Far IR Transmission Characteristics of Silicon Nitride ...

Far IR Transmission Characteristics of Silicon Nitride Films using Fourier Transform Spectroscopy D. Fermsca\ E. Castillo-Dominguez\ M. Velazquez^, D. Hughes^ A. Serrano^ A. Torres-Jacome^ 'Instituto Nacional de Astrofisica, Optica y Electronica, A.P. 51 y 216, Puebla, Puebla. C.P. 72000, Mexico ^Lawrence Livermore National Laboratory, 7000 East Ave., L-188, Livermore, CA 94550 Abstract. We are fabricating amorphous Silicon (a-Si) bolometers doped with boron with a measured NEP~1.5 x 10''* W/Hz"^ suitable for use in millimeter and sub-millimeter astronomy. In this paper we present the preliminary results of the absorber optimization for the a-Si bolometers. A fikn of Silicon Nitride (SiN), deposited by LPCVD (Low Pressure Chemical Vapor Deposition) process at INAOE, with or without metallic coating is used as a weak thermal link to the heat sink as well as an absorber. We have measured the transmission spectrum of thin films of SiN in the range of 200 to 1000 GHz using Fourier Transform Spectroscopy (FTS) and a bolometric system with a NEP~1.26 x 10''^. The transmission of thin films of SiN with a thickness of 0.4 |xm has been measured at temperatures of 290 K and 4 K. The uncoated SiN films have a transmission of 80%) and we expect a 50%) transmission for the metallic (e.g. Titanium) coated films. Keywords: Silicon Nitride, FTS. PACS: 78.20.-e

at INAOE to make the spectral measurements of the materials reported in this paper.

INTRODUCTION Fourier transform spectroscopy has been used widely to characterize qualitative and quantitative unknown samples, including molecular spectroscopy of gases, semiconductor studies, low-temperature experiments, emission studies, surface analysis, biochemical and biomedical apphcations among others. We have made deposits of SiN on Quartz samples and measured their transmission; these SiN films might be used as absorbers for a-Si bolometers with astronomical applications in the range of 200 GHz to 1 THz. In the following sections we describe the instrument used to make the spectroscopic measurements, the data processing procedure and the experimental results found.

Fourier Transform Spectrometer-MP The FTS-MP covers a frequency range of -200 GHz to 2.5 THz. The main parts of the instrument are shown in the schematic diagram of Figure 1. The FTSMP consists of two input ports, a beam splitter BS and the output port P2 (BS and P2 are wire-grid polarizers of 25 |im tungsten wire, with 75 |im period). The interfered beam is focused by the parabohc off-axis mirror PM2 and the flat M3 mirror into the Winston cone which feeds the bolometer. The interferogram is recorded by driving the roofmirror M2 at controlled constant velocity (21 |im/seg); the lag step size (typically from 75 to 150 |im) is defined by the samphng rate of the data acquisition system which can be changed in order to have different frequency resolutions. The typical total acquisition time for a 5 GHz resolution spectrum is about 12 minutes. The linear stage has a high accuracy lead screw (0.635 |im/step) driven by a servo motor and the position along the track is provided by the control

SPECTRAL MEASUREMENT DETECTION SYSTEM We use a Fourier Transform Spectrometer of the Martin-Puplett [1] type (FTS-MP) designed and built

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system of the servo (Animatics, SmartMotor). All the functions, including bolometer digitization, motor control and thermometry readout are performed by a personal computer running our own developed Labview 8 code.

(operating at room temperature) and sent to a lock-in amplifier to demodulate the signal and reject noise; this process allows us to recover an interferogram which is a time domain signal. The interferogram is digitized (using a DAQ National Instruments board) and processed in real time by our Lab View code to obtain a preliminary view of the spectrum; final calibration is performed offline with a Matlab code.

2?0K sample holder

Data Processing The recorded interferogram S(5) contains information about the radiant energy at a given frequency. If the interferometer is illuminated by a polychromatic source, all the wave trains will be registered at the detector. The interferogram can be represented by the integral

Linear Stage

Acrylic hood

FIGURE 1. schematic.

Fourier Transform Spectrometer layout

S(S)=

JB(v)cos(2m^S)dv

(1)

