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Thin metal films as absorbers for infrared sensors
Sensors and Actuators A, 37-38 (1993) 497-501
497
Thin metal films as absorbers for infrared sensors S Bauer, S Bauer-Gogonea,
W Becker, R Fettlg, B Ploss* and W Ruppel*
Instttut fur Angewandte Phystk, Umversltat Karlsruhe, Katserstrasse
12, D-W-7500 Karlsruhe (Germany)
W von Munch Instrtut fur Halbleltertechmk,
Umversrtat Stuttgart, flreltscheldstrasse
2, D- W-7tXM Stuttgart (Germany)
Abstract In order to achieve the maximal sensltlvlty of a thermal IR sensor, the incident hght must be efficiently absorbed This may be done either by the sensor matenal with its own electrodes or by an additIonal absorber structure Freely suspended thm metal films are known to act as wde-band absorbers for IR radiation Prepared on the sohd surface of the sensor, the absorbing propertles of the metal layer are, however, strongly influenced by the dlelectnc function
of the sensor matenal Results are presented for thermoelectnc and pyroeleetnc sensors For the thermoelectnc sensor, a 400 nm thick sIllcon layer 1s used as a support for the absorbing silver film urlth a sheet resistance of 150 61/O, evaporated onto the rear side of the &con layer This absorber has an absorbance of nearly 50% independent of the wavelength over the whole IR range For pyroelectnc sensors two examples of absorbers on PVDF are presented The first 1s a broad-band absorber and has an absorbance of about 50% w&m the wavelength regon 3-200 pm The other IS a selective absorber and has an absorbance of about 90% wthm the repon 7-15 pm The selective absorber 1scompletely opaque and can be realized by means of a PVDF film ulth a tluckness of about 2 pm, covered on both sides by metal films This structure may serve as an excellent absorber for a crystalhne pyroelectnc sensor
1. Introduction
The detectlon and measurement of infrared radiation are gammg increasing nnportance m various areas Detectors working at ambient temperature are of speclal interest, as expensive cooling apparatus IS avoided Thermal infrared detectors are used m a wde field of apphcatlons, starting from measurement devices, where the highest sensltlvity and performance are essential, to consumer apphcatlons, where cheap and simple manufacture of the sensors is the most Important cntenon Infrared sensors are used as detectors m measurmg devices hke Fourier spectrometers, for gas analysis, for thermal imaging, and for laser beam charactenzation All these applications are used on Earth and m space satelhtes Snnple infrared sensors have found mass apphcatlons m consumer products hke fire alarms and intruder detectors [l] The spectral sensltivlty of the sensor for the mcommg radlatlon 1s determined by its apphcatlon For spectrometer apphcatlons a broadband sensitivity 1s demable, while for consumer apphcations the sensor should have a selective sensltlvlty for radlatlon m the wavelength region 7-15 pm Sensors like photomultlphers or photoconductors are senslttve only wlthm a hrmted wavelength region, they
*Authors to whom correspondence should be addressed
0924-4247/93/$6 00
have an upper lmt for the wavelength of detectable radiation In contrast to these quantum detectors, thermal sensors hke thermoelectnc or pyroelectrlc sensors act as detectors for the entire radiation Intensity Provided with an appropnate absorber structure, they may work over a wide spectral range Thin metal films are known to act as wide-band absorbers for IR radlatlon urlth a very small heat capacity A freely suspended metal tihn can absorb up to 50% of the incident radratlon [Z] The absorbance 1s independent of the wavelength of the radiation, as long as the ra&atlon frequency is smaller than the reciprocal colhslon tune of the electrons m the metal film The apphcatlon to radlatlon sensors requires the deposition of the absorbing metal films on the solid surface of the sensor The absorbing properties of the metal layer are, however, strongly influenced by the dlelectnc function of the sensor matenal [3] In this paper the optlcal propertles of thm metal films on a dlelectnc substrate are discussed for selected substrates mth dtfferent
kinds of dlelectnc
functions
Ab-
sorber structures for thermoelectnc and pyroelectnc sensors are given For the thermoelectrrc sensor, a broad-band absorber has been reahzed This absorber has an absorbance of nearly 50% Independent of the wavelength over the whole IR range Two examples of absorbers for pyroelectnc detectors wth polyvmyhdenefluonde (PVDF) are presented The first IS a
@ 1993 -
Elsevler Sequoia All nghts reserved
498
broad-band absorber, the second 1s a selective absorber for the wavelength region 7- 15 pm
