- Email: [email protected]
Ion-beam milled, high-detectivity pyroelectric detectors
ION-BEAM MILLED, HIGH-DETECTIVITY PYROELECTRIC DETECTORS S. E. STOKOWSKI,J. D. VENABLES,N. E. BYER and T. C. ENSIGN? Martin
Marietta
Laboratories,
1450 South
Rolling
Road,
Baltimore,
MA 21227, U.S.A
abstract--Self-supporting wafers of pyroelectric materials have been prepared in thicknesses as small as 4 iurn using ion-beam milling. Normalized detectivities (D*) of 1 mm’ LiTaO, detcctots fabricated from these very thin wafers have been measured to be X-S x 10” cm Hz’:’ We ’ at 3OHz. It is shown that this technique has great promise in realizing detectors with D* values significantly above IO9 cm Hz ’ ’ W-‘. In addition, the ion milling technique provides the advantages of clean, relatively damage-free surfaces, while permitting the fabrication of detector wafers with complex geometrical structures.
The increasing number of applications for pyroelectric materials in radiometers.“’ pyroimproving the performance, and. in particular, the normalized detectivity (D*) of pyroelectric detectors. Values of D* greater than IO* cm Hz’!’ W- ’ have been obtained by several researchers, employing such materials as TGSo’ SBNj4’ PVFZ,(‘) and of I)* is the ability to employ thin LiTaO 3161 One factor governing the ma~itude pyroelectric wafers in the detector; for, as the detector thickness is reduced, the D* is increased for modulation frequencies in the 10-1000 Hz range. This effect is due to the fact that the noise voltages from most noise sources present in the pyroelectric detector or in its preamp are inverse functions of the thickness.@’ For instance, an increase in D* of up to a factor of five could be realized by decreasing the thickness from 25 to 5 pm, depending on the particular combination of material properties and FET preamp parameters. Commonly, mechanical polishing has been used to thin pyroelectric detector wafers, but the technique usually has been iimited to thicknesses greater than 20,~m because of the difficulties of handling thinner wafers. Although thinner wafers have been prepared by supporting the wafer on a rigid substrate, the response at low frequencies is degraded by the effects of thermal shunting by the substrate material.“’ By employing ion-beam milling to thin the pyroelectric detector wafers. we have succeeded in fabricating LiTaO, detectors with thicknesses as small as 4 pm and D” values of 85 x 10’ cm Hzl” W-t. This technique has allowed us to obtain LiTaO, detector wafers that are thinned below 10 pm only in the sensitive area of the detector, maintaining a larger thickness of 25-35 pm in those regions of the wafer used for support. LiTaO, was chosen for the present study because of its mechanical strength, stability in ambient atmospheres, high Curie temperature, and high pyroelectric figure of merit. The detector construction on which our results were obtained is illustrated in Fig. 1. The 1 mm square detector element was prepared at the center of a 3 mm dia wafer of LiTaO,, which had been aligned and cut in a c-axis plane to a thickness of 0.25-05 mm. One face was mechanically polished and a thin nickel electrode 1 mm square was evaporated on it to form the uhimate backside of the detector. The wafer was then mechanically polished on the other side to a thickness of 30-50 pm, and the center portion (2 1.8 mm dia) was thinned to less than 10 IAm by ion-beam milling at a rate of 1.5 prn,hr using 500eV Ar ions. During this milling,’ the edge of the wafer was masked to maintain a thick rim, which is necessary for good wafer support, and ease in handling. t Present
address:
Central
Rearch
Laboratory,
3M Company,
P.O. Box 33221, St. Paul,
MI 55133, U.S.A.
332
S. t. STOKOWShl. .1. D. VI U./.I%LI j. N. f
t311 I< and T. (‘. t.NsI(;I\
FRONT ELECTRODE
R
CONTACTVIRE
/
EPOXY
SUPPO
CARBON IFRONTELECTRODEI N, Fllfrl IFront Electrodei
NiFILMIBac
SUPPORTS Fig.
I. Construction
details
of the ion-milled
BASE p)rocIcciric
dctcctol
The thickness contours of a detector wafer after ion-beam milling are revealed in a photomicrograph, Fig. 2, of the interference fringes generated under illumination by 595 nm light. The minimum thickness of this wafer is 6.3 pm. After milling, a thin Ni front electrode was applied by evaporation. In some detectors, a thin carbon black coating is superimposed on the front electrode for enhanced optical
Fig. 2. Photomicrograph of the interfcrcnce an ion-hum milled LlTaO, dctcctor wafer. and has a minimum thickness of 6.3 pm 1 ach 0.14 um
fringes gcncratcd under 595 nm illumination by The central thinned arca is I.8 mm in diameter fringe corresponds to a change of approximately of thlchncss.