which is one-half of a cosine Fourier transform pair, where the other is

Bolometer and Readout We use a composite bolometer [2] as a detector with an operating bath temperature of 3.98 K (hquid helium boihng point at our ambient pressure at an altitude of 2100 meters). The bolometer used in the detection system has the following parameters: G = 18.04 |rW/K, S = 3.04 X 10^ VAV and an NEP = 1.26 x 10"^^ W/Hz^'^. Radiation is coupled to the bolometer via a gold- plated Winston cone with an entrance aperture of 10.16 mm and an exit aperture of 1.0922 mm which defines a collecting angle of ~12 degrees. The bolometer is connected to a DC voltage bias circuit in series with a 10 MQ. resistor which is physically close to the bolometer; both the bolometer and the resistor are at the bath temperature to minimize the Johnson noise contributions. The bolometer output signal is passed to a JFET (operating at 120 K) to couple the bolometer impedance (27 MQ.) with the room temperature amplifier electronics. In our FTS-MP setup port 1 views a hot load (blackbody source at 1150 K) and the other port views a room temperature source (290 K) which consists of millimeter wave absorbing material (Ecossorb), thus producing a differential signal at the output port. The interfered beam into the FTS-MP is modulated at 12 Hz with a chopper at the exit port of the interferometer and then enters into the cryostat through a series of low pass filters which consists of a Zotefoam vacuum window, two 0.25 mm thickness black polyethylene sheets (at 77 K and 50 K) and a 2.5 mm thickness GoreTex sheet (4 K). The output signal from the cold JFET module is amplified 1000 times by an instrumentation amplifier

B(v) = 2 jS(S)cos(2m^S)dS

(2)

B(v) is the spectrum of the source modified by the instrumental characteristics, v is the frequency of the radiation and 5 is the optical path difference. Apodization is performed before the Fourier transform of the interferograms to reduce sidelobes by a 65 % in the instrumental resolution function, our interferograms are apodized using the Norton-Beer [3, 4] medium function. However it is common that interferograms suffer from asymmetries because of phase errors introduced during the digitization process, alignment errors or asymmetries in the optical components. To solve this problem phase correction is performed following the Mertz method [5, 6]. Using this technique a small double-sided section of the full interferogram centered about the peak is used to calculate the phase correction term, Im^(v)

0,, = arctan—^^^^ Re,(v)

(3)

where Re/v) and Im/v) are the real and imaginary parts of the Fourier transformed interferogram. The interferogram used for the phase correction calculation is apodized with a triangular function. Finally 6y is used in the computation of the true spectrum B(v) by Fourier transforming the full interferogram.


B(v) = R e ^ (v) • cos^^ + Im^ (v) • sin 0^

(4)

However the addition of this support to the film modifies its absorbance due to dispersion and reflections for different refraction indexes of air, vacuum, substrate and the film under test. In order to compensate measurements for these effects, we compare the beam intensity for SiN films deposited on the Quartz substrates against measurements only with the substrate. Transmission differential measurements were used to calculate the absorbance spectra of the film under investigation. For experimental characterization a SiN film with a 0.4 |rm thickness and 2.01 [8] refraction index was deposited over a Quartz substrate of 1 mm thickness by LPCVD at INAOE. Quartz was chosen because its relative transparency at milhmeter and sub millimeter wavelengths [9, 10]. We recorded interferograms with samples at 290 K in broad band (300 to 1000 GHz.) and narrow band mode (200 to 300 GHz). Future optimized detectors with substrates under analysis will operate at 4 K at wavelengths of 1.1 mm, thus samples were placed inside the cryostat in front of the detector using a band-pass metal mesh filter centered at 240 GHz and a 50 GHz bandwidth.

Where Re/v) and Im/v) are the real and imaginary parts of the Fourier transform.

EXPERIMENTAL METHODOLOGY Due to photon and material absorbing particle interactions, the intensity of a radiation beam is attenuated from lo to li, where lo is the intensity without an intervening sample and li is the recorded intensity with some sample. The transmission (T) and absorbance (A) are given by expressions (5) and (6) respectively according to the Lambert-Beer law [7], where T =^

-logr:

(5)

-logf

(6)

In our experimental setup the FTS-MP system produces a converging beam with an area of 1 inctf at the location where the samples are mounted. Absorbance of SiN thin films can not be measured easily due to the fragility of these films on such large areas. For this reason we use a Quartz substrate to provide good mechanical support to the film. — \ —

RESULTS Recorded FTS-MP interferograms (Figure 2) for wide-band and narrow-band spectral measurements were taken using maximum frequencies of 1000 GHz and 500 GHz and resolutions of 10 GHz and 5 GHz respectively. Band-pass interferograms (200-300 GHz) have an intensity twenty five times smaller than wideband measurements. Following the data reduction procedures as described above, we obtain absorbance spectra shown in figure 3. Measurements at 290 K were made in order to see the trend of the spectrum over the range of frequency of the FTS. Later measurements were taken at 4 K using a metal mesh band-pass filter on the frequency region where the a-Si bolometers will operate. No overlap was possible on the 200 - 300 GHz region due to the very low signal to noise data (in this specific range) obtained in the 290 K broad-band measurement set imposed by the instrumental setup which includes Winston cone response, blackbody source, blocking filters and lock-in amplifier. SiN is a dielectric typically used as mechanical support film with a refraction index ~2 and low absorbance. As reported elsewhere [2, 11] SiN films can be coated with Bismuth to match the vacuum surface resistance and improve the absorbance properties. As an alternative to Bismuth coatings we have calculated the absorption spectra in the range of

Broad band inter erogram 1000 GHz

< 0 •-—'>/iV«-n/}/~^ ^V-^-^^^ÂA-W*^nO(^^/i|lf/V•^^

MfNllMM/^ ffVJtfl,IV-^ ^vv^V'tf*^'^ /ffv^Siyr:^..-

-1 300

350

400

450

Narrow band interferogram F=240 GHz Band 50 GHz

FIGURE 2. Interferograms for Silicon Nitride films. Top one (290 K) is for a broad band spectral measurement while inferior one (4 K) is using a band pass filter.


