Birkelund, Karen

Dam, Henrik Friis

Duun, Sune

Haahr, Rasmus Grønbek

Nørgaard, Lars

Petersen, P.

Raahauge, P.

Thomsen, Erik Vilain

Duun, Sune Bro

Raahauge, Palle

Petersen, Peter Christian

Copenhagen University Research Information System

A Novel Photodiode for Reflectance Pulse Oximetry in low-power applications

The amount of light collected is crucial for low-power applications of pulse oximetry. In this work a novel ring-shaped backside photodiode has been developed for a wearable reflectance pulse oximeter. The photodiode is proven to work with a dual LED with wavelengths of 660 nm and 940 nm. For the purpose of continuously monitoring vital signs of a human, a temperature sensor is integrated onto the chip containing the photodiode. This biomedical multisensor chip is made for integration into "the electronic patch", an autonomous monitoring system for humans

Online Research Database In Technology

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Downloaded from orbit.dtu.dk on: Dec 18, 2017A Novel Photodiode for Reflectance Pulse Oximetry in low-power applicationsHaahr, Rasmus Grønbek; Duun, Sune Bro; Birkelund, Karen; Raahauge, P.; Petersen, P.; Dam, HenrikFriis; Nørgaard, Lars; Thomsen, Erik VilainPublished in:Proceedings of 29th  IEEE Engineering in Medicine and Biology ConferenceLink to article, DOI:10.1109/IEMBS.2007.4352798Publication date:2007Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Haahr, R. G., Duun, S., Birkelund, K., Raahauge, P., Petersen, P., Dam, H. F., ... Thomsen, E. V. (2007). ANovel Photodiode for Reflectance Pulse Oximetry in low-power applications. In Proceedings of 29th  IEEEEngineering in Medicine and Biology Conference (pp. 2350-2353). IEEE. DOI: 10.1109/IEMBS.2007.4352798A Novel Photodiode for Reflectance Pulse Oximetry in low-powerapplicationsRasmus G. Haahr, Sune Duun, Karen Birkelund, Palle Raahauge, Peter Petersen,Henrik Dam, Lars Nørgaard, and Erik V. ThomsenAbstract— The amount of light collected is crucial for low-power applications of pulse oximetry. In this work a novelring-shaped backside photodiode has been developed for awearable reflectance pulse oximeter. The photodiode is provento work with a dual LED with wavelengths of 660 nm and940 nm. For the purpose of continuously monitoring vitalsigns of a human, a temperature sensor is integrated onto thechip containing the photodiode. This biomedical multisensorchip is made for integration into “the Electronic Patch”, anautonomous monitoring system for humans.I. INTRODUCTIONMicrosystems utilized in medico technology have a per-spective of attaching small measuring or dosing systems onhumans. Theses devices are small compared to conventionalmedical equipment which force patients to stay at hospitalsand often also to lie in bed such that instruments can beattached. This is both inconvenient for the patients and ex-pensive for the health care system. Small integrated systemsallow patients to keep their mobility and open up for homecare nursing using telemedicine solutions.Pulse oximetry is an intensively used non-invasive methodfor measuring the arterial oxygen saturation (SpO2) with thepurpose of detecting hypoxemia which can result in braindamage or death.We have previously described a new patient monitoringsystem - the Electronic Patch (EP) [1], which is an in vivoplatform for continuously monitoring body parameters, Fig.1. The system is attached to the body with a sticking patchcontaining sensors, an ASIC solution for signal processing,and wireless communication based on a SWM1601 radiochip. The electronic patch is powered by a CR-2025 coinbattery and is intended for use on heart disease patientshospitalized in their own home and for professionals in highrisk working environments.In this work we have developed a pulse oximetry sensorfor the electronic patch. Since the patch is planar, reflectionmode pulse oximetry is employed. The problem of artifactsin the signal due to motion is often mentioned in the literature[2]. In the EP the photodiode and light source are fixed tothe body by a sticking patch which minimizes the motionartifacts due to the movement of the photo detector and lightsource relative to each other.This work was supported by the Danish Ministry of Science, Technologyand Innovation.The authors are all with the Department of Micro and Nanotech-nology, Technical University of Demark, 2800 Kgs. Lyngby, Denmark.rasmus.haahr@mic.dtu.dkFig. 