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Antenna length and polarization response of antenna-coupled MOM diode infrared detectors
Infrared Physics & Technology 53 (2010) 182–185
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Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Antenna length and polarization response of antenna-coupled MOM diode infrared detectors Jeffrey A. Bean *, Badri Tiwari, Gergo Szakmány, Gary H. Bernstein, P. Fay, Wolfgang Porod Center for Nano Science and Technology, Department of Electrical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA
a r t i c l e
i n f o
Article history: Received 12 February 2009 Available online 22 November 2009 Keywords: Infrared detector Dipole antenna Metal-oxide-metal diode
a b s t r a c t This work focuses on the fabrication and response of dipole antenna-coupled metal–oxide–metal diode detectors to long-wave infrared radiation. The detectors are fabricated using a single electron beam lithography step and a shadow evaporation technique. The detector’s characteristics are presented, which include response as a function of incident infrared power and polarization angle. In addition, the effect of dipole antenna length on detection characteristics for 10.6 lm radiation has been measured to determine resonant lengths. The response of the detector shows a first resonance at a dipole length of 3.1 lm, a second resonance at 9.3 lm, and third at 15.5 lm. The zeros intermediate to the resonances are also evident. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The thermal or long-wave infrared (LWIR) band is an important frequency range for imaging because objects near 300 K, such as humans, animals, or other heat sources (e.g. engines, vehicles, machinery), can be detected without extrinsic illumination. Thermal detectors could be used for target tracking or image recognition, and robust uncooled imaging systems could be fashioned with a multi-spectral imaging approach. A plethora of LWIR detectors are commercially available – photon (quantum) detectors such as HgCdTe and quantum-well infrared photodetectors and thermal detectors such as thermopiles and bolometers. However, this paper discusses detectors that offer characteristics not provided by other detector technologies, namely the antenna-coupled metal–oxide–metal diode (ACMOMD) radiation-field infrared detector. ACMOMDs are composed of a planar antenna structure, a dipole antenna in this case, that detects the radiation field of incident infrared radiation, coupled to a nonlinear junction that rectifies the radiation-induced antenna currents. The rectifying element is a metal–oxide–metal (MOM) diode in this case. ACMOMDs differ from photon detectors in that they do not absorb photons and generate electron–hole pairs, but rather work by the rectification of antenna currents. These devices offer CMOS compatible fabrication, a small ‘footprint’, functionality without an applied bias, and operate at room temperature (i.e. do not require cooling). The first devices of this kind, based on point-contact or catwhisker diodes, were introduced in the 1960s and were capable
* Corresponding author. E-mail address: [email protected] (J.A. Bean). 1350-4495/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.infrared.2009.11.004
of functioning as infrared detectors [1,2]. The advancement of lithographic technologies allowed thin-film antenna-coupled diodes to be fabricated [3,4] in a more reliable and repeatable fashion. More recently, devices defined using electron beam lithography have further advanced this type of infrared detector [5–7]. In this paper, design elements and the fabrication process of antenna-coupled metal–oxide–metal diode detectors are detailed. The experimental testing arrangement of ACMOMDs is then discussed and the detection characteristics are presented.
2. Design and fabrication A half-wavelength dipole antenna is used as the element that couples to the LWIR radiation. In this case, the antenna is located at the interface of the substrate, which is SiO2 on top of a silicon wafer, and air. The devices presented in this paper are fabricated with electron beam lithography (EBL) and a shadow evaporation process that has been previously utilized to make nanoscale tunnel junctions [8], single-electron transistors [9], and most recently ACMOMDs [10]. The shadow evaporation process consists of two metal evaporations at opposing angles through a developed, undercut electron resist with an intermediate oxidation step. It is in this manner that the MOM diode in an ACMOMD is formed. While the fabrication process is outlined in this section, full details can be found in Ref. [10]. For this work, the chosen resist is PMMA on top of a copolymer MMA layer. A top-view and cross-section of the developed resist pattern is shown in Fig. 1. The PMMA layer is approximately 700 Å thick and the MMA layer is approximately 4500 Å thick. The undercut of the developed
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experiments. For an ACMOMD with a 3.1 lm dipole, the detector’s effective area is represented by a 12 lm 6.5 lm ellipse, with the larger dimension in the along-arm direction [12].
lead to Ti/Au contact
cross-section along dipole antenna lead to Ti/Au contact
PMMA bridge
Platinum
Aluminum
3. Experimental method
MOM diode
PMMA
Copolymer
SiO2
Si
Fig. 1. Top-view of developed resist (top) and cross-section of the developed resist along the antenna arms (bottom). The MOM diode is formed after two metal evaporations at opposing 7° angles and an intermediate oxidation step, and its position is noted.