200 to 300 GHz using deposited Titanium films on SiN. 0.4|

1000 GHz at a temperature of 290 K, and the second set in the range of 200-300 GHz at 4 K. The frequency coverage was defined in the first case by using blackpolyethylene, Gortex and Zotefoam low-pass filters, and in the second case by using a metal-mesh bandpass filter. We found a level of 10 to 18% absorption across the 200-1000 GHz frequency range on the SiN films after removing reflection effects (17%) through the materials using Fresnel equations from the conversion of absorbance to transmission of figure 3. Calculations suggest that the absorption behavior of our Silicon Nitride films can be increased to 50% by depositing Titanium films of 6 to 12 nm thickness over the substrate, improving their ability to work as bolometers in the (sub-)millimeter regime.

^

0.35 ^Êxpected absorbance for a 6 nm Titanium film 0.3 > Expected absorbance for a 12 nm Titanium film i — 0.2Sî^v— ^ ^ ^ ^ . ^ . . . . ^ ^ . H - - * - - ^ ^

>^.

+ 290K data ° 4K data 200

300

400

500 600 700 Frequency [GHz]

800

900

1000

FIGURE 3. Absorbance data for Silicon Nitride films. Data were taken at 290 K for the 300 - 1000 GHz range (black crosses) and at 4 K for the 200 - 300 GHz range (black circles) using a metal-mesh band-pass filter.

REFERENCES

For this calculation we have used the average broad band method [12] to determine the amount of absorption as a function of metalhc films thickness. Results of calculation for Titanium films with thicknesses of 6 and 12 nm are shown in figure 4. We have chosen to make deposits only on the back side of the films since similar absorption values can be obtained when using very thin thicknesses (of atomic order) on the front of the film which are technologically more difficult to control. Tests on SiN samples with Titanium films are now in progress, and the results will be reported elsewhere.

1. D.H. Martin, Infrared and Millimeter Waves, Vol. 6,pp. 66-148, Academic Press, New York, 1982. 2. P..L. Richards, J. Appl Physics, 76, p. 1, 1994 3. Norton, R.H. & Beer, R., J. Opt. Soc. Am, Vol. 66, pp. 259, 1976 4. Naylor, D.A., Tahic, M.K., J. Opt Soc. Am., Vol.24, pp. 3644,2007 5. Xing, T., Soc. Of Photo-Optical Instrumentation Eng., Vol 39(2), 393-395, 2000 6. P.R. Griffiths and J.A. de Haseth, "Fourier transforms" in, Fourier Transform Infrared Spectrometry, John Wiley & Sons, New York, 1986. 7. D.A.Scoog, F.J.HoUer, T.A. Nieman ."Introduccion a los metodos espectrometricos", in Principios de andlisis instrumental 5"ed. McGrawHill, 2001, pp 146-147. 8. A. Heredia "Fabricacion y caracterizacion de un bolometro utilizando un pelicula de a-Si:H dopada con Boro para la deteccion de IR en el rango de los milimetros", PhD Thesis INAOE 2004 9. D. KoUer, G. Eddis, J. Hesler, C. Cunningham, Electronics Division Technical Note No. 184, (August, 1999) 10. D. Bendford, J.W. Kooi, E. Serabyn, Ninth International Symposium on Space Terahertz Technology, pp.405, (1998) 11. A.D. Turner, J.J. Bock, J.W. Beeman, J. Glenn, P.C.Hargrave, V.V. Hristov, H. T. Nguyen, F. Rahman, S. Sethuraman, A.L. Woodcraft, Applied Optics, Vol. 40, No. 28, 4921-4932 (October 2001). 12. Carli & lorio-FiliJ. Opt Soc. Am., Vol. 71, No. 8, 10201025 (August 1981)

£ -7

-11

-10 Front film thicttness log [m]

FIGURE 4. Metal film thickness and absorption calculation results for Titanium and substrate with refraction index of 2.01 corresponding to INAOE's Silicon Nitride films.

CONCLUSIONS We have fabricated Sihcon Nitride films by LPCVD in our laboratories and measured their transmission spectra at submillimeter wavelengths with an FTS following the method of phase correction in the reduction analysis. Two sets of spectral measurements were taken: the first in the range of 300-