2. Absorptiou properties of thin metal films on dielectric substrates In this Section a metal film with thickness d and electnc conductlvlty u is considered, which is embedded between two dlelectnc materials with complex refractive mdlces ii, and i& Ra&abon mcldent from medmm 1 onto the metal film 1s partially reflected, transmitted and absorbed The reflection and transmlsslon coefficients r, t for the electnc field strength are [3] r=
ii,-ii,+y n”*+ n”,+ y
t=
26, ii,+fi,+y
y = 377 n/R, is the quotient of the vacuum nnpedance and the sheet resistance of the film R, = I/(@ These simple formulae are valid If the radlatlon frequency 1s smaller than the reciprocal colhslon time of the electrons m the metal film Due to surface scattenng and gram-boundary scattermg m thm polycrystalhne metal films with sheet resistances appropnate for absorber films, the collision time 1s drastically smaller than m bulk metal, and eqns (1) hold over the whole infrared wavelength region The refractive mdex fi of a dlelectrlc matenal IS a complex function of the wavelength It 1s usually described by the dlelectnc function EIvia n”= (C)“* The dlelectnc function is given by a sum of Lorentz osclllator terms [6]
urlth i,=dlelectnc constant due to the valence electrons, Cl= = oscillator strength, f&n = resonance frequency, Q” = dampmg frequency, w = 2~~11 is the angular frequency of the hght, c the velocity of hght m a vacuum, and 1, the wavelength of the mcldent hght m vacuum In wavelength intervals where the dlelectnc materials do not absorb, I e , where the refractive indices Cl, fiZ are real, the absorbance A of the metal film calculated from eqns (1) is 4fi,Y ‘4 =(ii, +fi,+# The metal lihu has a maximum absorbance A,,, = n,/(n, + nz) for y = n, + n, For a freely suspended metal film the absorbance can maximally be 50% If the refractive mdlces are real and If A, > eZ, then the reflection vamshes for a metal film with y = ii, - ii2 With such an appropnate metal fihn, the reflection can be suppressed completely if hght passes the interface
between two non-absorbmg dlelectnc matenals, commg from the medium with the higher optical density An apphcatlon 1s the suppresslon of multiple beam mterferences m dlelectnc plates over a wde range of wavelengths [4] Further, eqns (1) show that strong reflection occurs If the absolute value of the refractive index of the second matenal 1s high, I e , d the real or the imaginary part is high Typically such a sltuatlon arises m a dlelectnc matenal near the resonance frequency of a Lore& oscillator urlth a large oscdlator strength Typical substrates used m thermal infrared sensors are semiconductors hke sdlcon, crystalline SIO~, crystalhne ferroelectncs like LINbO, and ferroelectnc polymers like PVDF Each of these materials represents a class of dlelectncs wth very different &electnc functlons m the infrared The dlelectrlc function of silicon In the infrared 1s determined by the background of the valence electrons, leading to a high relative dlelectnc constant ls, z 11 2 The bonds between the &con atoms are non-polar, so that no slgmficant oscillators occur m the IR The opt& properties of &con over the Infrared wavelength region are well described by a nearly constant relatively high real refractive mdex nB x 3 3 In crystals, where the chemical bonds are polar or lomc, especially m ferroelectrlc crystals, the dlelectnc function m the infrared is characterized by few broad oscillators with a large osallator strength [3] Moreover, m some wavelength mtervals the real part of the dlelectnc function 1s negative In these parts of the spectrum the reflectlvlty IS high accordmg to eqns (1), and a metal film does not act as a good absorber In contrast to polar crystals, the dlelectnc function of polymers is characterized by many oscillators m the infrared, but each of them has only a small oscillator strength [3] These oscillators have little mfluence on the absorption properties of a metal film on a polymer substrate, and it becomes posnble to realize broad-band absorbers with a metal film on a polymer substrate [3]
3. Broad-band absorber structure for tbermoelectnc sensors A schematlc view of the thermoelectnc sensor realized 1s show-n m Fig l(a) Two n- and p-doped semtconductors form a thermocouple mth a very small contact area of the two matenals An absorber for this type of sensor must have a tigh absorbance and a low heat capacity, like any absorber for thermal sensors In addltlon, the absorber used for this apphcatlon must have a high heat conductlvlty, to provide an efficient transport of the heat from the absorbmg area to the contact zone of the thermocouple The absorber for the
metal
film
(a)
Fig 1 Schematic yIew of a thermoelectnc sensor consists of a substrate covered on senuconductors formmg a thermocouple electrodes on both sides, possibly placed
thermal Infrared sensor (a) and of a pyroelectnc thennal mfrared sensor (b) The thermoekctnc its rear side with a thm metal film The substrate IS m contact vnth two p- and n-conductmg as the sensor The pyroelectnc sensor consists of a pyroelectr~c element covered with metal on a substrate, for example, on the surface of a sdlcon chop
