Boreman, Glenn D.

Gritz, Michael A.

Puscasu, Irina

Spencer, David

English

University of Central Florida (UCF): STARS (Showcase of Text, Archives, Research & Scholarship)

University of Central Florida STARS Faculty Bibliography 2000s Faculty Bibliography 1-1-2003 Fabrication of an infrared antenna-coupled microbolometer linear array using chrome as a mask Michael A. Gritz University of Central Florida Irina Puscasu David Spencer Glenn D. Boreman University of Central Florida Find similar works at: https://stars.library.ucf.edu/facultybib2000 University of Central Florida Libraries http://library.ucf.edu This Editorial Material is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 2000s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Gritz, Michael A.; Puscasu, Irina; Spencer, David; and Boreman, Glenn D., "Fabrication of an infrared antenna-coupled microbolometer linear array using chrome as a mask" (2003). Faculty Bibliography 2000s. 3787. https://stars.library.ucf.edu/facultybib2000/3787 Fabrication of an infrared antenna-coupled microbolometer linear array using chromeas a maskMichael A. Gritz, Irina Puscasu, David Spencer, and Glenn D. BoremanCitation: Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures Processing,Measurement, and Phenomena 21, 2608 (2003); doi: 10.1116/1.1629714View online: https://doi.org/10.1116/1.1629714View Table of Contents: https://avs.scitation.org/toc/jvn/21/6Published by the American Institute of PhysicsSHOP NOTESThese are ‘‘how to do it’’ papers. They should be written and illustrated so that the reader may easily follow whateverinstruction or advice is being given.Fabrication of an infrared antenna-coupled microbolometer linear arrayusing chrome as a mask*Michael A. Gritza)School of Optics/CREOL, University of Central Florida, 4000 Central Florida Blvd., Orlando,Florida 32816-2700Irina PuscasuIon Optics Inc., 411 Waverly Oaks Rd., Suite 144, Waltham, Massachusetts 02452David SpencerCornell Nanofabrication Facility, Cornell University, Ithaca, New York 14853-540Glenn D. BoremanSchool of Optics/CREOL, University of Central Florida, 4000 Central Florida Blvd., Orlando,Florida 32816-2700~Received 4 March 2003; accepted 6 October 2003; published 26 November 2003!@DOI: 10.1116/1.1629714#I. INTRODUCTIONConventional bolometers have been used as thermal de-tectors for infrared ~IR! radiation by making use of thechange of the bolometer resistance with an increase in thetemperature either by absorbing the irradiation directly orabsorbing the heat power by a radiation absorber.1 For thecase of an antenna-coupled microbolometer, the irradiationpower is then received by the antenna. Then the inducedcurrent on the antenna passes through the microbolometer,which causes a change in the resistance of the bolometerfrom Joule heating. The advantages of antenna-coupled mi-crobolometers are their room temperature operation as wellas their tunablity for wavelength and polarization response.Antenna-coupled microbolometers and metal-oxide-metaldetectors have been demonstrated in the infrared at wave-lengths near 10 mm.2–4 Work has been done in fabricating athin-film niobium antenna-coupled microbolometer elementusing spiral antennas fabricated on a silicon ~Si! substrate.5This work suggested a method of fabricating a bolometricdetector using dipole elements.In this article, we present our work on the fabricationprocess developed for a dipole antenna-coupled microbolom-eter linear array. The array was fabricated with a direct writeelectron beam ~e-beam! lithography tool. We have demon-strated a process which uses a reactive ion etcher ~RIE! withchromium as a mask for the fabrication of the aluminumdipole antennas. A traditional lift-off process yielded alumi-num dipole antenna arms with widths of 200 nm. Our pro-cess allowed for a single layer of resist to be used, whichenhances our linewidth control to 90 nm for the dipole an-tenna arms.II. DESIGNResults from previous work indicated that the effectivewavelength of our system was 2.5 mm.6 In order to obtain alinear array with a beam maximum at 90°, the spacing be-tween elements should be an integer multiple of the effectivewavelength. Therefore, we designed a linear array with acenter-to-center spacing of 5 mm between dipole antennas.The receivers were fabricated on a 3 in. silicon ~Si! substrate,which was coated on both sides with 200 nm of silicon di-oxide (SiO2). Previous work has found that a high resistivitysubstrate in the range of 2335–3941 V cm yielded a 52%power transmission.7 