuit is the restricted resolution of the spot detection. The de-sign goal with respect to sensitivity ( pW per spot) hasbeen achieved. The practical limit for the minimum pixel size(and position resolution) in the chosen CMOS process tech-nology is m. Though in the static case the position reso-lution is limited to the pixel pitch, this limit can be passed underin the dynamic case, by adequate temporal post-processing ofthe spot position time series [9], e.g., through adequate Kalmanor other filtering techniques. A spatial resolution well below thepixel dimension can be achieved and no different chip architec-ture is required. In addition, it would be useful to increase thefunctionality of the sensor by adding a modal wavefront-recon-struction module directly on the chip. A possible implementa-tion would be a hardware artificial neural network [10], whichcould provide a direct analog output signal for some low orderaberrations, like tilt, defocus and astigmatism, useful for modalwavefront correctors. This could result in miniaturized AO sys-tems, which do not need a host computer.

For the chip based on quad cells (QC-WFS), the goal is toimprove the light sensitivity of this sensor by three orders ofmagnitude (microwatt nanowatt per spot), while maintaininga good wavefront accuracy ( ). The main limitation isnow capacitive noise ( ). Several basic measures can con-tribute toward this improvement, still preserving the benefits ofthe quad-cell architecture and the supra-micron fabrication tech-nology (1.6 m): substitution of the double-junction photodi-odes with n-well/p-epi photodiodes, of passive pixels with activepixels, and of simple transimpedance amplifiers with capacitiveshunted ones. The junction substitution leads to a capacitanceone order of magnitude lower; active pixels, which include asource follower and a reset transistor, ensure the decoupling of


the photodiode capacitance from the line capacitance and en-able the cancellation of FPN and noise; and a properlydesigned shunted amplifier reduces the amplitude of the noisespectral density within a frequency range of interest. These mea-sures will also improve the sensor intrinsic frequency.

VI. CONCLUSION

CMOS-based wavefront sensors are growing mature andcan be beneficial for a number of applications in industry andmedicine, e.g., laser diagnostics, direct optical data links andophthalmology. Off-the-shelf CMOS cameras can substitutethe long-used CCD cameras for H–S sensors. However, thechoice of an appropriate CMOS camera can be tricky; often,an improved speed performance comes at the cost of lowerresolution and/or sensitivity. The most important advantages ofCMOS technology is the capability to realize custom designsand the possibility of implementation of reliable digital andanalog circuits as well as photosensitive elements on a singlechip. The use of CMOS favors the development of customwavefront sensors with embedded functionality.

The custom CMOS wavefront sensors developed so far arebased on integrated matrices of PSDs. This approach enables afaster readout of the spot centroids and circumvents the needfor an image-processing step, yielding frame rates in excessof 1 kHz. The sensors independently developed in Delft, TheNetherlands, and in Heidelberg, Germany, have also shown thatCMOS-based solutions can offer both a good resolution and agood light sensitivity. The challenge is now to combine thesetwo characteristics on a single chip.

ACKNOWLEDGMENT

The authors would like to thank the staff of the Delft Instituteof Microelectronics and Submicron Technology (DIMES), aswell as Dr. D. Droste and Prof. J. Bille.

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Davies William de Lima Monteiro (M’98) was born in Volta Redonda, Riode Janeiro, Brazil, in 1972. He received the B.Sc. degree in physics and theM.Sc. degree in solid-state physics from the Universidade Federal do EspíritoSanto, Vitória, Brazil, in 1994 and 1997, respectively, and the Ph.D. degree inmicroelectronics and adaptive optics from the Delft University of Technology(TU), Delft, The Netherlands, in 2002.

He was an Associate Researcher at the Electronic Instrumentation Labora-tory, TU Delft, in the field of CMOS integrated photosensors, microoptics, andadaptive optics. He is now an Associate Professor with the Electrical Engi-neering Department, Universidade Federal de Minas Gerais, Belo Horizonte,Brazil, where his main research interests include low-cost adaptive and microop-tical components and systems.

Dr. de Lima Monteiro is a member of the International Society for OpticalEngineering (SPIE).

Thomas Nirmaier was born in Ludwigshafen, Germany, in 1971. He receivedthe Diploma and the Ph.D. degree in physics from the University of Heidelberg,Heidelberg, Germany, in 1999 and 2003, respectively.

He was an ERASMUS Fellow at the University of Santiago de Compostela,Spain, in 1996 and did an internship at Volkswagen de Mexico, Puebla, Mexico.Between 1999 and 2001, he worked on signal processing concepts for subma-rine sonar systems with ATLAS Elektronik, Bremen, Germany. He has beendeveloping CMOS-based wavefront sensors for ophthalmology at the Univer-sity of Heidelberg, Germany, since February 2001. In 2003, he joined the De-partment of Physics, University of Heidelberg. In 2004, he joined the MemoryProducts Division at Infineon Technologies, Munich, Germany. His research in-terests include integrated CMOS sensors, signal processing with artificial neuralnetworks, and CMOS technology.

Gleb V. Vdovin was born in Russia. He received the M.Sc. degree in opticsfrom the Leningrad Institute of Fine Mechanics and Optics, Leningrad, Russia,in 1986, and the Ph.D. degree in electronics from the Delft University of Tech-nology (TU), Delft, The Netherlands, in 1996.

He was a Research Associate in various adaptive optics projects, first forthe Russian space industry and, later, for the Russian Academy of Sciences.Since 1993, he has been with the TU Delft as a Research Assistant, PostdoctoralFellow, and Assistant and Associate Professor. He has published more than 40articles in the field of adaptive optics and microoptics. He is a founder and CEO(1997) of Flexible Optical BV, which is a producer of deformable mirrors andwavefront sensors.

Dr. Vdovin is a member of the Optical Society of America, and he servedas a member of the programme Committee for CLEO and various conferencesorganized by SPIE.

Albert J. P. Theuwissen (F’02) was born in Maaseik, Belgium, on December20, 1954. He received the degree in electrical engineering and the Ph.D. in elec-trical engineering from the Catholic University of Leuven, Leuven, Belgium, in1977 and 1983, respectively. His M.S. thesis work was based on the develop-ment of supporting hardware around a linear CCD image sensor, and his Ph.D.dissertation was based on the implementation of transparent conductive layersas gate material in the CCD technology.

From 1977 to 1983, his work at the ESAT Laboratory, Catholic University ofLeuven, focused on semiconductor technology for linear CCD image sensors. In1983, he joined the Micro-Circuits Division of the Philips Research Laborato-ries, Eindhoven, The Netherlands, as a member of the scientific staff. Since thattime, he has been involved in research in the field of solid-state image sensing,which resulted in the project leadership of, respectively, SDTV- and HDTV-im-agers. In 1991, he became Department Head of the Division Imaging Devices,including CCD as well as CMOS solid-state imaging activities. In 1995, he au-thored the textbook Solid-State Imaging with Charge-Coupled Devices (Ams-terdam, The Netherlands: Kluwer, 1995). He is a member of the editorial boardof the magazine Photonics Spectra. In March 2001, he became part-time Pro-fessor at the Delft University of Technology, Delft, The Netherlands, where heteaches courses in solid-state imaging and coaches Ph.D. students with theirresearch on CMOS image sensors. In April 2002, he joined DALSA, Inc., Eind-hoven, where he is currently the Chief Technology Officer.

Prof. Theuwissen is member of SPIE. In 1998, he became an IEEE Distin-guished Lecturer.

976 ieee sensors journal, vol. 5, no. 5, …...976 ieee sensors journal, vol. 5, no. 5, october 2005...

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