sensor consists of a dlelectnc substrate mth a wavelength-mdependent real refractive index n, covered on the rear surface by a thm metal film The sheet resistance of this metal film 1s selected as R, = 377 n/(n - 1) In this case no reilectlon occurs at the rear surface of the substrate and no mterferences occur Consldermg the transnusslon of the radlatlon through the front surface of the slhcon substrate, the absorber has a wavelength-independent absorbance A gwen by thermoelectnc
A=4(n-l) -
(n + 1)’
A maximum absorbance of 50% can be achieved when the refractive index of the substrate IS n = 3 An absorber conslstmg of a 400 nm thick silicon fdm (n z 3 4), covered on the rear side with a thm chromegold film (R, x 150 Q) has been reahzed Figure 2 shows the calculated absorbance for this absorber The absorbance 1s about 50% and nearly independent of the wavelength over the entue infrared wavelength regon
“:: 4
6
810
40 20 wavelength X $ml
Fig 2 Calculated absorbance thermoelectnc sensor of Fig 2mlun
en 60100
200
for the absorber structure for the l(a) m the wavelength repon J-
At wavelengths shorter than 5 w the absorbance decreases Thus decrease LScaused by the fitute colhslon time of the electrons m the metal film The colhslon time used for the calculation 1s of the order of lo-” s It has been determmed by a fit of the transnusslon computed by the Drnde-Lore& theory to the measured results Sticon has been chosen as the substrate matenal because of its high heat conductlvlty The best-smted &electnc substrate for such an apphcatlon would be a thm freely suspended diamond fihn, as diamond 1s the msulator with the h@est thermal conductlvlty It IS not possible to increase the absorbance by an additional metal film at the front side of the substrate if the refractive index of the substrate 1s higher than three For substrates wth n Q 3 the broad-band absorbance 1s smaller than 50%, and m this case an additional metal film at the front side of the substrate w&h y = 3 -n increases the total absorbance to 500/o An nnportant condition for the operation of broadband absorbers urlth thm metal films 1s that no reflection of the radiation transrmtted through the absorber occurs, which could cause Interference tis means that there must be free space behmd the absorber The extension of this free space must be larger than the coherence length of the mcommg radiation, I e , larger than the longest wavelength of interest m the spectrum for thermal rtiatlon It should be noted that it 1s possible to construct absorbers mth a bgher averaged broad-band absorbance However, all these absorbers have an absorbance that 1s wavelength dependent [7’j
4. Absorber stmctm~ for pyroelectric infrared sensors The pyroelectnc sensor element consists of a pyroelectnc layer, covered v&h metal electrodes for the
500
pyroelectnc signal on both surfaces Used m an mtegrated sensor, the pyroelectrlc element 1s placed on a substrate [5] The prmclpal arrangement 1s shown m Fig l(b) To achieve a mmlmum heat capacity, it 1s favourable to use this arrangement simultaneously as the absorber for the mcommg radiation The refractive mdex of pyroelectrlc matenals m the infrared wavelength range 1s wavelength dependent and complex m general Thus, the calculation of the absorbance of a pyroelectnc sensor 1s more complicated than for a thermoelectric sensor A standard matnx formalism [8] has been used for this purpose Two different absorber configurations for pyroelectnc sensors are discussed the first a broad-band absorber, the second a selective absorber for the wavelength range 7-15 pm The broad-band absorber consists of a 25 pm thick film of the pyroelectrlc polymer PVDF, covered with metal fihns on both surfaces The refractive index of PVDF 1s II z 1 4 Used as a free-standing sensor, the metal film at the rear side wth y = n - 1 suppresses reflection at that surface A further metal film at the front side wth y = 3 - n increases the total absorbance, as the refractive index of PVDF is smaller than three Curve 1 of Fig 3 shows the absorbance calculated for a 25 pm tbck PVDF fihn, covered on the front side with a metal film having a sheet resistance R. = 188 a, and on the rear surface mth a metal film having a sheet resistance R, = 800 n The parameters for the dlelectnc function of the PVDF polyer film are gven elsewhere
Rg 3 Calculated absorbance for the proposed broad-band and selectwely absorbmg pyroelectnc sensor elements accordmg to Rg l(b) as a function of the wavelength of the mudent rachation Curve 1 shows the absorbance of the broad-band absorber, conustmg of a 25 pm tluck PVDF film covered on the front surface wth a metal film havmg a sheet resistance R, = 188 iI/ 0, and on the rear surface unth a metal film with R, = 800 n/O Curve 2 represents the absorbance of the selective absorber Hrlth a 1 7 pm thick PVDF film covered on the front side with a metal film with R, = 377 iI/ 0, and on the rear side wth a thick metal film actmg as a nurror