Therefore, the resistivity of the sub-strate was chosen to be in this range. The Si wafers werepolished on both sides to allow for illumination from eitherside of the substrate. The linear array consisted of 15 dipoleelements with a center-to-center spacing of 5 mm and with adipole length of 10 mm. Figure 1 displays a scanning elec-tron micrograph ~SEM! of three elements within our lineararray.III. FABRICATION AND RESULTSThe linear arrays were fabricated at the Cornell Nanofab-rication Facility using a Cambridge/Leica ~LeicaMicrosystems—www.leica-microsystems.com! ElectronBeam Micro-Fabricator ~EBMF! 10.5 e-beam lithographysystem. The field that was used to write the devices was819.2 mm. On each 10 mm310 mm Si chip ten devices were*No proof corrections received from author prior to publication.a!Author to whom correspondence should be addressed; electronic mail:mgritz@creol.ucf.edu2608 2608J. Vac. Sci. Technol. B 216, NovÕDec 2003 1071-1023Õ2003Õ216Õ2608Õ4Õ$19.00 ©2003 American Vacuum Societywritten with a 730 mm center-to-center spacing. A singleelement detector was fabricated on each chip in order toensure that all elements of the receivers were functioningproperly. There were four separate layers which were fabri-cated in the following order:~1! gold alignment marks using a bilayer resist lift-off pro-cess;~2! aluminum antenna arms using a chromium mask alumi-num etching process;~3! aluminum bond pads using a bilayer resist lift-off pro-cess; and~4! niobium detectors using a bilayer lift-off process.The alignment marks were made of gold to provide goodcontrast between the marks and Si substrate for detection bythe EBMF backscatter detector.The first step in the fabrication process was to write a setof alignment marks. Both local and global alignment markswere written. The global marks were used to correct for anyrotation errors while loading the substrates into the chuck,while the local marks were used to correct for any stage driftwhen moving from field to field. By using the two sets ofalignment marks, extremely precise alignment was possible.In Ref. 8, when 250 nm of Au was used, the overlay accu-racy was better than 25 nm.The lift-off technique is a common processing techniqueused among e-beam lithographers. In a lift-off process thethickness of the metal will determine the type of resist profileemployed. For applications that require a thickness greaterthan 50 nm, a bilayer lift-off process is used. A bilayer lift-off process uses two layers of resist. The bottom layer ispoly~methylmethacrylate and methacrylic acid! ~PMMA-MAA! @see Fig. 2~a!#. The PMMA-MAA layer has a largerundercut because of its higher sensitivity.9 The second layeris poly~methylmethacrylate! PMMA. If e-beam evaporationis used to deposit the metal, then the PMMA will be theultimate resolution of the entire system @see Fig. 2~b!#.E-beam evaporation is considered to be a point source,which implies that the material will fall onto the resist andsubstrate. This will provide a clean separation of the materialallowing for a solvent such as methylene chloride, whichdissolves the resist, to undercut the resist @see Fig. 2~c!#.The alignment marks were fabricated on a 3 in. Si sub-strate using a bilayer resist profile. First PMMA-MAA wasspun on the wafer at 3500 rpm. The substrates were baked ona hot plate set to 170 °C for 10 min to remove any solvents.Using an interferometer, the resist thickness was measured at561 nm. After the resist thickness was measured, the secondlayer was spun onto the substrates. For this layer, PMMAwas spun onto the substrates at 3500 rpm. Next, the sub-strates were baked on a hot plate set to 170 °C for 10 min.This yielded a total resist thickness of 710 nm. The align-ment marks were then exposed in the e-beam writer at abeam current of 30 nA with a dose of 200 mC/cm2. Thewafer was developed in Methyl Isobutyl Ketone: Isopropanol~MIBK:IPA! 1:1 for 2 min. After development the substrateswere rinsed with isopropanol and dried with N2 . The nextstep was to remove any unwanted resist, which may havebeen left behind after the development but not seen under anoptical microscope. The presence of such a thin layer ofresist was found to lead to bad contacts between elements inour bolometers. The thin layer of resist was removed byoxygen plasma in a barrel etcher.10 This was done each timeimmediately after the development of the substrates. Then a10-nm-thick adhesion layer of titanium and a 250-nm-thicklayer of gold were evaporated onto the substrate. The lift-offwas carried out using methylene chloride to remove theresist.The next step in the fabrication process was to fabricatethe antenna arms. For our process, the wafer was first coatedwith 120 nm of aluminum using a thermal evaporator @seeFig. 