resist is due to the two layer resist, and is a function of the sensitivity and thickness of the MMA layer as well as the beam acceleration voltage used for EBL. The first evaporated layer is aluminum, which forms a thin native oxide in the presence of oxygen. This oxide is believed to be Al2O3, but will be referred to as AlOx because the exact composition is difficult to accurately determine due to the very thin films used. The oxygen exposure is determined by the oxygen pressure in the evaporator chamber and the oxidation time. The second evaporated metal layer, platinum in this case, is then deposited, which creates an Al/AlOx/Pt MOM diode between the two arms of the dipole antenna. By utilizing the shadow evaporation procedure, these devices can be made with a single EBL step, providing a faster, more economical method of fabrication that avoids the need to align multiple EBL steps. A completed Al/AlOx/Pt ACMOMD is shown in Fig. 2. The device is connected to titanium/gold leads, which provide the electrical connections for current–voltage (I–V) characteristics and infrared response measurements. The MOM overlap area is approximately 50 nm 80 nm; the length of the dipole antenna is varied for the
A CO2 laser is used to determine the detection characteristics of the ACMOMDs presented in this paper. The experimental setup, shown in Fig. 3, consists of a 25 W Synrad 48-2 Series linearly polarized CO2 laser with 10.6 lm nominal emission wavelength. A beamsplitter is used to attenuate the beam so that no more than 100 mW is directed towards the device under test (DUT). A mechanical chopper is used to provide square-wave modulation of the beam. A power meter is used to continuously monitor the beam power with respect to the device output. A micrometer stage is used to position the DUT within the beam. The device current is amplified using a low-noise current preamplifier and read out using a lock-in amplifier. The ACMOMDs tested in this work were tested with the beam oriented at normal incidence to the substrate. Fig. 4 shows an illustration of an incident electromagnetic wave on an ACMOMD. In this case, the polarization of the electric field is parallel to the dipole antenna, and is denoted as Ell. In determining figures of merit, such as responsivity, signal to noise ratio (SNR), noise-equivalent power (NEP), and normalized detectivity (D*), the direction of the electric field of the incident radiation was aligned parallel to the dipole antenna, where the response is a maximum. This same orientation is used for detector response as a function of infrared input power, and for determining the length-dependence of ACMOMD performance. The polarization direction is also varied to determine the polarization dependence of ACMOMDs.
4. Experimental results The current response of an Al/AlOx/Pt ACMOMD with respect to the infrared input power has been measured and is shown in Fig. 5. The detector functions as a square law detector as expected for an antenna-coupled diode; the response (output current) is proportional to the power of the incident radiation. This detector signal is measured with the polarization of the incident infrared radiation parallel to the antenna, referred to as p-polarization.
beamsplitter
visible alignment laser 25W CO2 laser
beamstop brick mechanical chopper
chopper controller
laser power meter
laser controller
beamsplitter
chip socket and DUT SMA connectors
low-noise current preamplifier
lock-in amplifier
5-axis micrometer stage
Fig. 2. Scanning electron micrograph of an Al/AlOx/Pt ACMOMD connected to Ti/Au electrical leads. Inset: micrograph indicating the location of the MOM diode.
Fig. 3. Experimental infrared testing arrangement. The CO2 laser provides the 10.6 lm radiation used to test the ACMOMDs fabricated in this research. The output of the laser is attenuated and chopped to provide for square-wave modulation of the infrared radiation incident on the DUT.
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compatible and function at room temperature without cooling or applied bias. The polarization dependence of a detector can lend insight into its operation mechanisms. For an antenna-coupled MOM diode, the response should be strongest for p-polarization, i.e. where the electric field of the incident radiation is parallel to the antenna axis (u = 0°), and is referred to as the polarization-dependent response [5]. A smaller cross-polarization response for s-polarization, i.e. where the incident electric field is perpendicular to the antenna axis (u = 90°), is referred to as the polarization-independent contribution [5]. For the devices presented here, this cross-polarization signal is present regardless of the polarization of the incident radiation. The polarization-dependent response is represented by the signal response for p-polarization less that of the response for spolarization. The response of these devices can be represented by the sum of a constant, polarization-independent response, plus a cosine-squared polarization-dependent signal. The detected signal current I(u) can be expressed as:
IðuÞ ¼ Iip þ Ip ðuÞ ¼ Iip þ Ip cos2 ðu0 uÞ
Fig. 4. Electromagnetic wave incident on ACMOMD. The polarization angle of the electric field of the incident wave is represented by u and the angle of incidence is represented by h. For this work, the angle of incidence is normal to the sample.