[3] The result of this calculation shows that the absorbance over the wavelength range 3-200 pm 1s always greater than 50% The pyroelectrlc sensor wth a selective absorbance consists of a 1 7 pm thm pyroelectnc PVDF film covered on the front surface mth a thm metal film havmg a sheet resistance R, = 377 R, and on its rear surface with a thick metal film acting as a mirror The absorbance of this structure 1s given m curve 2 of l+g 3, showing an absorbance of about 90% m the wavelength regon 7-15 km If a pyroelectrlc element 1s mounted on a substrate, especially If It 1s used m an integrated sensor, the influence of the substrate on the optical properties must be considered as well The selective absorber structure 1s provided with a metal rmrror at the rear surface As this structure 1s completely opaque, the substrate does not influence the optical properties By this fact, tbs selective absorbmg pyroelectnc sensor element can be directly integrated on the surface of an integrated cu-cult Furthermore, this element 1s of interest as an absorber for other types of sensors The dlelectnc function of crystalline ferroelectnc matenals like LINbO, 1s determmed by Lorentz oscillators mth a high oscillator strength m the wavelength region 7- 15 pm The absorption properties of metal films deposited on such materials are msufficlent As the proposed selective sensor is completely opaque, it is well suited to be mounted on the surface of a crystalline pyroelectnc sensor, and to act as the absorber While the selective absorbing structure can be used on a substrate \nthout modficatlon, the use of the broad-band absorber 1s more complicated The absorption behavlour of this structure 1s strongly influenced by the substrate, as this absorber 1s partially transparent Mounted on the slhcon substrate of an integrated cucult, the reflection at the mterface from the polymer material to the silicon substrate cannot be suppressed by a metal film Even worse are materials like SlO, and Sl,N,, having strong oscillators m the near-infrared region from 3 to 20 m [9, lo] The broad-band absorbmg structure can then only be used on a substrate if the refractive index of this substrate 1s not larger than the refractive mdex of the PVDF layer, and d the optical thickness of this substrate exceeds the coherence length of the radiation wlttun the wavelength range of interest The use of a thick PVDF layer between the broad-band absorbmg pyroelectrlc element and the slhcon chip 1s appropnate for apphcatlon m integrated pyroelectrlc sensors In this case the sheet resistance of the metal films must be modtied according to y = 1 + n for the front-side layer, and to y
501
5. Conclusions It has been shown that thin metal fihns allow the realization of excellent absorber structures for thermoelectnc sensors and for pyroelectnc sensors v&h ferroelectrlc polymers For the thermoelectnc sensor a broad-band absorber structure has been reahzed mth an absorbance of approxnnately 50% For pyroelectic sensors two different absorber structures have been discussed a broad-band absorber for the whole infrared range and a selective absorber for the wavelength range 7-15 pm Furthermore, it has been shown that the absorbance of the broad-band absorber structure IS strongly mfluenced by the substrate, so that an appropriate selection of the substrate IS essential
References 1R W 2 W
3 4
5
6
7 8
Acknowledgements Fmanclal support by the Deutsch Forschungsgememschaft and by the Deutsche Agentur fur Raumfahrtangelegenhelten IS gratefully acknowledged
Whatmore,
Pyroelectnc devices and matenals, Rep
Prog Phys, 49 (1986) 1335-1386
9
10
Woltersdorf, uber die optlschen Konstanten dtier Metallschlchten nn langwelhgen Ultrarot, Z Phys , 91 (1934) 230-252 S Bauer, S Bauer-Gogonea and B Ploss, The physics of ovroelectnc de-, ADDI Phvs B, 54 (19921 544-551 i-W McKmght, K.P -.&wart, H b Diews and K Mooqam, Wavelength independent anu-mterference coatmg for the far Infrared, Infrared Phys , 5 (1987) 327-333 B Ploss, P Lehmann, H Schopf, T Lessle, S Bauer and U memann, Integrated pyroelectnc detector arrays wLth the sensor matenal PVDF, Ferroelectrrcs, 109 (1990) 223-228 See, for example, F Wooten, Optacal Propertws of Soha& Academic Press, New York, 1972 B Carh and D Iono-FIN, Absorption of composite bolometers, J Opt Sot Am, 71 (1981) 1020-1025 B Harbezke, Coherent and Incoherent retlectlon and transmlsslon of m&layer structures, Appl Phys B, 39 ( 1986) 165 - 170 P Grosse, B Harbecke, B Hemz and R Meyer, Infrared spectroscopy of oxide layers on techmcal Sl wafers, Appl Phys A, 39 (1986) 255-268 R Brendel, Quanhtatlve Infrared study of ultrathm MIS structures by grazmg mternal reflectlon, Appl Phys A, 50 (1990)
587-593
497