3~a!#. Then a single layer of PMMA was spun onto thesubstrates at 1700 rpm @see Fig. 3~b!#. The advantage ofusing a single layer of resist is there is less forward scatteringin the resist, thus enhancing line width control. Next thesubstrates were baked on a hot plate set to 170 °C for 10 min.This yielded a total resist thickness of 140 nm. The antennaarms were then exposed in the e-beam writer in positiveFIG. 1. SEM micrograph of three of the dipole elements in the linear array ofantenna-coupled niobium microbolometers.FIG. 2. Diagram of a bilayer e-beam resist profile. ~a! PMMA is spun on topof the copolymer PMMA-MAA. ~b! After the metal is e-beam evaporated.~c! The resist is then dissolved in methylene chloride or acetone.2609 Gritz et al.: Fabrication of an infrared antenna 2609JVST B - Microelectronics and Nanometer Structuresresist at a beam current of 0.50 nA with a line dose of 4.4mC/cm. The wafer was developed in MIBK:IPA 1:3 for 2min. After development, the substrates were rinsed with iso-propanol and dried with N2 , and an oxygen plasma etch wasperformed in a barrel etcher for 1 min. Then a 23-nm-thicklayer of chromium was thermally evaporated onto the sub-strate @see Fig. 3~c!#. The lift-off was carried out using me-thylene chloride. Next a RIE was used with 1.5 sccm Cl2 , 40sccm BCl3 , and 20 sccm CH4 to remove the aluminum @seeFig. 3~d!#. The chromium in our system served as an etchmask. The final step is to remove the chromium @see Fig.3~e!#. It was found that chrome etchant also removed alumi-num, so a resist strip using O2 in a barrel etcher with apressure of 1 Torr for 3 min at 1000 W was used to removethe chromium instead. In Ref. 6, a linewidth of 200 nm wasobtained. Our process yielded linewidths for the dipole an-tenna arms of 90 nm, which can be seen in Fig. 4.Next, the aluminum bond pads were fabricated on a 3 in.Si substrate using a bilayer resist profile as previously de-scribed. The bond pads were then exposed in positive resistin the e-beam writer at a beam current of 30 nA with a doseof 200 mC/cm2. The wafer was developed in MIBK:IPA 1:1for 2 min. After development, an oxygen plasma etch wasperformed for 1 min. Then a 100-nm-thick layer of alumi-num was evaporated onto the substrate. The lift-off of thealuminum was carried out using methylene chloride.The final step in the fabrication process was to fabricatethe niobium detectors with dimensions of 1 mm30.25 mm,which allowed for maximum contact area between the detec-tor and the antenna. First PMMA-MAA resist was spun onthe wafer at 3000 rpm. The substrates were baked on a hotplate set to 170 °C for 10 min to remove any solvents. Usingan interferometer, the resist thickness was measured at 305nm. After the resist thickness was measured, the second layerwas spun onto the substrates. For this layer PMMA was spunonto the substrates at 3500 rpm. Next the substrates werebaked on a hot plate set to 170 °C for 10 min. This yielded atotal resist thickness of 460 nm. The niobium detectors werewritten at 1 nA with a dose of 370 mC/cm2. After exposure,the substrates were developed in MIBK:IPA 1:1 for 1 min.After careful inspection to ensure proper development, thesubstrates were cleaned in oxygen plasma using a barreletcher for 1 min. Once the cleaning process was performed,FIG. 3. Details of the fabrication pro-cess to fabricate a linear array ofantenna-coupled microbolometers.FIG. 4. SEM micrograph showing a resolution of 90 nm of the individualdetector elements in the linear array.2610 Gritz et al.: Fabrication of an infrared antenna 2610J. Vac. Sci. Technol. B, Vol. 21, No. 6, NovÕDec 2003the niobium was ready to be sputtered onto the substrate.After several experiments it was found that a pre-sputteringtreatment of the chamber was required to obtain homoge-neous devices. This treatment consisted of the followingsteps seen in Table I. Upon completion of this treatment pro-cess, the substrates were ready to be loaded into the chamberof the sputtering system. The substrates were loaded into themachine as soon as the chamber lid was raised enough toallow for clearance. This kept the base pressure as low aspossible and limited the possibility of contamination of thesputtering system. The chamber was pumped down overnightto obtain the best possible vacuum, which