150
Device Response (pA)
120
90
60
30
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
2
Infrared Irradiance (W/cm )
Fig. 5. Response of an unbiased Al/AlOx/Pt ACMOMD as a function of infrared irradiance. The detector functions as a square law detector within the range of infrared input powers able to be tested in this work.
where Iip is the polarization-independent contribution, Ip is the polarization-dependent contribution, u0 is the location of the maximum polarization response, and u is the angle between the antenna axis and the polarization direction [5]. Fig. 6 shows the polarization response of an Al/AlOx/Pt ACMOMD, fabricated by the shadow-evaporation method used in this work. As expected, the maximum signal was obtained when the electric field of the incident infrared radiation was parallel to the antenna (90°, 270°), while the minimum was measured with the electric field perpendicular to the antenna (0°, 180°). In agreement with antenna theory [5,14], the device response follows a cosinesquared dependence, denoted by the dotted line. The variations in detector response are due to mode hopping in the CO2 laser used for testing and experimental error introduced in the rotational stage. The polarization ratio for this device, which is defined as the ratio between the maximum and minimum signal, is around 10. The response of an antenna-coupled detector depends upon the substrate, antenna geometry, and the wavelength of the incident electromagnetic irradiation. At 28.3 THz, which corresponds to 10.6 lm radiation in air, the dielectric constant of SiO2 is 4.84 [15], corresponding to an effective dielectric constant eeff [16] of
eeff ¼
eSiO2 þ eair 2
¼ 2:92:
The effective wavelength keff of 10.6 lm radiation in a dielectric is then [14]
Device Response (pA)
160
The figures of merit for ACMOMDs operating without bias were also measured using p-polarized 10.6 lm illumination. The detectors function without bias due to the asymmetric current–voltage characteristic with respect to zero bias. This asymmetry arises due to the different metals, and their respective work functions, used to fabricate the MOM diode, which are aluminum and platinum in this case [10,11]. For Al/AlOx/Pt ACMOMDs fabricated with shadow evaporation and a 50 mTorr intermediate oxygen exposure for 40 min, the signal to noise ratio (SNR) is 48.5 dB for an infrared input irradiance of 0.498 W/cm2. Noise-equivalent power (NEP), the amount of radiant power collected by a detector that produces a signal-to-noise ratio (SNR) of 1, was measured to be 1.15 nW/ Hz1/2. Specific detectivity (D*) is commonly used to compare optical and infrared detectors [13] and was measured to be 2.15 106 cm Hz1/2 W1. While D* is lower than that of state-ofthe-art HgCdTe, the detectors presented in the paper are CMOS
dipole antenna
120
80 parallel polarization perpendicular polarization
40
0 0
60
120
180
240
300
360
Polarization Angle (degrees)
Fig. 6. Polarization response of an unbiased Al/AlOx/Pt 3.1 lm ACMOMD to 10.6 lm radiation normal to the substrate. The polarization angle of the antenna with respect to the dipole antenna orientation is indicated.
J.A. Bean et al. / Infrared Physics & Technology 53 (2010) 182–185
4
surface attenuation. The theoretical attenuation is CC = 1.48 lm1 and the theoretical surface attenuation constant [6] due to the skin effect, is Csr = 22.7 lm1. Since the experimental attenuation constant is much smaller than the surface attenuation constant, it can be concluded that skin effect imparts a negligible attenuation and that the experimental antenna current attenuation is due to the Coleman effect [18]. This is the same result as previously found by Wilke [6] and Fumeaux [19].
2
5. Conclusions
10 Polarization Ratio pol 4ratio unity sin dependence
Polarization Ratio
8
6
0 0
3.1
6.2
9.3
12.4
15.5
18.6
Antenna Length (µm)
Fig. 7. Polarization ratio of Al/AlOx/Pt ACMOMDs as a function of the length of the dipole antenna. The data demonstrates the expected theoretical dependence on antenna length and signal attenuation due to the Coleman effect. The error bars represent the standard deviation of polarization ratios of the measured devices, which is calculated by measuring at least eight devices at each length.