Thin metal films as absorbers for infrared sensors S Bauer, S Bauer-Gogonea,
W Becker, R Fettlg, B Ploss* and W Ruppel*
Instttut fur Angewandte Phystk, Umversltat Karlsruhe, Katserstrasse
12, D-W-7500 Karlsruhe (Germany)
W von Munch Instrtut fur Halbleltertechmk,
Umversrtat Stuttgart, flreltscheldstrasse
2, D- W-7tXM Stuttgart (Germany)
Abstract In order to achieve the maximal sensltlvlty of a thermal IR sensor, the incident hght must be efficiently absorbed This may be done either by the sensor matenal with its own electrodes or by an additIonal absorber structure Freely suspended thm metal films are known to act as wde-band absorbers for IR radiation Prepared on the sohd surface of the sensor, the absorbing propertles of the metal layer are, however, strongly influenced by the dlelectnc function
of the sensor matenal Results are presented for thermoelectnc and pyroeleetnc sensors For the thermoelectnc sensor, a 400 nm thick sIllcon layer 1s used as a support for the absorbing silver film urlth a sheet resistance of 150 61/O, evaporated onto the rear side of the &con layer This absorber has an absorbance of nearly 50% independent of the wavelength over the whole IR range For pyroelectnc sensors two examples of absorbers on PVDF are presented The first 1s a broad-band absorber and has an absorbance of about 50% w&m the wavelength regon 3-200 pm The other IS a selective absorber and has an absorbance of about 90% wthm the repon 7-15 pm The selective absorber 1scompletely opaque and can be realized by means of a PVDF film ulth a tluckness of about 2 pm, covered on both sides by metal films This structure may serve as an excellent absorber for a crystalhne pyroelectnc sensor
1. Introduction
The detectlon and measurement of infrared radiation are gammg increasing nnportance m various areas Detectors working at ambient temperature are of speclal interest, as expensive cooling apparatus IS avoided Thermal infrared detectors are used m a wde field of apphcatlons, starting from measurement devices, where the highest sensltlvity and performance are essential, to consumer apphcatlons, where cheap and simple manufacture of the sensors is the most Important cntenon Infrared sensors are used as detectors m measurmg devices hke Fourier spectrometers, for gas analysis, for thermal imaging, and for laser beam charactenzation All these applications are used on Earth and m space satelhtes Snnple infrared sensors have found mass apphcatlons m consumer products hke fire alarms and intruder detectors [l] The spectral sensltivlty of the sensor for the mcommg radlatlon 1s determined by its apphcatlon For spectrometer apphcatlons a broadband sensitivity 1s demable, while for consumer apphcations the sensor should have a selective sensltlvlty for radlatlon m the wavelength region 7-15 pm Sensors like photomultlphers or photoconductors are senslttve only wlthm a hrmted wavelength region, they
*Authors to whom correspondence should be addressed
0924-4247/93/$6 00
have an upper lmt for the wavelength of detectable radiation In contrast to these quantum detectors, thermal sensors hke thermoelectnc or pyroelectrlc sensors act as detectors for the entire radiation Intensity Provided with an appropnate absorber structure, they may work over a wide spectral range Thin metal films are known to act as wide-band absorbers for IR radlatlon urlth a very small heat capacity A freely suspended metal tihn can absorb up to 50% of the incident radratlon [Z] The absorbance 1s independent of the wavelength of the radiation, as long as the ra&atlon frequency is smaller than the reciprocal colhslon tune of the electrons m the metal film The apphcatlon to radlatlon sensors requires the deposition of the absorbing metal films on the solid surface of the sensor The absorbing properties of the metal layer are, however, strongly influenced by the dlelectnc function of the sensor matenal [3] In this paper the optlcal propertles of thm metal films on a dlelectnc substrate are discussed for selected substrates mth dtfferent
kinds of dlelectnc
functions
Ab-
sorber structures for thermoelectnc and pyroelectnc sensors are given For the thermoelectrrc sensor, a broad-band absorber has been reahzed This absorber has an absorbance of nearly 50% Independent of the wavelength over the whole IR range Two examples of absorbers for pyroelectnc detectors wth polyvmyhdenefluonde (PVDF) are presented The first IS a
@ 1993 -
Elsevler Sequoia All nghts reserved
498
broad-band absorber, the second 1s a selective absorber for the wavelength region 7- 15 pm
2. Absorptiou properties of thin metal films on dielectric substrates In this Section a metal film with thickness d and electnc conductlvlty u is considered, which is embedded between two dlelectnc materials with complex refractive mdlces ii, and i& Ra&abon mcldent from medmm 1 onto the metal film 1s partially reflected, transmitted and absorbed The reflection and transmlsslon coefficients r, t for the electnc field strength are [3] r=