1. Illustration of the developed electronic patch. The patch has a sizeof 20 cm2 and is 5 mm thick.In the literature reflection mode pulse oximetry sensorshave previously been reported: Studies have been conductedusing one photo detector and one light source by Mendelsonet al. [3] and Dassel et al. [4]. An early design using multiplephoto detectors with a single light source was made byMendelson et al. [5] and more recently power optimizationhas been studied [6]. Other design approaches have also beenstudied: Takatani et al. have studied a design using 20 LEDsplaced concentrically around a single photo detector [7], [8];however, these designs were used to study the quality ofthe signal and not low power applications which is the casewith the EP. A finger-ring type of pulse oximeter intendedfor ambulatory use has been developed and studied by bothAsada et al. [9], [10] and Mendelson et al. [11].A physiologically monitoring system worn on the wristfor measuring several body parameters including SpO2 hasbeen reported by Yoo [12]. This system uses a conventionalfinger probe attached on the finger which is connected by awire to the monitoring unit on the wrist.We report a pulse oximeter based on a ring-shaped photo-diode with a hole in the middle for the light source as shownin Fig. 1. The photodiode collects backscattered light fromthe tissue all the way around the light source, therefore asmuch as possible of the backscattered light will be collectedby the photodiode. This allows for a very low LED drivecurrent which will lower the power consumption by thedevice. Furthermore, we have integrated an anti-reflectionoptical filter on the photodiode for enhanced sensitivity. InProceedings of the 29th Annual InternationalConference of the IEEE EMBSCité Internationale, Lyon, FranceAugust 23-26, 2007.FrP1C4.151-4244-0788-5/07/$20.00 ©2007 IEEE 2350Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on November 11, 2009 at 07:50 from IEEE Xplore.  Restrictions apply. 400 600 800 1000Wavelength (nm)024681012AbsorptionratioofHb/HbO2Fig. 2. Ratio between the absorption coefficients of de-oxygenated andoxygenated hemoglobin. The plot is based on data from [13].addition to the photodiode the chip also contains a thermistorfor measuring the body surface temperature, thereby creatinga multisensor for medical purposes on a single chip.A. Pulse OximetryPulse oximetry is a spectrophotometric method for non-invasive measurement of the arterial oxygen saturation andcan generally be performed in either transmission or re-flection modes. In the first, light is transmitted throughthe tissue e.g. a finger and in the latter light is backscat-tered from the tissue. The method relies on a differencein the absorption spectra of oxygenated and de-oxygenatedhemoglobin. The ratio between these is seen in Fig. 2. Itis seen that the ratio has a peak at approximately 660 nmand at higher wavelengths the ratio is lower than one. Twowavelengths must be used since we have two absorptionspectra. Conventionally, 660 nm (red) and 940 nm (IR) areused since the absorption ratio is large and small at thosewavelengths respectively. This minimizes the uncertainty ofthe SpO2 measurement. By rapidly blinking (a frequencymuch higher than the heart beat rate) with the red and IR lightsources two photoplethysmograms (optical recordings of thecardiovascular cycle), one for each wavelength, are recorded.The SpO2 is a function of the measured magnitude at thesystolic and diastolic states of the two photoplethysmograms:SpO2 ∼ ln(redsystolereddiastole) /ln(IRsystoleIRdiastole)(1)where redsystole and reddiastole are the magnitudes of thered light measured at the systolic state and diastolic staterespectively and likewise for IRsystole and IRdiastole. How-ever, measurement of SpO2 normally relies on empiricalcalibration [2].II. DESIGN OF CHIP AND PHOTODIODEThe multisensor chip with the photodiode and thermistordeveloped in this work has several novel features. The front-Frontside with optical filter, facing tissueDual LEDBackside metallization for light protectionAu thermistor locationMetal pad for flip chip6 mmFig. 3. Picture of the chip with the ring-shaped photodiode and the Authermistor. The diode junction is made on the opposite site of the lightentrance to avoid metallization and wiring to the frontside facing the tissue.and backside of the chip together with a Ledtronics dualLED with wavelengths of 660 nm and 940 nm is seen inFig. 3. The chip has a hole in the middle of 3.3× 3.6 mm2which fits to the dual LED. The photodiode is defined as asingle ring-shaped structure concentrically around the holewith an inner radius of 3.29 mm and an outer radius of 4.07mm yielding an active area of 18 mm2. The actual diodejunction is made on the opposite side of the light entrance toavoid metallization and wiring to the surface facing the skin.This therefore creates a backside photodiode where photonsare absorbed by generation of minority carriers in the basesubstrate and these then diffuse to the diode junction. Thediode is fabricated in p-type silicon since minority electronshave approximately a factor of 2 higher diffusivity comparedto holes.The front side of the chip has an