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Stabilisation of thin tin films
Thin Solid Films - Elsevier Sequoia
S.A., Lausanne
- Printed
in Switzerland
R43
Short Communication Stabilisation of thin tin films H. P. KEHRER Forschungslaboratorien der Siemens AG, Miinchen (Germany)
(Received
*
April 3. 1971)
Due to their high mobility at the surface of a substrate tin atoms agglomerate into islands. Caswell’ showed that under ultra-clean conditions it is difficult to deposit films less than 200 nm thick. If the substrate is cooled to liquid nitrogen temperature (- 196 “C), then continuous films of 15 nm thickness can be produced. However, if the substrate is heated from - 196°C to room temperature the tin atoms agglomerate into islands. Caswell and Budo’ showed that the room temperature diffusion of tin is reduced if, before heating to room temperature, oxygen is admitted to a pressure of low4 tort-. The effect of oxygen on the stabilisation of tin films was studied by means of electrical resistivity measurements. It was evident that the formation of an oxide film reduced the surface mobility of the tin atoms; a 260 nm layer of SiO has no effect on the surface mobility. Caswell and Budo* concluded that the activation energy for diffusion of tin atoms at the Sn-SiO interface is almost the same as the activation energy at the Sn-vacuum interface. In the present work, other methods for the stabilization of tin films. are described. Tin films having a thickness from 2CL80 mn were deposited on carbon layers on electron microscope sample holders. The substrate was cooled with liquid nitrogen. Deposition was carried out in a UHV-system at 5 x 10m8 torr and also in a high vacuum system which operated at either 2 X 10v6 torr or low4 torr/O,. The deposition rate was 3 non/s. Measurements of the blm thickness during deposition were made with a quartz oscillator. After the deposition experiments the film thickness was measured with an interference microscope. Films deposited in UHV exhibited an island structure as reported in the literature’ **(Fig. l(a)). An island structure also formed in 36 mn films even when * New
address:
ZFA WTP Werkstofftechnik
Thin Solid lilms, 7 (1971) R43-R47
1 der Siemens AG, Miinchen (Germany).
R44 oxygen
SHORT COMMUNICATIONS was admitted
if the film thickness
before was
heating
50 n m
to room
temperature
the structure
(a)
40 000 ×
(c)
10 000 x
(b)
(d)
became
(Fig, l(b)). However, continuous
(Fig.
l(c)).
40 000 x
10 000 x
Fig. 1. Tin films deposited at liquid nitrogen temperature at 5 x 10 8 torr; (a) approx. 30 nm; (b) approx. 36 nm; (c) approx. 53 nm; (d) approx. 80 nm. In the case of (b) and (c) oxygen was admitted before heating to room temperature. Thin Solid [ilms, 7 (1971) R43 R47
R45
SHORT COMMUNICATIONS
Extreme care was necessary during the electron microscope examination of this specimen, since electron beam heating caused rapid grain growth and island formation. The critical thickness for continuous film formation was between 50 and 70 nm. Films having a thickness of 80 nm were composed of fine grains (Fig. l(d)) and were stable in the electron microscope under normal working conditions. The above experiments showed that films < 50 nm are not stabilised by oxygen. However, a coating of AI or EuS stabilised tin films having a thickness of about 20 nm. An island structure was not observed in films with 5 nm layers of AI (Fig. 2(a)) and 1 nm layers of EuS (Fig. 2(b)). From these observations it was concluded that the tin films were continuous at liquid nitrogen temperatures. The diffusion of tin atoms appeared to be reduced by thin layers of AI or EuS. It should also be pointed out, that films stabilized by layers of AI and EuS were extremely stable in the electron microscope; grain growth was not observed at high electron beam intensities. Tin films deposited at pressures from 10 -5 torr differed in structure from films deposited in an UHV-system (Fig. 3(a)). A fine grain structure was observed ( ~ 25 nm) and it seemed that during evaporation tin reacts with oxygen to form an oxide which prohibits grain growth. The admission of oxygen to a pressure of 10-4 torr during evaporation reduced the grain size to ~ 10 nm (Fig. 3(b)), and this film was extremely stable in the electron microscope at high electron beam 1
(a)
10 000 x
(b)
10 000 x
Fig. 2. Tin films of approximately 20 nm, deposited at liquid nitrogen temperature at 3 × 10 -~ tort with a layer o f (a) approx. 5 n m A1, and (b) approx. 1 n m EuS.