ii,-ii,+y n”*+ n”,+ y
t=
26, ii,+fi,+y
y = 377 n/R, is the quotient of the vacuum nnpedance and the sheet resistance of the film R, = I/(@ These simple formulae are valid If the radlatlon frequency 1s smaller than the reciprocal colhslon time of the electrons m the metal film Due to surface scattenng and gram-boundary scattermg m thm polycrystalhne metal films with sheet resistances appropnate for absorber films, the collision time 1s drastically smaller than m bulk metal, and eqns (1) hold over the whole infrared wavelength region The refractive mdex fi of a dlelectrlc matenal IS a complex function of the wavelength It 1s usually described by the dlelectnc function EIvia n”= (C)“* The dlelectnc function is given by a sum of Lorentz osclllator terms [6]
urlth i,=dlelectnc constant due to the valence electrons, Cl= = oscillator strength, f&n = resonance frequency, Q” = dampmg frequency, w = 2~~11 is the angular frequency of the hght, c the velocity of hght m a vacuum, and 1, the wavelength of the mcldent hght m vacuum In wavelength intervals where the dlelectnc materials do not absorb, I e , where the refractive indices Cl, fiZ are real, the absorbance A of the metal film calculated from eqns (1) is 4fi,Y ‘4 =(ii, +fi,+# The metal lihu has a maximum absorbance A,,, = n,/(n, + nz) for y = n, + n, For a freely suspended metal film the absorbance can maximally be 50% If the refractive mdlces are real and If A, > eZ, then the reflection vamshes for a metal film with y = ii, - ii2 With such an appropnate metal fihn, the reflection can be suppressed completely if hght passes the interface
between two non-absorbmg dlelectnc matenals, commg from the medium with the higher optical density An apphcatlon 1s the suppresslon of multiple beam mterferences m dlelectnc plates over a wde range of wavelengths [4] Further, eqns (1) show that strong reflection occurs If the absolute value of the refractive index of the second matenal 1s high, I e , d the real or the imaginary part is high Typically such a sltuatlon arises m a dlelectnc matenal near the resonance frequency of a Lore& oscillator urlth a large oscdlator strength Typical substrates used m thermal infrared sensors are semiconductors hke sdlcon, crystalline SIO~, crystalhne ferroelectncs like LINbO, and ferroelectnc polymers like PVDF Each of these materials represents a class of dlelectncs wth very different &electnc functlons m the infrared The dlelectrlc function of silicon In the infrared 1s determined by the background of the valence electrons, leading to a high relative dlelectnc constant ls, z 11 2 The bonds between the &con atoms are non-polar, so that no slgmficant oscillators occur m the IR The opt& properties of &con over the Infrared wavelength region are well described by a nearly constant relatively high real refractive mdex nB x 3 3 In crystals, where the chemical bonds are polar or lomc, especially m ferroelectrlc crystals, the dlelectnc function m the infrared is characterized by few broad oscillators with a large osallator strength [3] Moreover, m some wavelength mtervals the real part of the dlelectnc function 1s negative In these parts of the spectrum the reflectlvlty IS high accordmg to eqns (1), and a metal film does not act as a good absorber In contrast to polar crystals, the dlelectnc function of polymers is characterized by many oscillators m the infrared, but each of them has only a small oscillator strength [3] These oscillators have little mfluence on the absorption properties of a metal film on a polymer substrate, and it becomes posnble to realize broad-band absorbers with a metal film on a polymer substrate [3]
3. Broad-band absorber structure for tbermoelectnc sensors A schematlc view of the thermoelectnc sensor realized 1s show-n m Fig l(a) Two n- and p-doped semtconductors form a thermocouple mth a very small contact area of the two matenals An absorber for this type of sensor must have a tigh absorbance and a low heat capacity, like any absorber for thermal sensors In addltlon, the absorber used for this apphcatlon must have a high heat conductlvlty, to provide an efficient transport of the heat from the absorbmg area to the contact zone of the thermocouple The absorber for the
metal
film
(a)
Fig 1 Schematic yIew of a thermoelectnc sensor consists of a substrate covered on senuconductors formmg a thermocouple electrodes on both sides, possibly placed
thermal Infrared sensor (a) and of a pyroelectnc thennal mfrared sensor (b) The thermoekctnc its rear side with a thm metal film The substrate IS m contact vnth two p- and n-conductmg as the sensor The pyroelectnc sensor consists of a pyroelectr~c element covered with metal on a substrate, for example, on the surface of a sdlcon chop