k0 keff ¼ pffiffiffiffiffiffiffi ¼ 6:2 lm:
eeff
A half-wavelength dipole is chosen because the rectifying MOM diode is at the center of the dipole where a current antinode forms at the resonant frequency [14]. For dipole antennas fabricated on a silicon substrate with 1.5 lm silicon dioxide quarter-wave matching layers exposed to 10.6 lm laser radiation, the optimum dipole length has been found to be 3.1 lm [6,17]. Fig. 7 shows the polarization ratio as a function of antenna length for Al/AlOx/Pt shadow-evaporation ACMOMDs. Polarization ratio, rather than polarization-dependent signal, is shown in an effort to normalize response from numerous detectors of the same length and minimize the impacts of mode-hopping of the CO2 laser. A zero polarization-dependent response corresponds to a polarization ratio of one in this case, and therefore the theoretical dependence on antenna length still holds and is shown by the dotted line in the figure. The first maximum, or resonance, occurs at the expected antenna length of 3.1 lm. However, for shorter antenna lengths, the electrical leads can have an effect on the detected polarizationdependent signal. The leads can act as antennas themselves and contribute a signal that is independent of the antenna geometry and orientation, which decreases the polarization ratio. However, this impact is small because the leads are longer than 10 lm and currents are attenuated due to the significant conduction losses at these frequencies [18]. In addition, since the leads are wide, transverse currents would dominate and hinder resonance. The measurements closely match the theory for the first and second maxima and minima, and the third maximum. However, beyond the third maximum, the response is significantly attenuated. By fitting the measured data to a theoretical attenuated antenna response [6], which assumes the form of 4
sin
Cexp L 2 e ;
pL 2k
185
the attenuation constant Cexp is found to be 0.40 lm1. This takes into account all forms of attenuation such as Coleman effect and
ACMOMD devices capable of detecting long-wave infrared radiation have been fabricated and characterized. The fabrication offers CMOS compatibility, a small detector area, and produces devices that function without cooling or biasing at room temperature. The devices show a polarization dependence that indicates that the response is due to resonant currents in the antenna. Further, the resonant lengths for infrared dipole antennas to 10.6 lm radiation were experimentally determined. References [1] S.I. Green, Point contact MOM tunneling detector analysis, Journal of Applied Physics 42 (1971) 1166. [2] Y. Yasuoka, T. Sakurada, D.P. Siu, T.K. Gustafson, Resistance dependence of detected signals of MOM diodes, Journal of Applied Physics 50 (1979) 5860. [3] M. Heiblum, S.Y. Wang, T.K. Gustafson, J.R. Whinnery, Edge-MOM diode: an integrated optical, nonlinear device, IEEE Transactions on Electron Devices ED24 (1977) 1199. [4] S.Y. Wang, T. Izawa, T.K. Gustafson, Coupling characteristics of thin-film metal–oxide–metal diodes at 10.6 lm, Applied Physics Letters 27 (1975) 481. [5] C. Fumeaux, W. Herrmann, F.K. Kneubuhl, H. Rothuizen, Nanometer thin-film Ni–NiO–Ni diodes for detection and mixing of 30 THz radiation, Infrared Physics & Technology 39 (1998) 123. [6] I. Wilke, W. Herrmann, F.K. Kneubuhl, Integrated nanostrip dipole antennas for coherent 30 THz infrared radiation, Applied Physics B (Lasers and Optics) B58 (1994) 87. [7] I. Wilke, Y. Oppliger, W. Herrmann, F.K. Kneubuhl, Nanometer thin-film Ni– NiO–Ni diodes for 30 THz radiation, Applied Physics A 58 (1994) 329. [8] G.J. Dolan, Offset works for lift-off photoprocessing, Applied Physics Letters 31 (1977) 337. [9] A.O. Orlov, I. Amlani, R.K. Kummamuru, R. Ramasubramaniam, G. Toth, C.S. Lent, G.H. Bernstein, G.L. Snider, Experimental demonstration of clocked single-electron switching in quantum-dot cellular automata, Applied Physics Letters 77 (2000) 295. [10] J.A. Bean, B. Tiwari, G.H. Bernstein, P. Fay, W. Porod, Long wave infrared detection using dipole antenna-coupled metal–oxide–metal diodes, Journal of Vacuum Science & Technology B 27 (2009) 11. [11] B. Tiwari, J.A. Bean, G. Szakmany, G.H. Bernstein, P. Fay, W. Porod, Controlled etching and regrowth of tunnel oxide for antenna-coupled metal–oxide–metal diodes, Journal of Vacuum Science & Technology B 27 (2009) 2153. [12] C. Fumeaux, G.D. Boreman, W. Herrmann, F.K. Kneubühl, H. Rothuizen, Spatial impulse response of lithographic antennas, Applied Optics 38 (1999) 37. [13] E.L. Dereniak, G.D. Boreman, Infrared Detectors and Systems, John Wiley & Sons Inc., New York, 1996. [14] C.A. Balanis, Antenna Theory: Analysis and Design, John Wiley & Sons, Hoboken, 2005. [15] E.D. Palik, Handbook of Optical Constants of Solids, Academic Press, 1985. [16] G. Hasnain, G. Arjavalingam, A. Dienes, J.R. Whinnery, Dispersion of Picosecond Pulses on Microstrip Transmission Lines, San Diego, CA, USA, 1983, pp. 159. [17] B. Rakos, Investigation of Metal–oxide–metal Structures for Optical Sensor Applications, in Electrical Engineering, Ph.D. Dissertation, University of Notre Dame, Notre Dame, 2006. [18] B.L. Coleman, Propagation of electromagnetic disturbances along a thin wire in a horizontally stratified medium, Philosophical Magazine 41 (1950) 276. [19] C. Fumeaux, M.A. Gritz, I. Codreanu, W.L. Schaich, F.J. González, G.D. Boreman, Measurement of the resonant lengths of infrared dipole antennas, Infrared Physics & Technology 41 (2000) 271.