anti-reflection filter whichallows transmission of 98 % for wavelengths of 660 nmand 940 nm which enhance the sensitivity of the device,Fig. 4. The filter is developed so that other wavelengths aresuppressed to screen off stray light. The peak at 515 nm isless important due to a shorter photon penetration depth inthe base material and a high absorbance by tissue at shorterwavelengths [13]. The filter is made by a stack of 50 nmsilicon oxide and 550 nm silicon nitride.An Au metal thermistor, made in a meander structure witha total length of 17.8 cm, a width of 3 µm and a height of200 nm, is integrated on the chip for measurement of bodytemperature on the skin surface.III. RESULTSA. Diode characteristicsA current-voltage characteristic for the photodiode isshown in Fig. 5. The photodiode has a forward saturationcurrent density of Js = 167 pA/cm2 and an ideality factorof n = 1.19 showing that the diode is close to ideal. Somegeneration current is observed as seen from the deviationfrom the fit in the range 0 to 0.12 V. The series resistanceof the mounted chip on a printed circuit board is Rs = 84Ω. We have obtained a quantum efficiency of 0.65 and 0.72351Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on November 11, 2009 at 07:50 from IEEE Xplore.  Restrictions apply. 500 600 700 800 900Wavelength (nm)405060708090100Transmission(%)R = 0 mmR = 15 mmR = 30 mmR = 40 mmFig. 4. Transmission of the SiO2 and SiN interference filter measured witha Filmtek reflectometer. The transmission uniformity across a wafer is seento be good since there is no significant deviation in transmission betweenthe four plots measured at a radius, R, from the wafer center.0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7Voltage [V]10-1210-1010-810-610-410-2Current[A]MeasuredFitIs and nIs=2.1×10-11 ± 8×10-13 An=1.19 ± 0.01R=84 ± 3 ΩFig. 5. Current-voltage characteristic of the fabricated photodiode. Thephotodiode ring has inner radius 3.29 mm and outer radius 4.07 mm yieldingan active area of 18 mm2.at wavelengths of 660 nm and 940. This is reasonable forthe current design of the photodiode, but less than that ofhigh-end commercial photodiodes and solar cells.B. Au thermistor characteristicsA measurement of resistance as a function of temperatureis shown in Fig. 6. The thermistor is found to have aresistance of 13.6 kΩ at 37◦C and a temperature of resistanceratio (TCR) of 2.6 · 10−3K−1. This is a little less than thatof pure Au (3.4 · 10−3K−1) and it is believed to be due to anon lattice structure of the deposited Au compared to bulkAu.34 36 38 40 42Temperature (oC)13.613.713.8Resistance(kΩ)Fig. 6. Resistance dependance of temperature for the Au thermistor.C. PhotoplethysmogramsThe fabricated photodiode packaged with a dual LED, asshown in Fig. 3, has been tested as a pulse oximeter on atest person’s pulp of the left hand index finger. The result isshown in Fig. 7. The two recorded photoplethysmograms forthe red and IR light both clearly display the cardiovascularcycle. The data are obtained by blinking with the red andIR LED elements with a frequency of 4 kHz and 180◦ outof phase. Data are sampled with 8 kHz. The data collectionis done using Labview and the data have been processedwith a moving average using a rectangular window witha width of 401 data points. The photoplethysmograms arealso corrected for drift. The photoplethysmograms using redand IR light are plotted against the left and right abscissasrespectively. From (1) a ratio of 0.7 is obtained. In [2] asimple theoretical model for the calibration is derived. Usingthis it is found that the ratio in (1) of 0.7 corresponds toa SpO2 of approximately 85 %. Even though the pulseoximeter has not been empirically calibrated in a clinicalsetup the result indicates that it works properly.IV. FABRICATIONThe process fabrication sequence is shown in Fig. 8. Thestarting point is a 300 µm thick 4” p-type silicon floatzonewafer with a boron doping of 5·1015 cm−3 and 1 ms minoritycarrier lifetime. A 765 nm thermal wet silicon oxide isgrown. Alignment marks and the opening of the SiO2 at thephotodiode area are done by photolithography and etching inbuffered hydrofluoric acid (bHF) with the backside protectedby a photoresist. A 1 h phosphor pre-deposition is done at1000◦C. Another wet oxidation for 1 h at 1000◦C is per-formed to make alignment marks and a masking silicon oxidefor boron pre-deposition at the contacting areas. The maskingsilicon oxide are opened with bHF using photolithography.The boron pre-deposition is performed for 1 h at 975◦Cgiving a 1 µm deep doping profile, Fig. 8(a). The maskingsilicon oxide is removed in bHF and a 50 nm dry silicon2352Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on November 11, 2009 at 07:50 from IEEE Xplore.  Restrictions apply. 