sensor consists of a dlelectnc substrate mth a wavelength-mdependent real refractive index n, covered on the rear surface by a thm metal film The sheet resistance of this metal film 1s selected as R, = 377 n/(n - 1) In this case no reilectlon occurs at the rear surface of the substrate and no mterferences occur Consldermg the transnusslon of the radlatlon through the front surface of the slhcon substrate, the absorber has a wavelength-independent absorbance A gwen by thermoelectnc
A=4(n-l) -
(n + 1)’
A maximum absorbance of 50% can be achieved when the refractive index of the substrate IS n = 3 An absorber conslstmg of a 400 nm thick silicon fdm (n z 3 4), covered on the rear side with a thm chromegold film (R, x 150 Q) has been reahzed Figure 2 shows the calculated absorbance for this absorber The absorbance 1s about 50% and nearly independent of the wavelength over the entue infrared wavelength regon
“:: 4
6
810
40 20 wavelength X $ml
Fig 2 Calculated absorbance thermoelectnc sensor of Fig 2mlun
en 60100
200
for the absorber structure for the l(a) m the wavelength repon J-
At wavelengths shorter than 5 w the absorbance decreases Thus decrease LScaused by the fitute colhslon time of the electrons m the metal film The colhslon time used for the calculation 1s of the order of lo-” s It has been determmed by a fit of the transnusslon computed by the Drnde-Lore& theory to the measured results Sticon has been chosen as the substrate matenal because of its high heat conductlvlty The best-smted &electnc substrate for such an apphcatlon would be a thm freely suspended diamond fihn, as diamond 1s the msulator with the h@est thermal conductlvlty It IS not possible to increase the absorbance by an additional metal film at the front side of the substrate if the refractive index of the substrate 1s higher than three For substrates wth n Q 3 the broad-band absorbance 1s smaller than 50%, and m this case an additional metal film at the front side of the substrate w&h y = 3 -n increases the total absorbance to 500/o An nnportant condition for the operation of broadband absorbers urlth thm metal films 1s that no reflection of the radiation transrmtted through the absorber occurs, which could cause Interference tis means that there must be free space behmd the absorber The extension of this free space must be larger than the coherence length of the mcommg radiation, I e , larger than the longest wavelength of interest m the spectrum for thermal rtiatlon It should be noted that it 1s possible to construct absorbers mth a bgher averaged broad-band absorbance However, all these absorbers have an absorbance that 1s wavelength dependent [7’j
4. Absorber stmctm~ for pyroelectric infrared sensors The pyroelectnc sensor element consists of a pyroelectnc layer, covered v&h metal electrodes for the
500
pyroelectnc signal on both surfaces Used m an mtegrated sensor, the pyroelectrlc element 1s placed on a substrate [5] The prmclpal arrangement 1s shown m Fig l(b) To achieve a mmlmum heat capacity, it 1s favourable to use this arrangement simultaneously as the absorber for the mcommg radiation The refractive mdex of pyroelectrlc matenals m the infrared wavelength range 1s wavelength dependent and complex m general Thus, the calculation of the absorbance of a pyroelectnc sensor 1s more complicated than for a thermoelectric sensor A standard matnx formalism [8] has been used for this purpose Two different absorber configurations for pyroelectnc sensors are discussed the first a broad-band absorber, the second a selective absorber for the wavelength range 7-15 pm The broad-band absorber consists of a 25 pm thick film of the pyroelectrlc polymer PVDF, covered with metal fihns on both surfaces The refractive index of PVDF 1s II z 1 4 Used as a free-standing sensor, the metal film at the rear side wth y = n - 1 suppresses reflection at that surface A further metal film at the front side wth y = 3 - n increases the total absorbance, as the refractive index of PVDF is smaller than three Curve 1 of Fig 3 shows the absorbance calculated for a 25 pm tbck PVDF fihn, covered on the front side with a metal film having a sheet resistance R. = 188 a, and on the rear surface mth a metal film having a sheet resistance R, = 800 n The parameters for the dlelectnc function of the PVDF polyer film are gven elsewhere
Rg 3 Calculated absorbance for the proposed broad-band and selectwely absorbmg pyroelectnc sensor elements accordmg to Rg l(b) as a function of the wavelength of the mudent rachation Curve 1 shows the absorbance of the broad-band absorber, conustmg of a 25 pm tluck PVDF film covered on the front surface wth a metal film havmg a sheet resistance R, = 188 iI/ 0, and on the rear surface unth a metal film with R, = 800 n/O Curve 2 represents the absorbance of the selective absorber Hrlth a 1 7 pm thick PVDF film covered on the front side with a metal film with R, = 377 iI/ 0, and on the rear side wth a thick metal film actmg as a nurror