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Antenna length and polarization response of antenna-coupled MOM diode infrared detectors Jeffrey A. Bean *, Badri Tiwari, Gergo Szakmány, Gary H. Bernstein, P. Fay, Wolfgang Porod Center for Nano Science and Technology, Department of Electrical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA
a r t i c l e
i n f o
Article history: Received 12 February 2009 Available online 22 November 2009 Keywords: Infrared detector Dipole antenna Metal-oxide-metal diode
a b s t r a c t This work focuses on the fabrication and response of dipole antenna-coupled metal–oxide–metal diode detectors to long-wave infrared radiation. The detectors are fabricated using a single electron beam lithography step and a shadow evaporation technique. The detector’s characteristics are presented, which include response as a function of incident infrared power and polarization angle. In addition, the effect of dipole antenna length on detection characteristics for 10.6 lm radiation has been measured to determine resonant lengths. The response of the detector shows a first resonance at a dipole length of 3.1 lm, a second resonance at 9.3 lm, and third at 15.5 lm. The zeros intermediate to the resonances are also evident. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The thermal or long-wave infrared (LWIR) band is an important frequency range for imaging because objects near 300 K, such as humans, animals, or other heat sources (e.g. engines, vehicles, machinery), can be detected without extrinsic illumination. Thermal detectors could be used for target tracking or image recognition, and robust uncooled imaging systems could be fashioned with a multi-spectral imaging approach. A plethora of LWIR detectors are commercially available – photon (quantum) detectors such as HgCdTe and quantum-well infrared photodetectors and thermal detectors such as thermopiles and bolometers. However, this paper discusses detectors that offer characteristics not provided by other detector technologies, namely the antenna-coupled metal–oxide–metal diode (ACMOMD) radiation-field infrared detector. ACMOMDs are composed of a planar antenna structure, a dipole antenna in this case, that detects the radiation field of incident infrared radiation, coupled to a nonlinear junction that rectifies the radiation-induced antenna currents. The rectifying element is a metal–oxide–metal (MOM) diode in this case. ACMOMDs differ from photon detectors in that they do not absorb photons and generate electron–hole pairs, but rather work by the rectification of antenna currents. These devices offer CMOS compatible fabrication, a small ‘footprint’, functionality without an applied bias, and operate at room temperature (i.e. do not require cooling). The first devices of this kind, based on point-contact or catwhisker diodes, were introduced in the 1960s and were capable
* Corresponding author. E-mail address: [email protected] (J.A. Bean). 1350-4495/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.infrared.2009.11.004
of functioning as infrared detectors [1,2]. The advancement of lithographic technologies allowed thin-film antenna-coupled diodes to be fabricated [3,4] in a more reliable and repeatable fashion. More recently, devices defined using electron beam lithography have further advanced this type of infrared detector [5–7]. In this paper, design elements and the fabrication process of antenna-coupled metal–oxide–metal diode detectors are detailed. The experimental testing arrangement of ACMOMDs is then discussed and the detection characteristics are presented.
2. Design and fabrication A half-wavelength dipole antenna is used as the element that couples to the LWIR radiation. In this case, the antenna is located at the interface of the substrate, which is SiO2 on top of a silicon wafer, and air. The devices presented in this paper are fabricated with electron beam lithography (EBL) and a shadow evaporation process that has been previously utilized to make nanoscale tunnel junctions [8], single-electron transistors [9], and most recently ACMOMDs [10]. The shadow evaporation process consists of two metal evaporations at opposing angles through a developed, undercut electron resist with an intermediate oxidation step. It is in this manner that the MOM diode in an ACMOMD is formed. While the fabrication process is outlined in this section, full details can be found in Ref. [10]. For this work, the chosen resist is PMMA on top of a copolymer MMA layer. A top-view and cross-section of the developed resist pattern is shown in Fig. 1. The PMMA layer is approximately 700 Å thick and the MMA layer is approximately 4500 Å thick. The undercut of the developed
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J.A. Bean et al. / Infrared Physics & Technology 53 (2010) 182–185
experiments. For an ACMOMD with a 3.1 lm dipole, the detector’s effective area is represented by a 12 lm 6.5 lm ellipse, with the larger dimension in the along-arm direction [12].
lead to Ti/Au contact
cross-section along dipole antenna lead to Ti/Au contact
PMMA bridge
Platinum
Aluminum
3. Experimental method
MOM diode
PMMA
Copolymer
SiO2
Si
Fig. 1. Top-view of developed resist (top) and cross-section of the developed resist along the antenna arms (bottom). The MOM diode is formed after two metal evaporations at opposing 7° angles and an intermediate oxidation step, and its position is noted.