was 1.831027 Torr. Then the shutter was closed and titanium wassputtered for 10 min. The exact processing requirements forthis step are given in Table II. Titanium is a getter material,which means it absorbs oxygen and other gases once it isevaporated. This causes the pressure to become lower in thechamber, allowing a better quality film of niobium to beobtained. The chamber was allowed to cool for approxi-mately 40 min, which yielded a final pressure of 1.531027 Torr. Then niobium was sputtered with the shutterclosed for 5 min. This was done to ensure that all the con-taminants on the niobium target were removed. The shutterwas then opened and niobium was sputtered onto the sub-strates for 5.1 min. This yielded approximately a 70-nm-thick film of niobium. The substrates were allowed to coolfor 30 min inside the chamber. Then the resist and excessniobium were removed with methylene chloride. SEM mi-crographs of the linear array, showing the results of the fab-rication process, are shown in Fig. 1.IV. SUMMARYThe fabrication of a linear array of antenna-coupled mi-crobolometer with critical linewidths of 90 nm has beendemonstrated using electron beam lithography in conjunctionwith a RIE with chromium as a mask. There are two issuesthat must be addressed to further improve the performance ofthe linear array. The first is that the effective wavelength of2.5 mm from Ref. 6 needs to be more precisely determined,which will lead to an enhancement of the signal to noiseratio. The second is that a study on the antenna couplingbetween the individual antenna elements needs to be inves-tigated to improve the performance of the array. This workdemonstrates a unique method of fabrication for a linear ar-ray of antenna-coupled microbolometers with a resolution of90 nm. Improvements on this device could lead to the devel-opment of an uncooled two-dimensional focal plane array ofantenna-coupled IR sensors.ACKNOWLEDGMENTSThis work was performed in part at the Cornell Nanofab-rication facility ~a member of the National NanofabricationUsers Network!, which is supported by the National ScienceFoundation under Grant No. ECS-9731293, Cornell Univer-sity, and industrial affiliates. This material is based upon re-search supported by NASA Grant No. NAG5-10308.1E. L. Dereniak and G. D. Boreman, Infrared Detectors and Systems~Wiley, New York, 1996!, Chap. 9.2C. Fumeaux, W. Herrmann, F. K. Kneubu¨hl, and H. Rothuizen, InfraredPhys. 41, 123 ~1998!.3I. Wilke, W. Herrmann, and F. K. Kneubu¨hl, Appl. Phys. B: Lasers Opt.58, 87 ~1994!.4I. Wilke, Y. Oppliger, W. Herrmann, and F. K. Kneubu¨hl, Appl. Phys. A:Solids Surf. 58, 329 ~1994!.5M. E. MacDonald and E. Grossman, IEEE Trans. Microwave TheoryTech. Pt. 1, 893 ~1995!.6C. Fumeaux, M. A. Gritz, I. Codreanu, W. L. Schaich, F. J. Gonza´lez, andG. Boreman, Infrared Phys. 41, 271 ~2000!.7J. Alda, C. Fumeaux, M. A. Gritz, D. Spencer, and G. Boreman, InfraredPhys. 41, 1 ~2000!.8M. Gritz, M. Metzler, J. Moser, D. Spencer, and G. Boreman, J. Vac. Sci.Technol. B 21, 332 ~2003!.9P. Rai-Choudhury, Handbook of Microlithography, Micromachining, andMicrofabrication, Vol. 1: Microlithography ~SPIE, Bellingham, WA,1997!, Chap. 2.10M. N. Webster, A. Tuinhout, B. Lochel, A. H. Verbruggen, J. Romijn, E.van der Drift, S. Radelaar, H. Jos, and P. M. A. Moors, Microelectron.Eng. 23, 441 ~1994!.TABLE I. Pre-sputtering process.1. Install niobium and titanium targets.2. Close shutter over niobium target.Attach power supply to niobium target.3. Pump argon into the chamber.Set pressure to 3 mTorr ~21.1 sccm!.4. Apply 2 kW dc power.5. Keep shutter closed 5 min.6. Open shutter 10 min.TABLE II. Niobium sputtering process.1. Close shutter over titanium target.Attach power supply to titanium target.2. Pump argon into the chamber.Set pressure to 1.6 mTorr ~9.2 sccm!.3. Apply 2 kW dc power.4. Keep shutter closed 10 min.5. Turn off dc power. Allow chamber to cool for 40 min.6. Close shutter over niobium target.Attach power supply to niobium target.7. Set argon pressure to 3.1 mTorr ~21.1 sccm!.8. Apply 2 kW dc power.9. Keep shutter closed for 5 min.10. Open shutter for 5.1 min.2611 Gritz et al.: Fabrication of an infrared antenna 2611JVST B - Microelectronics and Nanometer Structures

Fabrication of an infrared antenna-coupled microbolometer linear array using chrome as a mask

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Fabrication of an infrared antenna-coupled microbolometer linear array using chrome as a mask

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