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Crystal oscillator film thickness monitor
NUCLEAR INSTRUMENTS AND METHODS 18
(1964)
242-20 ; (0, NORTH-HOLLAND PUBLISHING CO .
*STAL OSCILLATOR FIL H . F . Nü ~CY
ON and F. A,
AN" i~ea;xans "rch Fstablishment, Aldermaston,
nd
Received 7 November 1963 r monitoring the thickness and deposition is described . The instrumcnt
betwé-.en two similar crystals
Ti7ilu
ration of thin films and nuclear targets oration, it is often necessary to monitor and the mass of material us means of achieving this have been vported,, all of which suffer from one disadvantage or fans elegant technique - ') utilizes the fact that increasir4; the mass of a vibrating piezo-clectric c-iystal by depositing a thin layer of foreign matter upon alters its resonant frequency. -hrand! and LoVe2) :showed that for thin films this frequency shift duo is proportional to a change in man data. f02 d in
when the surface of one has a target material oapomted on The advantages of this system over mo enumerated .
variations in crystal temperature . portant to select crystals with a low tore coefficient. AT-type crystals with a 35'2 f cut and a near zero coefficient from - FC to + 55"r, arc in the apparatus described. The temperature effect due to heat radiation from the evaporator crucible minimised by mounting the two cry, crystals in close proximity. 2. Experimental apparatus A schematic drawing of the experimental assembly show -i in fig. 1, Two I Mc/s quartz crystals*, separated by approximately I", are mounted on a brass plate.
N, F
AT-cut crysta
n the characteristics ~.irement sensitivity ) will incr and Ous rit a largernnasss () S. lui the system to bc of
output lioni each crystai ~s Lik- cil &
Ws and Nd imo a ckcWt box mouwl t(-)P Plate. th
SUM-)Iied by SaM)rd I,`Iedrical
CRYS
243
OSCILLATOR FILM THICKNESS MONITOR
ept w short w possible. Both serves to area crysta to the t°~apora t bcam, when required cryitaisare mounted vertically urce in such
"P40 ur a way mj b i cm . Two shwtec, . opzratcd t h r ougl1 rota ~;N,, oic to K- -,pwi~J or isol.ttcd. i(cam ot n0
Img . 3
Each crystal forms part of an oscillatory circuit,, g. 3 . The output from . each oscillator is fed into a long tailed pair mixer and the resulting difference beat y is displayed on a simple frequency -nieter . ike similar systems the instrument described does not utilize an external standard frequency source to obtain the frequency difference . The crystal mounting is supported on a 6" diameter Quickfit glass pipe ich in turn is mounted on the baseplate of a vacuum evaporator . ral P"Ure Borosilicate miscroscope slides, cleaned in an isopropyl vapour degreaser, are used in the preparation of self supporting nuclear targets. To facilitate subsequent stripping of the deposited film in a water bath, a spot of Teepol is placed onto the exposed substrate surface which is then polished vigorously with a clean selvyt cloth"). The substrate is placed in position between the crystals, the system evacuated to - I x 10" Torr and the evaporation element degassed with both crystals shielded. After switching on the crystal power supply and allowing sufficient time for the system to stabilize, minimum beat frequency is obtained by adjustmera of the appropriate trimming condensers . One crystal is shielded from the vapour source by the rotary shutter poration commenced. The resonant frequency the exposed crystal decreases as the mass of evaporated deposit builds up . This change is recorded by crysL"!' the niver. By s&Wbk selcctioa Of orant the out of balance frequency niay bc kept within the range of the ratter and the sensitivity of nwa,',urcment rel'ainc,i constant . It W bc c)bservcd from h- -1 1hat ch~fllgc frequency with deposited mass is linear for films up to mg/cm2 pet, crystal). Above this value the - constants contribute to frequency shift and the the
g.
scals.
cillatm and niiv_` r orcuit .
~
A. H. F. MUGGLETON AND F. A. HOWE
a) By using two crysta'Is mounted adjacent to each other temperature frequency drift is minimised, b )There is no necessity for an expensive standard ow-Ilator. '
A km
Fig. 4. Change caf frequency with mass of material deposited, for lead, gold. silver and alvininium.
nship becomes non-lin;~ar. Sensitivity of approxi2 cycles change per uglcm'' was the same far all rnaterials used (i .e. gold, aluminium, lead and silver). This obscrvatii:ln shows good agreement with equation 1. 4. CmAusiow oc: instrument described may not be as a, 0-.c more elaborate systems reNrted, this re;onant frequency film thickness monitor to WnAmg a&-ntages for the vacuum evapura-
c) The frequency difference to be measured remains smell and can thus be monitored by a single range meter. d) The transistorised oscillator and mixing cricuits are compact and inexpensive to construct. e) By using each crystal alternately the range of instrument is twice that of a system using a single crystal sensing element. f) For nuclear target applications, where weight/unit area i of primary importance, an instrument r"easuring the mass of material deposited has advantages over other systems. g) As the frequency change is directly proportional to mass it is 'necmar to celibmte the instmment for one material only . ' The authors would like to thank Mr. C. A. Campbell for constructing the electrical circuits and making some of tt.te. initial measurements . References
P. Oberg and J. Lingenijo, Rev. Sci. Instr. 31 (1959) 1049 . K, H. Behrandt and R. W. Love, Vacuum 12 (1962) 1 . 3) S. I Lips and Harpy S. Kukuk, 'Vacuum Sympos . Trans., 1960 Tq~rjamon Press . London 1961) p. 333 . -1) A. H. F Muggleton anki F . A lio%ve . Nu,:I . Instr . and Meth . 13 (19M) 211 . 1) 2)
(1964)
242-20 ; (0, NORTH-HOLLAND PUBLISHING CO .