resist is due to the two layer resist, and is a function of the sensitivity and thickness of the MMA layer as well as the beam acceleration voltage used for EBL. The first evaporated layer is aluminum, which forms a thin native oxide in the presence of oxygen. This oxide is believed to be Al2O3, but will be referred to as AlOx because the exact composition is difficult to accurately determine due to the very thin films used. The oxygen exposure is determined by the oxygen pressure in the evaporator chamber and the oxidation time. The second evaporated metal layer, platinum in this case, is then deposited, which creates an Al/AlOx/Pt MOM diode between the two arms of the dipole antenna. By utilizing the shadow evaporation procedure, these devices can be made with a single EBL step, providing a faster, more economical method of fabrication that avoids the need to align multiple EBL steps. A completed Al/AlOx/Pt ACMOMD is shown in Fig. 2. The device is connected to titanium/gold leads, which provide the electrical connections for current–voltage (I–V) characteristics and infrared response measurements. The MOM overlap area is approximately 50 nm 80 nm; the length of the dipole antenna is varied for the
A CO2 laser is used to determine the detection characteristics of the ACMOMDs presented in this paper. The experimental setup, shown in Fig. 3, consists of a 25 W Synrad 48-2 Series linearly polarized CO2 laser with 10.6 lm nominal emission wavelength. A beamsplitter is used to attenuate the beam so that no more than 100 mW is directed towards the device under test (DUT). A mechanical chopper is used to provide square-wave modulation of the beam. A power meter is used to continuously monitor the beam power with respect to the device output. A micrometer stage is used to position the DUT within the beam. The device current is amplified using a low-noise current preamplifier and read out using a lock-in amplifier. The ACMOMDs tested in this work were tested with the beam oriented at normal incidence to the substrate. Fig. 4 shows an illustration of an incident electromagnetic wave on an ACMOMD. In this case, the polarization of the electric field is parallel to the dipole antenna, and is denoted as Ell. In determining figures of merit, such as responsivity, signal to noise ratio (SNR), noise-equivalent power (NEP), and normalized detectivity (D*), the direction of the electric field of the incident radiation was aligned parallel to the dipole antenna, where the response is a maximum. This same orientation is used for detector response as a function of infrared input power, and for determining the length-dependence of ACMOMD performance. The polarization direction is also varied to determine the polarization dependence of ACMOMDs.
4. Experimental results The current response of an Al/AlOx/Pt ACMOMD with respect to the infrared input power has been measured and is shown in Fig. 5. The detector functions as a square law detector as expected for an antenna-coupled diode; the response (output current) is proportional to the power of the incident radiation. This detector signal is measured with the polarization of the incident infrared radiation parallel to the antenna, referred to as p-polarization.
beamsplitter
visible alignment laser 25W CO2 laser
beamstop brick mechanical chopper
chopper controller
laser power meter
laser controller
beamsplitter
chip socket and DUT SMA connectors
low-noise current preamplifier
lock-in amplifier
5-axis micrometer stage
Fig. 2. Scanning electron micrograph of an Al/AlOx/Pt ACMOMD connected to Ti/Au electrical leads. Inset: micrograph indicating the location of the MOM diode.
Fig. 3. Experimental infrared testing arrangement. The CO2 laser provides the 10.6 lm radiation used to test the ACMOMDs fabricated in this research. The output of the laser is attenuated and chopped to provide for square-wave modulation of the infrared radiation incident on the DUT.
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compatible and function at room temperature without cooling or applied bias. The polarization dependence of a detector can lend insight into its operation mechanisms. For an antenna-coupled MOM diode, the response should be strongest for p-polarization, i.e. where the electric field of the incident radiation is parallel to the antenna axis (u = 0°), and is referred to as the polarization-dependent response [5]. A smaller cross-polarization response for s-polarization, i.e. where the incident electric field is perpendicular to the antenna axis (u = 90°), is referred to as the polarization-independent contribution [5]. For the devices presented here, this cross-polarization signal is present regardless of the polarization of the incident radiation. The polarization-dependent response is represented by the signal response for p-polarization less that of the response for spolarization. The response of these devices can be represented by the sum of a constant, polarization-independent response, plus a cosine-squared polarization-dependent signal. The detected signal current I(u) can be expressed as:
IðuÞ ¼ Iip þ Ip ðuÞ ¼ Iip þ Ip cos2 ðu0 uÞ
Fig. 4. Electromagnetic wave incident on ACMOMD. The polarization angle of the electric field of the incident wave is represented by u and the angle of incidence is represented by h. For this work, the angle of incidence is normal to the sample.