[3] The result of this calculation shows that the absorbance over the wavelength range 3-200 pm 1s always greater than 50% The pyroelectrlc sensor wth a selective absorbance consists of a 1 7 pm thm pyroelectnc PVDF film covered on the front surface mth a thm metal film havmg a sheet resistance R, = 377 R, and on its rear surface with a thick metal film acting as a mirror The absorbance of this structure 1s given m curve 2 of l+g 3, showing an absorbance of about 90% m the wavelength regon 7-15 km If a pyroelectrlc element 1s mounted on a substrate, especially If It 1s used m an integrated sensor, the influence of the substrate on the optical properties must be considered as well The selective absorber structure 1s provided with a metal rmrror at the rear surface As this structure 1s completely opaque, the substrate does not influence the optical properties By this fact, tbs selective absorbmg pyroelectnc sensor element can be directly integrated on the surface of an integrated cu-cult Furthermore, this element 1s of interest as an absorber for other types of sensors The dlelectnc function of crystalline ferroelectnc matenals like LINbO, 1s determmed by Lorentz oscillators mth a high oscillator strength m the wavelength region 7- 15 pm The absorption properties of metal films deposited on such materials are msufficlent As the proposed selective sensor is completely opaque, it is well suited to be mounted on the surface of a crystalline pyroelectnc sensor, and to act as the absorber While the selective absorbing structure can be used on a substrate \nthout modficatlon, the use of the broad-band absorber 1s more complicated The absorption behavlour of this structure 1s strongly influenced by the substrate, as this absorber 1s partially transparent Mounted on the slhcon substrate of an integrated cucult, the reflection at the mterface from the polymer material to the silicon substrate cannot be suppressed by a metal film Even worse are materials like SlO, and Sl,N,, having strong oscillators m the near-infrared region from 3 to 20 m [9, lo] The broad-band absorbmg structure can then only be used on a substrate if the refractive index of this substrate 1s not larger than the refractive mdex of the PVDF layer, and d the optical thickness of this substrate exceeds the coherence length of the radiation wlttun the wavelength range of interest The use of a thick PVDF layer between the broad-band absorbmg pyroelectrlc element and the slhcon chip 1s appropnate for apphcatlon m integrated pyroelectrlc sensors In this case the sheet resistance of the metal films must be modtied according to y = 1 + n for the front-side layer, and to y
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5. Conclusions It has been shown that thin metal fihns allow the realization of excellent absorber structures for thermoelectnc sensors and for pyroelectnc sensors v&h ferroelectrlc polymers For the thermoelectnc sensor a broad-band absorber structure has been reahzed mth an absorbance of approxnnately 50% For pyroelectic sensors two different absorber structures have been discussed a broad-band absorber for the whole infrared range and a selective absorber for the wavelength range 7-15 pm Furthermore, it has been shown that the absorbance of the broad-band absorber structure IS strongly mfluenced by the substrate, so that an appropriate selection of the substrate IS essential
References 1R W 2 W
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Acknowledgements Fmanclal support by the Deutsch Forschungsgememschaft and by the Deutsche Agentur fur Raumfahrtangelegenhelten IS gratefully acknowledged
Whatmore,
Pyroelectnc devices and matenals, Rep
Prog Phys, 49 (1986) 1335-1386
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Woltersdorf, uber die optlschen Konstanten dtier Metallschlchten nn langwelhgen Ultrarot, Z Phys , 91 (1934) 230-252 S Bauer, S Bauer-Gogonea and B Ploss, The physics of ovroelectnc de-, ADDI Phvs B, 54 (19921 544-551 i-W McKmght, K.P -.&wart, H b Diews and K Mooqam, Wavelength independent anu-mterference coatmg for the far Infrared, Infrared Phys , 5 (1987) 327-333 B Ploss, P Lehmann, H Schopf, T Lessle, S Bauer and U memann, Integrated pyroelectnc detector arrays wLth the sensor matenal PVDF, Ferroelectrrcs, 109 (1990) 223-228 See, for example, F Wooten, Optacal Propertws of Soha& Academic Press, New York, 1972 B Carh and D Iono-FIN, Absorption of composite bolometers, J Opt Sot Am, 71 (1981) 1020-1025 B Harbezke, Coherent and Incoherent retlectlon and transmlsslon of m&layer structures, Appl Phys B, 39 ( 1986) 165 - 170 P Grosse, B Harbecke, B Hemz and R Meyer, Infrared spectroscopy of oxide layers on techmcal Sl wafers, Appl Phys A, 39 (1986) 255-268 R Brendel, Quanhtatlve Infrared study of ultrathm MIS structures by grazmg mternal reflectlon, Appl Phys A, 50 (1990)
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