Thin Solid Films, 7 (1971) R 4 3 - R 4 7
R46
SHORT COMMUNICATIONS
.
¢¢'
•
'
~
'%i
.rJ
~
'
(a)
40 000 x
(c)
20 000 x
(bl
40 000 x
Fig. 3. Tin films approximately 30-40 nm, deposited at liquid nitrogen temperature (a) at 2 x 10 -~' torr, (b) at 10 -4 torr/oxygen, (c) at 4 x 10 -4 tort/oxygen.
intensities. Under the same pressure conditions as above, but without cooling the substrate, irregular tin films of uneven thickness were produced (Fig. 3(c)). A Thin Solid I ilms, 7 (1971) R43-R47
SHORT COMMUNICATIONS
R47
fine grain size of approximately 35 nm was observed in this specimenn, but the film was not continuous. Additional electrical resistivity measurements were used to indicate the presence o f oxygen in the films. A resistivity value of 54pf2 cm was observed for a film deposited at 10-4 torr on a glass substrate; this value was four times as much as the resistivity of a similar film deposited at 10 - 6 torr. None of the films described above, exhibited a preferred orientation when examined by transmission electron diffraction. These observations differed from reflection electron diffraction investigations o f Preece, Wilman and Stoddard 3 who reported preferred orientations of tin films deposited on glass substrates at room temperature. The present work shows that the grain-size of tin films is influenced by the presence of oxygen during evaporation. However, unlike the observations of Caswell and Budo 2 the admission of oxygen after UHV-evaporation, but before heating to room temperature is not sufficient to avoid agglomeration in tin films < 50 nm. Better results are obtained i f a thin layer of A1 or EuS is evaporated. In this case the activation energy for diffusion of tin atoms at the interface between Sn and the layer seems to be increased compared with that at the Sn-vacuum or Sn-SiO interface. REFERENCES 1 H . L . CASWELL,J. Appl. Phys., 32 (1961) 105. 2 H . L . CAS~/ELLAND Y. BUt:O, J. Appl. Phys., 35 (1964) 644. 3 J.B. PREECE, H. WILraAr~AND C. T. H. STODDARD, Phil. Mag., 16 (1967) 447.
Thin Solid Films, 7 (1971) R43-R47
S.A., Lausanne
- Printed
in Switzerland
R43
Short Communication Stabilisation of thin tin films H. P. KEHRER Forschungslaboratorien der Siemens AG, Miinchen (Germany)
(Received
*
April 3. 1971)
Due to their high mobility at the surface of a substrate tin atoms agglomerate into islands. Caswell’ showed that under ultra-clean conditions it is difficult to deposit films less than 200 nm thick. If the substrate is cooled to liquid nitrogen temperature (- 196 “C), then continuous films of 15 nm thickness can be produced. However, if the substrate is heated from - 196°C to room temperature the tin atoms agglomerate into islands. Caswell and Budo’ showed that the room temperature diffusion of tin is reduced if, before heating to room temperature, oxygen is admitted to a pressure of low4 tort-. The effect of oxygen on the stabilisation of tin films was studied by means of electrical resistivity measurements. It was evident that the formation of an oxide film reduced the surface mobility of the tin atoms; a 260 nm layer of SiO has no effect on the surface mobility. Caswell and Budo* concluded that the activation energy for diffusion of tin atoms at the Sn-SiO interface is almost the same as the activation energy at the Sn-vacuum interface. In the present work, other methods for the stabilization of tin films. are described. Tin films having a thickness from 2CL80 mn were deposited on carbon layers on electron microscope sample holders. The substrate was cooled with liquid nitrogen. Deposition was carried out in a UHV-system at 5 x 10m8 torr and also in a high vacuum system which operated at either 2 X 10v6 torr or low4 torr/O,. The deposition rate was 3 non/s. Measurements of the blm thickness during deposition were made with a quartz oscillator. After the deposition experiments the film thickness was measured with an interference microscope. Films deposited in UHV exhibited an island structure as reported in the literature’ **(Fig. l(a)). An island structure also formed in 36 mn films even when * New
address:
ZFA WTP Werkstofftechnik
Thin Solid lilms, 7 (1971) R43-R47
1 der Siemens AG, Miinchen (Germany).