150
Device Response (pA)
120
90
60
30
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
2
Infrared Irradiance (W/cm )
Fig. 5. Response of an unbiased Al/AlOx/Pt ACMOMD as a function of infrared irradiance. The detector functions as a square law detector within the range of infrared input powers able to be tested in this work.
where Iip is the polarization-independent contribution, Ip is the polarization-dependent contribution, u0 is the location of the maximum polarization response, and u is the angle between the antenna axis and the polarization direction [5]. Fig. 6 shows the polarization response of an Al/AlOx/Pt ACMOMD, fabricated by the shadow-evaporation method used in this work. As expected, the maximum signal was obtained when the electric field of the incident infrared radiation was parallel to the antenna (90°, 270°), while the minimum was measured with the electric field perpendicular to the antenna (0°, 180°). In agreement with antenna theory [5,14], the device response follows a cosinesquared dependence, denoted by the dotted line. The variations in detector response are due to mode hopping in the CO2 laser used for testing and experimental error introduced in the rotational stage. The polarization ratio for this device, which is defined as the ratio between the maximum and minimum signal, is around 10. The response of an antenna-coupled detector depends upon the substrate, antenna geometry, and the wavelength of the incident electromagnetic irradiation. At 28.3 THz, which corresponds to 10.6 lm radiation in air, the dielectric constant of SiO2 is 4.84 [15], corresponding to an effective dielectric constant eeff [16] of
eeff ¼
eSiO2 þ eair 2
¼ 2:92:
The effective wavelength keff of 10.6 lm radiation in a dielectric is then [14]
Device Response (pA)
160
The figures of merit for ACMOMDs operating without bias were also measured using p-polarized 10.6 lm illumination. The detectors function without bias due to the asymmetric current–voltage characteristic with respect to zero bias. This asymmetry arises due to the different metals, and their respective work functions, used to fabricate the MOM diode, which are aluminum and platinum in this case [10,11]. For Al/AlOx/Pt ACMOMDs fabricated with shadow evaporation and a 50 mTorr intermediate oxygen exposure for 40 min, the signal to noise ratio (SNR) is 48.5 dB for an infrared input irradiance of 0.498 W/cm2. Noise-equivalent power (NEP), the amount of radiant power collected by a detector that produces a signal-to-noise ratio (SNR) of 1, was measured to be 1.15 nW/ Hz1/2. Specific detectivity (D*) is commonly used to compare optical and infrared detectors [13] and was measured to be 2.15 106 cm Hz1/2 W1. While D* is lower than that of state-ofthe-art HgCdTe, the detectors presented in the paper are CMOS
dipole antenna
120
80 parallel polarization perpendicular polarization
40
0 0
60
120
180
240
300
360
Polarization Angle (degrees)
Fig. 6. Polarization response of an unbiased Al/AlOx/Pt 3.1 lm ACMOMD to 10.6 lm radiation normal to the substrate. The polarization angle of the antenna with respect to the dipole antenna orientation is indicated.
J.A. Bean et al. / Infrared Physics & Technology 53 (2010) 182–185
4
surface attenuation. The theoretical attenuation is CC = 1.48 lm1 and the theoretical surface attenuation constant [6] due to the skin effect, is Csr = 22.7 lm1. Since the experimental attenuation constant is much smaller than the surface attenuation constant, it can be concluded that skin effect imparts a negligible attenuation and that the experimental antenna current attenuation is due to the Coleman effect [18]. This is the same result as previously found by Wilke [6] and Fumeaux [19].
2
5. Conclusions
10 Polarization Ratio pol 4ratio unity sin dependence
Polarization Ratio
8
6
0 0
3.1
6.2
9.3
12.4
15.5
18.6
Antenna Length (µm)
Fig. 7. Polarization ratio of Al/AlOx/Pt ACMOMDs as a function of the length of the dipole antenna. The data demonstrates the expected theoretical dependence on antenna length and signal attenuation due to the Coleman effect. The error bars represent the standard deviation of polarization ratios of the measured devices, which is calculated by measuring at least eight devices at each length.