*STAL OSCILLATOR FIL H . F . Nü ~CY
ON and F. A,
AN" i~ea;xans "rch Fstablishment, Aldermaston,
nd
Received 7 November 1963 r monitoring the thickness and deposition is described . The instrumcnt
betwé-.en two similar crystals
Ti7ilu
ration of thin films and nuclear targets oration, it is often necessary to monitor and the mass of material us means of achieving this have been vported,, all of which suffer from one disadvantage or fans elegant technique - ') utilizes the fact that increasir4; the mass of a vibrating piezo-clectric c-iystal by depositing a thin layer of foreign matter upon alters its resonant frequency. -hrand! and LoVe2) :showed that for thin films this frequency shift duo is proportional to a change in man data. f02 d in
when the surface of one has a target material oapomted on The advantages of this system over mo enumerated .
variations in crystal temperature . portant to select crystals with a low tore coefficient. AT-type crystals with a 35'2 f cut and a near zero coefficient from - FC to + 55"r, arc in the apparatus described. The temperature effect due to heat radiation from the evaporator crucible minimised by mounting the two cry, crystals in close proximity. 2. Experimental apparatus A schematic drawing of the experimental assembly show -i in fig. 1, Two I Mc/s quartz crystals*, separated by approximately I", are mounted on a brass plate.
N, F
AT-cut crysta
n the characteristics ~.irement sensitivity ) will incr and Ous rit a largernnasss () S. lui the system to bc of
output lioni each crystai ~s Lik- cil &
Ws and Nd imo a ckcWt box mouwl t(-)P Plate. th
SUM-)Iied by SaM)rd I,`Iedrical
CRYS
243
OSCILLATOR FILM THICKNESS MONITOR
ept w short w possible. Both serves to area crysta to the t°~apora t bcam, when required cryitaisare mounted vertically urce in such
"P40 ur a way mj b i cm . Two shwtec, . opzratcd t h r ougl1 rota ~;N,, oic to K- -,pwi~J or isol.ttcd. i(cam ot n0
Img . 3
Each crystal forms part of an oscillatory circuit,, g. 3 . The output from . each oscillator is fed into a long tailed pair mixer and the resulting difference beat y is displayed on a simple frequency -nieter . ike similar systems the instrument described does not utilize an external standard frequency source to obtain the frequency difference . The crystal mounting is supported on a 6" diameter Quickfit glass pipe ich in turn is mounted on the baseplate of a vacuum evaporator . ral P"Ure Borosilicate miscroscope slides, cleaned in an isopropyl vapour degreaser, are used in the preparation of self supporting nuclear targets. To facilitate subsequent stripping of the deposited film in a water bath, a spot of Teepol is placed onto the exposed substrate surface which is then polished vigorously with a clean selvyt cloth"). The substrate is placed in position between the crystals, the system evacuated to - I x 10" Torr and the evaporation element degassed with both crystals shielded. After switching on the crystal power supply and allowing sufficient time for the system to stabilize, minimum beat frequency is obtained by adjustmera of the appropriate trimming condensers . One crystal is shielded from the vapour source by the rotary shutter poration commenced. The resonant frequency the exposed crystal decreases as the mass of evaporated deposit builds up . This change is recorded by crysL"!' the niver. By s&Wbk selcctioa Of orant the out of balance frequency niay bc kept within the range of the ratter and the sensitivity of nwa,',urcment rel'ainc,i constant . It W bc c)bservcd from h- -1 1hat ch~fllgc frequency with deposited mass is linear for films up to mg/cm2 pet, crystal). Above this value the - constants contribute to frequency shift and the the
g.
scals.
cillatm and niiv_` r orcuit .
~
A. H. F. MUGGLETON AND F. A. HOWE
a) By using two crysta'Is mounted adjacent to each other temperature frequency drift is minimised, b )There is no necessity for an expensive standard ow-Ilator. '
A km
Fig. 4. Change caf frequency with mass of material deposited, for lead, gold. silver and alvininium.
nship becomes non-lin;~ar. Sensitivity of approxi2 cycles change per uglcm'' was the same far all rnaterials used (i .e. gold, aluminium, lead and silver). This obscrvatii:ln shows good agreement with equation 1. 4. CmAusiow oc: instrument described may not be as a, 0-.c more elaborate systems reNrted, this re;onant frequency film thickness monitor to WnAmg a&-ntages for the vacuum evapura-
c) The frequency difference to be measured remains smell and can thus be monitored by a single range meter. d) The transistorised oscillator and mixing cricuits are compact and inexpensive to construct. e) By using each crystal alternately the range of instrument is twice that of a system using a single crystal sensing element. f) For nuclear target applications, where weight/unit area i of primary importance, an instrument r"easuring the mass of material deposited has advantages over other systems. g) As the frequency change is directly proportional to mass it is 'necmar to celibmte the instmment for one material only . ' The authors would like to thank Mr. C. A. Campbell for constructing the electrical circuits and making some of tt.te. initial measurements . References
P. Oberg and J. Lingenijo, Rev. Sci. Instr. 31 (1959) 1049 . K, H. Behrandt and R. W. Love, Vacuum 12 (1962) 1 . 3) S. I Lips and Harpy S. Kukuk, 'Vacuum Sympos . Trans., 1960 Tq~rjamon Press . London 1961) p. 333 . -1) A. H. F Muggleton anki F . A lio%ve . Nu,:I . Instr . and Meth . 13 (19M) 211 . 1) 2)