0 1 2 3 4 5Time (s)34.034.134.234.3Current[µA],redLEDon62.863.063.263.4Current[µA],IRLEDonRed LED onIR LED onFig. 7. Photoplethysmograms measured with the developed ring-shapedphotodiode.oxide is grown on both sides of the wafer. On the skin sideit serves as the first layer for the optical filter and to give alow surface recombination velocity. On the side of the diodejunction it serves as electrical isolation. Hereafter, a 550 nmPECVD silicon nitride is deposited for the second filter layerand electric passivation, Fig. 8(b). Contact holes are definedusing photolithography and opened with bHF. Contacts, Authermistor, and metallization for reflection of light from thetop side of the wafer are made by evaporation of 20 nm Crand 200 nm Au using an Alcatel e-beam evaporator, Fig.8(c). The chromium is used to give a good adhesion to thesilicon nitride. In the last step the hole for the dual LED isdefined using backside photolithography and a hole is etchedthrough the wafer from the skin side (lower side on Fig. 8(d))with a STS Advanced Silicon Etcher, ASE.V. CONCLUSION AND FUTURE WORKSA. ConclusionsA multisensor chip for measurements of temperature andpulse oximetry (SpO2) is reported. The chip has a novel ring-shaped backside photodiode with a special designed opticalfilter for pulse oximetry in low-power applications, and anAu thermistor. The design and fabrication of the photodiodeand the thermistor has been presented. The photodiode andthe thermistor has been characterized and proven to workas a pulse oximeter; however, more advanced photodiodedesigns could be considered to further enhance the quantumefficiency. The chip has been packaged together with a dualLED of 660 nm and 940 nm into a compact pulse oximeterwhich is compatible with integration into the ElectronicPatch, Fig. 1.B. Future WorksThe pulse oximeter should be completely integrated intothe Electronic Patch for optimizing the systems with respectto measuring sites on the body and power consumption. Themonitoring systems should then be evaluated in clinical trials.Phosphor Boron SiO2 SiN Au(a) First phosphor doping then boron doping.(b) SiO2 and SiN deposition for optical filter.(c) Contact and thermistor metallization.(d) ASE etch of hole for LED’s.Fig. 8. Process sequence for the ring shaped photodiode for pulseoximetry.VI. ACKNOWLEDGMENTSThe authors gratefully acknowledge the The Danish Min-istry of Science, Technology and Innovation for financing.REFERENCES[1] E. V. Thomsen, A. Hyldga˚rd, K. Birkelund, R. G. Haahr, S. Duun, R.Elsubaihi, J. Branebjerg, “Distributed sensors: Applications, fabrica-tion and challenges”, Proc. Eurosensors XX, 2006[2] J. G. Webster, Design of Pulse Oximeters, IOP Publishing, UK; 1997.[3] Y. Mendelson, “Noninvasive pulse oximetry utilizing skin reflectancephotoplethysmography”, IEEE Transactions on Biomedical Engineer-ing, vol. 35, nr. 10, pp 798-805, 1988[4] A.C. Dassel, “Reflectance Pulse oximetry in fetal lambs”, Pediatr. Res.,vol. 31, nr. 6 pp 266-269, 1992[5] Y. Mendelson, “Design and evaluation of a new reflectance pulseoximeter sensor”, Medical Instrumentation , vol. 22, nr. 4, pp 167-173, 1988[6] Y. Mendelson, “Measurement site and photodetector size considera-tions in optimizing power consuption of a wearable reflectance pulseoximeter”, Proc. 25th Annual IEEE EMBS conf., pp 3016-3019, 2003[7] S. Takatani, “A new hybrid reflectance optical pulse oximetry sensorfor lower oxygen saturation measurements and for broader clinicalapplications”, Proc. SPIE, nr. 2976, pp 78-87, 1997[8] S. Takatani, “A novel hybrid reflectance pulse oximeter sensor withimproved linearity and general applicability to various portions of thebody”, Proc. 20th Annual IEEE EMBS conf., vol. 20, nr. 4, pp 1858-1861, 1998[9] H. Asada, “The Ring Sensor: a New Ambulatory Wearable Sensorfor Twenty-Four Hour Patient Monitoring”, Proc. 20th Annual IEEEEMBS conf., vol. 4, pp 1906-1909, 1998[10] H. Asada, “Artifact-resistant power-efficient design of finger-ringplethysmographic sensors”, IEEE Transactions on Biomedical Engi-neering, vol. 48, nr. 7, pp 795-805, 2001[11] Y. Mendelson, “Minimazation of LED power consumption in thedesign of a wearable pulse oximeter”, Proc. IASTED Conf. on Biomed-ical Engineering, pp 249-254, 2003[12] T. Yoo, “A wrist-worn integrated health monitoring instrument witha tele-reporting device for telemedicine and telecare”, IEEE Transac-tions on Instrumentation and Measurement, vol. 55, nr. 5, pp 1655-1661, 2006[13] Scott Prahl, Oregon Medical Laser Center, http://omlc.ogi.edu, Port-land, OR, 1998.2353Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on November 11, 2009 at 07:50 from IEEE Xplore.  Restrictions apply. 

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A Novel Photodiode for Reflectance Pulse Oximetry in low-power applications

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