High-dctcctivit)
pi roclcctt-ic
333
dctcctors
absorption. Subsequently, the detectors usually have been mounted on TO-5 headers by three epoxy supports, one of which makes the electrical connection to the back electrode. The front electrode was electrically connected by a gold wire attached with silver epoxy. This structure provides good thermal isolation, with thermal time constants in the range of 30-50 msec. Detectors of this construction have withstood shock loadings of 5000 g.
10-6
t
-IQ“
-10-8
Fig. 3. Measured pm thick LiTaO,
detectivity (D*), voltage responsi\ity (R,). and noise voltage (V,) of a 7.5 detector of I x I mm’ area in the 10-2000 Hr frequency range. The theoretical curves for D* and I’,Yare indicated by dashed lines.
In addition to the ease in thinning material that ion-beam milling provides, it also gives a clean, relatively damage-free surface. In particular, some of the damage introduced by mechanical polishing is removed by ion-milling, as was evidenced by significant reductions in the density of invested microdomains.@’ Measurements of the responsivity and noise of detectors fabricated by the above procedure have been made for the frequency range 10-2000 Hz. The results presented in Fig. 3, which were obtained on a 7.5 pm thick detector, are typical of several 1 x 1 mm’ detectors. The responsivity and noise measurements were obtained by employing a modulated tungsten source and an Ithaca 391A lock-in amplifier. The LiTaO, detector was coupled electronically to an FET follower amplifier of 10” 0 input resistance. A comparison is provided in Fig. 3 bctwecn the D” dcrivcd from rcsponsivity and noise data and the theoretical D” derived from measured materials properties and FET parameters. The experimentally mcasurcd and the calculated noise curves agree very well except in the region below 20 Hz. It is hclievcd that the additional noise voltage at low frequencies is due to airborne vibrations. In calculating the noise voltage, we have included the effects of Johnson noise of the 10”R input resistance, a.c. dielectric noise of the LiTaO, detector material. the FET voltage and current noise. and thermal fluctuations of the detector wafer due to its thermal conductance to the surrounding environment. The functional relationship between the noise voltage from these sources and the detector parameters may be found in a review article by Putley.‘9’ The following experimentally determined values wcrc LISC~: the dielectric loss tangent (tan 6). 0.43. 0.28, and 0.20x, at 10. 100 and 1000 Hz. respectively: the FET gate leakage current. @2 pA; the FET voltage noise. 10 nV/(Hz)’ ‘; and the detector capacitance. 49.5 pF. Theoretically, a 5 pm thick LiTaO, detector of 1 mm’ area should attain a D* of 1.6 x lo9 cmHz’.” W-’ at 10 Hz if the d‘ie 1ec t ric 1oss tangent were equal to the
bulk value of 0.3”/;,.However, in our finished detectors, both those thinned by ion-milling and by mechanical polishing, we have observed losses in the range of 044~5’!$ These higher values of the Ioss tangent have resulted in D* values less than 10” cm Hz”~ W-l. The source of this additional loss is being iil~~estigated. If the dielectric loss can be reduced to a level of QOY’,,, which has been previously observedC6) in a high purity boule of LiTaO,, a D* of 3 x 10” cm Hz’ ’ W- I at 10 Hz would result. In conclusioI1. WC have shown that the tcchniyuc of ion-beam milling to ~~cco~~plisll the final stages of thiilnin~ pyrocfcctric detector materials has great promise in fabricating detectors with D* values significantly above 10” cm H7’ ” W I. With LiTaO, matcrial of rclativcty high dielectric loss. WC have obtilincd a D” of 8.5 x lo8 cm Hz”’ W- ’ in detectors of 1 mm’ area. An improvcmcnt of more than a factor of three is prcdictcd if low loss material is employed. Likcwisc. application of the ion-milling technique to the thinning of other pyroclectric dctcctor materials could also result in significant P improvement. In addition to the cast in tllil~nillg material. the ion-milling technique provides the advantages of clean. rclativoly damage-fret s~~rf‘imx fiu- superior to those produced by mechani~t~ly polishing. and it permits the ready ~~bric~~tion of detector wafers with complex gconletric~~1structure. This tcchlli~l~~cmay- he of value. thcrcforc. for I-eticLllatiIlg pyroelectric vidicon targets or linear detector arrays. and thus permit attaining higher resolution than has been possihlc previously from pyroclectric thermal imaging systems. .4~kflf,~~l(,ri~~~,/fi~,/i~.sStimulatin, o WC also acknowledge
discussions the technical ;wisbnw
~1111 and the sncouragcment CA’R. G. Ly arc of A. ban dzr Jagt. W. :A. Beck and W. H&on.
ucknowlcdgcd.
