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Submitted on 1 Jan 1978
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A < 0.1 µs RESPONSE TIME CARBON FILM
BOLOMETER DETECTION SYSTEM
Y. Korczynskyj
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C6, supplkment au
no
8, Tome 39, aotit 1978, page C6-1188
A
<
0.1 psRESPONSE
TIME
CARBON
FILM BOLOMETER
DETECTION
SYSTEM
Y.
KorczynskyjDeparfment o f P h y s i c s , U n i v e r s i t y o f Nottingham, U n i v e r s i t y Park, N o t t i n g h a m NG7 2RD, EngZemd.
RQsum&.- Nous avons obtenu un temps de rgponse global d'environ % 50 ns pour un systsme deddtection
utilisant un bolomstre B film de carbone, par la modification du facteur de forme rectangulaire usuel et par l'utilisation d'un dtage MOSFET
2
transformation d1imp6dance op6rant B la temperature de l'hdlium liquide.Abstract.- An overall response time of % 50 ns is achieved for a carbon film bolometer detection
system through the modification of the usual rectangular form factor and by the use of a MOSFET impedance transforming stage operating at liquid helium temperatures.
INTRODUCTION.- Thin carbon film bolometers havebeen used for many years in the study of phonon propoga- tion in liquid 4 ~ e using heat pulse techniques. These bolometers respond to all phonon frequencies, are cheap, very simple to fabricate and require no special operating environments. In the form tradi- tionally used the overall response time of these bolometers is dominated by their thermal time cons- tant ". 50~s.
In a previous paper / I / the factors deter- mining this time constant were discussed in some de- tail and a bolometer design was described that re- duced this time constant by an order of magnitude. These fast bolometers have proved most useful in our investigations of the 'He dispersion curve /2/ and indeed the desirability of a bolometer with a time constant ,$ 0 . 1 ~ ~ for these experiments has provided the impetus for the work reported in this paper. Improved fabrication techniques coupled with a MOSFET impedance transforming stage, operating at liquid 4 ~ e temperatures, have enabled a reduction of the overall bolometer response time to as little as 20-50ns.
BOLOMETER FABRICATION.- The geometry of the bolome- ters is that described previously in /I/, as is the general fabrication technique i.e. a long thin zig- zag track is sratched out in a vacuum deposited me-
0
tal film % 400
A
thick over which a colloidal gra- phite dispersion is sprayed ; thus producing a long narrow, convoluted track of graphite through which the bias current is passed across the narrow di- mension. A reduction of the track width allows the graphite film to be made thinner (for the same total bolometer resistance) with a concomitant reductionof its response time.
Previously several factors limited the track width to 30-50pm. Perhaps the most serious of these was the difficulty in scratching entirely uniform tracks in the evaporated metal film. In the case of Au, for which adhesion to the sapphire substratewas poor, flaking tended to occur at the track edges ;
while in the case of Cu-Ni, its relative hardness and good adhesion to the sapphire made the scrat- ching itself more difficult and microscopic bridges tended to be left spanning the track. However, Ag and A1 films haveprovedeasy to scratch leaving a well defined track. A sapphire phonograph stylus is now used to produce tracks 7-15um wide (depending on stylus pressure) showing no edge structure under magnifications of up to 500X.
A further factor determining the track width is the graphite dispersion grain size. For the dis- persion used in / 1 / the grain size was measured using an optical microscope and found to be % 1511m.
However, a subsequent electron micrograph investi- gation revealed that the actual grain size % 2um and that these particles coalesced into the large
15pm size agglomorations. Thus using this disper- sion /3/ track widths ". 10um are feasible. Further,
it has been possible to obtain a graphite powder having an average particle size of 70
11
( 4 ) , and this clearly puts no restriction on any practicable track width.With the above improvements in techniques and materials, track lengths of up to 40 mm have been produced within a total area of 1 mm2, with a r06m temperature resistance of a few m / ~ t o give a to- tal room temperature resistance % Ika.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786526
CRYOGENIC AMPLIFIER STAGE.- Once the bolometer thermal time constant had been reduced to < Ips, the electronic 'RC' time constant, which in our casewas
2
0.3ps, made a significant contribution to the overall response time. This contribution was redu- ced to << 0 . 1 ~ ~ by using a simple impedance trans- formation circuit situated close to the bolometer, in the 'He. The output impedance of this stage was% 50Q, which coupled with the 150pF capacitance of the signal lines leading to the room temperature amplifier gave an electronic time constant % 7.511s.