k0 keff ¼ pffiffiffiffiffiffiffi ¼ 6:2 lm:
eeff
A half-wavelength dipole is chosen because the rectifying MOM diode is at the center of the dipole where a current antinode forms at the resonant frequency [14]. For dipole antennas fabricated on a silicon substrate with 1.5 lm silicon dioxide quarter-wave matching layers exposed to 10.6 lm laser radiation, the optimum dipole length has been found to be 3.1 lm [6,17]. Fig. 7 shows the polarization ratio as a function of antenna length for Al/AlOx/Pt shadow-evaporation ACMOMDs. Polarization ratio, rather than polarization-dependent signal, is shown in an effort to normalize response from numerous detectors of the same length and minimize the impacts of mode-hopping of the CO2 laser. A zero polarization-dependent response corresponds to a polarization ratio of one in this case, and therefore the theoretical dependence on antenna length still holds and is shown by the dotted line in the figure. The first maximum, or resonance, occurs at the expected antenna length of 3.1 lm. However, for shorter antenna lengths, the electrical leads can have an effect on the detected polarizationdependent signal. The leads can act as antennas themselves and contribute a signal that is independent of the antenna geometry and orientation, which decreases the polarization ratio. However, this impact is small because the leads are longer than 10 lm and currents are attenuated due to the significant conduction losses at these frequencies [18]. In addition, since the leads are wide, transverse currents would dominate and hinder resonance. The measurements closely match the theory for the first and second maxima and minima, and the third maximum. However, beyond the third maximum, the response is significantly attenuated. By fitting the measured data to a theoretical attenuated antenna response [6], which assumes the form of 4
sin
Cexp L 2 e ;
pL 2k
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the attenuation constant Cexp is found to be 0.40 lm1. This takes into account all forms of attenuation such as Coleman effect and
ACMOMD devices capable of detecting long-wave infrared radiation have been fabricated and characterized. The fabrication offers CMOS compatibility, a small detector area, and produces devices that function without cooling or biasing at room temperature. The devices show a polarization dependence that indicates that the response is due to resonant currents in the antenna. Further, the resonant lengths for infrared dipole antennas to 10.6 lm radiation were experimentally determined. References [1] S.I. Green, Point contact MOM tunneling detector analysis, Journal of Applied Physics 42 (1971) 1166. [2] Y. Yasuoka, T. Sakurada, D.P. Siu, T.K. Gustafson, Resistance dependence of detected signals of MOM diodes, Journal of Applied Physics 50 (1979) 5860. [3] M. Heiblum, S.Y. Wang, T.K. Gustafson, J.R. Whinnery, Edge-MOM diode: an integrated optical, nonlinear device, IEEE Transactions on Electron Devices ED24 (1977) 1199. [4] S.Y. Wang, T. Izawa, T.K. Gustafson, Coupling characteristics of thin-film metal–oxide–metal diodes at 10.6 lm, Applied Physics Letters 27 (1975) 481. [5] C. Fumeaux, W. Herrmann, F.K. Kneubuhl, H. Rothuizen, Nanometer thin-film Ni–NiO–Ni diodes for detection and mixing of 30 THz radiation, Infrared Physics & Technology 39 (1998) 123. [6] I. Wilke, W. Herrmann, F.K. Kneubuhl, Integrated nanostrip dipole antennas for coherent 30 THz infrared radiation, Applied Physics B (Lasers and Optics) B58 (1994) 87. [7] I. Wilke, Y. Oppliger, W. Herrmann, F.K. Kneubuhl, Nanometer thin-film Ni– NiO–Ni diodes for 30 THz radiation, Applied Physics A 58 (1994) 329. [8] G.J. Dolan, Offset works for lift-off photoprocessing, Applied Physics Letters 31 (1977) 337. [9] A.O. Orlov, I. Amlani, R.K. Kummamuru, R. Ramasubramaniam, G. Toth, C.S. Lent, G.H. Bernstein, G.L. Snider, Experimental demonstration of clocked single-electron switching in quantum-dot cellular automata, Applied Physics Letters 77 (2000) 295. [10] J.A. Bean, B. Tiwari, G.H. Bernstein, P. Fay, W. Porod, Long wave infrared detection using dipole antenna-coupled metal–oxide–metal diodes, Journal of Vacuum Science & Technology B 27 (2009) 11. [11] B. Tiwari, J.A. Bean, G. Szakmany, G.H. Bernstein, P. Fay, W. Porod, Controlled etching and regrowth of tunnel oxide for antenna-coupled metal–oxide–metal diodes, Journal of Vacuum Science & Technology B 27 (2009) 2153. [12] C. Fumeaux, G.D. Boreman, W. Herrmann, F.K. Kneubühl, H. Rothuizen, Spatial impulse response of lithographic antennas, Applied Optics 38 (1999) 37. [13] E.L. Dereniak, G.D. Boreman, Infrared Detectors and Systems, John Wiley & Sons Inc., New York, 1996. [14] C.A. Balanis, Antenna Theory: Analysis and Design, John Wiley & Sons, Hoboken, 2005. [15] E.D. Palik, Handbook of Optical Constants of Solids, Academic Press, 1985. [16] G. Hasnain, G. Arjavalingam, A. Dienes, J.R. Whinnery, Dispersion of Picosecond Pulses on Microstrip Transmission Lines, San Diego, CA, USA, 1983, pp. 159. [17] B. Rakos, Investigation of Metal–oxide–metal Structures for Optical Sensor Applications, in Electrical Engineering, Ph.D. Dissertation, University of Notre Dame, Notre Dame, 2006. [18] B.L. Coleman, Propagation of electromagnetic disturbances along a thin wire in a horizontally stratified medium, Philosophical Magazine 41 (1950) 276. [19] C. Fumeaux, M.A. Gritz, I. Codreanu, W.L. Schaich, F.J. González, G.D. Boreman, Measurement of the resonant lengths of infrared dipole antennas, Infrared Physics & Technology 41 (2000) 271.