Marietta
Laboratories,
1450 South
Rolling
Road,
Baltimore,
MA 21227, U.S.A
abstract--Self-supporting wafers of pyroelectric materials have been prepared in thicknesses as small as 4 iurn using ion-beam milling. Normalized detectivities (D*) of 1 mm’ LiTaO, detcctots fabricated from these very thin wafers have been measured to be X-S x 10” cm Hz’:’ We ’ at 3OHz. It is shown that this technique has great promise in realizing detectors with D* values significantly above IO9 cm Hz ’ ’ W-‘. In addition, the ion milling technique provides the advantages of clean, relatively damage-free surfaces, while permitting the fabrication of detector wafers with complex geometrical structures.
The increasing number of applications for pyroelectric materials in radiometers.“’ pyroimproving the performance, and. in particular, the normalized detectivity (D*) of pyroelectric detectors. Values of D* greater than IO* cm Hz’!’ W- ’ have been obtained by several researchers, employing such materials as TGSo’ SBNj4’ PVFZ,(‘) and of I)* is the ability to employ thin LiTaO 3161 One factor governing the ma~itude pyroelectric wafers in the detector; for, as the detector thickness is reduced, the D* is increased for modulation frequencies in the 10-1000 Hz range. This effect is due to the fact that the noise voltages from most noise sources present in the pyroelectric detector or in its preamp are inverse functions of the thickness.@’ For instance, an increase in D* of up to a factor of five could be realized by decreasing the thickness from 25 to 5 pm, depending on the particular combination of material properties and FET preamp parameters. Commonly, mechanical polishing has been used to thin pyroelectric detector wafers, but the technique usually has been iimited to thicknesses greater than 20,~m because of the difficulties of handling thinner wafers. Although thinner wafers have been prepared by supporting the wafer on a rigid substrate, the response at low frequencies is degraded by the effects of thermal shunting by the substrate material.“’ By employing ion-beam milling to thin the pyroelectric detector wafers. we have succeeded in fabricating LiTaO, detectors with thicknesses as small as 4 pm and D” values of 85 x 10’ cm Hzl” W-t. This technique has allowed us to obtain LiTaO, detector wafers that are thinned below 10 pm only in the sensitive area of the detector, maintaining a larger thickness of 25-35 pm in those regions of the wafer used for support. LiTaO, was chosen for the present study because of its mechanical strength, stability in ambient atmospheres, high Curie temperature, and high pyroelectric figure of merit. The detector construction on which our results were obtained is illustrated in Fig. 1. The 1 mm square detector element was prepared at the center of a 3 mm dia wafer of LiTaO,, which had been aligned and cut in a c-axis plane to a thickness of 0.25-05 mm. One face was mechanically polished and a thin nickel electrode 1 mm square was evaporated on it to form the uhimate backside of the detector. The wafer was then mechanically polished on the other side to a thickness of 30-50 pm, and the center portion (2 1.8 mm dia) was thinned to less than 10 IAm by ion-beam milling at a rate of 1.5 prn,hr using 500eV Ar ions. During this milling,’ the edge of the wafer was masked to maintain a thick rim, which is necessary for good wafer support, and ease in handling. t Present
address:
Central
Rearch
Laboratory,
3M Company,
P.O. Box 33221, St. Paul,
MI 55133, U.S.A.
332
S. t. STOKOWShl. .1. D. VI U./.I%LI j. N. f
t311 I< and T. (‘. t.NsI(;I\
FRONT ELECTRODE
R
CONTACTVIRE
/
EPOXY
SUPPO
CARBON IFRONTELECTRODEI N, Fllfrl IFront Electrodei
NiFILMIBac
SUPPORTS Fig.
I. Construction
details
of the ion-milled
BASE p)rocIcciric
dctcctol
The thickness contours of a detector wafer after ion-beam milling are revealed in a photomicrograph, Fig. 2, of the interference fringes generated under illumination by 595 nm light. The minimum thickness of this wafer is 6.3 pm. After milling, a thin Ni front electrode was applied by evaporation. In some detectors, a thin carbon black coating is superimposed on the front electrode for enhanced optical
Fig. 2. Photomicrograph of the interfcrcnce an ion-hum milled LlTaO, dctcctor wafer. and has a minimum thickness of 6.3 pm 1 ach 0.14 um
fringes gcncratcd under 595 nm illumination by The central thinned arca is I.8 mm in diameter fringe corresponds to a change of approximately of thlchncss.