A schematicof the arrangement is shown in figure 1.
Fig. 1 : The cryogenic impedance transforming ar- rangement. Rb is the bolometer resistance, I the bolometer current bias, Vo is the output, S are two MOSFET s switched by the potential Vs, thus providing access to Rb for the digital voltmeter
(DVM). Also shown is the LED reponse time measure- ment circuit.
The two MOSFET 'switches' allow access to the bolo- meter for temperature calibration purposes. The 3N225 dual gate, N channel, depletion-type MOSFET, in a source follower configuration, provides an improved performance over a similar circuit des- cribed previously 151. All bias supplies were ad- justable to allow for in-situ optimization of the circuit. Our experiments are performed at a 4 ~ e temperature
6
0.1 K and the power dissipated by the cryo-amp is far in excess of the cooling capacity of the 3 ~ e - 'He dilution refrigerator used to main- tain these temperatures. Thus the cryo-amp must be situated outside the experimental cell, thermally anchored at either the 1.2 K or 4.2 K stages. In either case, a further significant electronic com- ponent to the overall response time then appears via %C where F$, is the bolometer resistance and C is the capacitance of the bolometer-cryo-amp leads. For twisted pair leads 10-20 cm long, the capaci- tance is 'L few pF. This then provides an upper limit to the bolometer resistance, at its working temperature, determined by the acceptable lowerlimit for the R,,C product.
RESPONSE TIME MEASUREMENTS.- The total response time was measured using an LED signal source situa- ted % 5 mm in front of the carbon film bolometer.
A Lyons PG71 pulse generator was used to pulse the LED for 1-lops. Figure 2.(a) showsaschematic dia- gram of the input pulse to the LED (some'ringing' could not be avoided) and (b) shows the bolometer response for a bolometer with a room temperature resistance of 0.27kS2 and % Tm/n, operating in 4 ~ e
at 1.2 K with a 2yA bias current. Note that the bolometer signal is photon induced and thus the response is 'immediate' with no delay due to any flight time. Although the initial rise time is to some extent masked by that of the electronic 'break- through', it is clear that the bolometer response can be no greater than % 5Gns and is probably of the order of that of the PG71 i.e. 10-2011s. For comparison figure 2.(c) shows the response, under similar conditions, of a conventionally constructed carbon film bolometer 161.
Fig. 2. Schematic diagram of (a) the input pulse to the LED, (b) bolometer response as described in the text and (c) the response of a conventional carbon film bolometer with a thermal time constant % 5 0 ~ s .
As discussed in
11
/,
the order of magnitude reduction in the bolometer response time (from Q50 to % 5ys) is achieved at the expense of the sen-
temperature is that when used at 0.1 K, the bolo- meters are insensitive to relatively large ambient
temperature changes. Acknowledgements - The author would like to thank R.A.Sherlock for much helpful correspondance and encouragement, A,F.G,Wyatt for his support, D.G.Blair for his advice and assis- tance with the cryogenic amplifier stage, G.N.Crisp for many discussions, aid and support and J.Middle- ton for his technical assistance. This work was supported by the Science Research Council.
References
-
/I/ Korczynskyj, Y., Sherlock, R.A. and Wyatt, A.F. G.; Proc. 14th Int, Conf. Low Temp. Phys.,
4
(1975) 313./2/ Sherlock, R.A., Wyatt, A.F.G. and Lockerbie, N.A.; J. phys. C : Solid State Phys.,
10
(1977) 2567.
/3/ DAG580 (In reference (1) this was misprinted as DAG850), Acheson Colloids Co., Prince Rock, Plymouth, Devon, England.
/ 4 / The author is grateful to 8. BALIBAR who provi- ded a sample of this graphite powder.
/5/ Snavely, B.B andYutzy,J.C.; Rev. Sci. Inst., 38 (1967) 703.