High-dctcctivit)
pi roclcctt-ic
333
dctcctors
absorption. Subsequently, the detectors usually have been mounted on TO-5 headers by three epoxy supports, one of which makes the electrical connection to the back electrode. The front electrode was electrically connected by a gold wire attached with silver epoxy. This structure provides good thermal isolation, with thermal time constants in the range of 30-50 msec. Detectors of this construction have withstood shock loadings of 5000 g.
10-6
t
-IQ“
-10-8
Fig. 3. Measured pm thick LiTaO,
detectivity (D*), voltage responsi\ity (R,). and noise voltage (V,) of a 7.5 detector of I x I mm’ area in the 10-2000 Hr frequency range. The theoretical curves for D* and I’,Yare indicated by dashed lines.
In addition to the ease in thinning material that ion-beam milling provides, it also gives a clean, relatively damage-free surface. In particular, some of the damage introduced by mechanical polishing is removed by ion-milling, as was evidenced by significant reductions in the density of invested microdomains.@’ Measurements of the responsivity and noise of detectors fabricated by the above procedure have been made for the frequency range 10-2000 Hz. The results presented in Fig. 3, which were obtained on a 7.5 pm thick detector, are typical of several 1 x 1 mm’ detectors. The responsivity and noise measurements were obtained by employing a modulated tungsten source and an Ithaca 391A lock-in amplifier. The LiTaO, detector was coupled electronically to an FET follower amplifier of 10” 0 input resistance. A comparison is provided in Fig. 3 bctwecn the D” dcrivcd from rcsponsivity and noise data and the theoretical D” derived from measured materials properties and FET parameters. The experimentally mcasurcd and the calculated noise curves agree very well except in the region below 20 Hz. It is hclievcd that the additional noise voltage at low frequencies is due to airborne vibrations. In calculating the noise voltage, we have included the effects of Johnson noise of the 10”R input resistance, a.c. dielectric noise of the LiTaO, detector material. the FET voltage and current noise. and thermal fluctuations of the detector wafer due to its thermal conductance to the surrounding environment. The functional relationship between the noise voltage from these sources and the detector parameters may be found in a review article by Putley.‘9’ The following experimentally determined values wcrc LISC~: the dielectric loss tangent (tan 6). 0.43. 0.28, and 0.20x, at 10. 100 and 1000 Hz. respectively: the FET gate leakage current. @2 pA; the FET voltage noise. 10 nV/(Hz)’ ‘; and the detector capacitance. 49.5 pF. Theoretically, a 5 pm thick LiTaO, detector of 1 mm’ area should attain a D* of 1.6 x lo9 cmHz’.” W-’ at 10 Hz if the d‘ie 1ec t ric 1oss tangent were equal to the
bulk value of 0.3”/;,.However, in our finished detectors, both those thinned by ion-milling and by mechanical polishing, we have observed losses in the range of 044~5’!$ These higher values of the Ioss tangent have resulted in D* values less than 10” cm Hz”~ W-l. The source of this additional loss is being iil~~estigated. If the dielectric loss can be reduced to a level of QOY’,,, which has been previously observedC6) in a high purity boule of LiTaO,, a D* of 3 x 10” cm Hz’ ’ W- I at 10 Hz would result. In conclusioI1. WC have shown that the tcchniyuc of ion-beam milling to ~~cco~~plisll the final stages of thiilnin~ pyrocfcctric detector materials has great promise in fabricating detectors with D* values significantly above 10” cm H7’ ” W I. With LiTaO, matcrial of rclativcty high dielectric loss. WC have obtilincd a D” of 8.5 x lo8 cm Hz”’ W- ’ in detectors of 1 mm’ area. An improvcmcnt of more than a factor of three is prcdictcd if low loss material is employed. Likcwisc. application of the ion-milling technique to the thinning of other pyroclectric dctcctor materials could also result in significant P improvement. In addition to the cast in tllil~nillg material. the ion-milling technique provides the advantages of clean. rclativoly damage-fret s~~rf‘imx fiu- superior to those produced by mechani~t~ly polishing. and it permits the ready ~~bric~~tion of detector wafers with complex gconletric~~1structure. This tcchlli~l~~cmay- he of value. thcrcforc. for I-eticLllatiIlg pyroelectric vidicon targets or linear detector arrays. and thus permit attaining higher resolution than has been possihlc previously from pyroclectric thermal imaging systems. .4~kflf,~~l(,ri~~~,/fi~,/i~.sStimulatin, o WC also acknowledge
discussions the technical ;wisbnw
~1111 and the sncouragcment CA’R. G. Ly arc of A. ban dzr Jagt. W. :A. Beck and W. H&on.
ucknowlcdgcd.