R44 oxygen
SHORT COMMUNICATIONS was admitted
if the film thickness
before was
heating
50 n m
to room
temperature
the structure
(a)
40 000 ×
(c)
10 000 x
(b)
(d)
became
(Fig, l(b)). However, continuous
(Fig.
l(c)).
40 000 x
10 000 x
Fig. 1. Tin films deposited at liquid nitrogen temperature at 5 x 10 8 torr; (a) approx. 30 nm; (b) approx. 36 nm; (c) approx. 53 nm; (d) approx. 80 nm. In the case of (b) and (c) oxygen was admitted before heating to room temperature. Thin Solid [ilms, 7 (1971) R43 R47
R45
SHORT COMMUNICATIONS
Extreme care was necessary during the electron microscope examination of this specimen, since electron beam heating caused rapid grain growth and island formation. The critical thickness for continuous film formation was between 50 and 70 nm. Films having a thickness of 80 nm were composed of fine grains (Fig. l(d)) and were stable in the electron microscope under normal working conditions. The above experiments showed that films < 50 nm are not stabilised by oxygen. However, a coating of AI or EuS stabilised tin films having a thickness of about 20 nm. An island structure was not observed in films with 5 nm layers of AI (Fig. 2(a)) and 1 nm layers of EuS (Fig. 2(b)). From these observations it was concluded that the tin films were continuous at liquid nitrogen temperatures. The diffusion of tin atoms appeared to be reduced by thin layers of AI or EuS. It should also be pointed out, that films stabilized by layers of AI and EuS were extremely stable in the electron microscope; grain growth was not observed at high electron beam intensities. Tin films deposited at pressures from 10 -5 torr differed in structure from films deposited in an UHV-system (Fig. 3(a)). A fine grain structure was observed ( ~ 25 nm) and it seemed that during evaporation tin reacts with oxygen to form an oxide which prohibits grain growth. The admission of oxygen to a pressure of 10-4 torr during evaporation reduced the grain size to ~ 10 nm (Fig. 3(b)), and this film was extremely stable in the electron microscope at high electron beam 1
(a)
10 000 x
(b)
10 000 x
Fig. 2. Tin films of approximately 20 nm, deposited at liquid nitrogen temperature at 3 × 10 -~ tort with a layer o f (a) approx. 5 n m A1, and (b) approx. 1 n m EuS.
Thin Solid Films, 7 (1971) R 4 3 - R 4 7
R46
SHORT COMMUNICATIONS
.
¢¢'
•
'
~
'%i
.rJ
~
'
(a)
40 000 x
(c)
20 000 x
(bl
40 000 x
Fig. 3. Tin films approximately 30-40 nm, deposited at liquid nitrogen temperature (a) at 2 x 10 -~' torr, (b) at 10 -4 torr/oxygen, (c) at 4 x 10 -4 tort/oxygen.
intensities. Under the same pressure conditions as above, but without cooling the substrate, irregular tin films of uneven thickness were produced (Fig. 3(c)). A Thin Solid I ilms, 7 (1971) R43-R47
SHORT COMMUNICATIONS
R47
fine grain size of approximately 35 nm was observed in this specimenn, but the film was not continuous. Additional electrical resistivity measurements were used to indicate the presence o f oxygen in the films. A resistivity value of 54pf2 cm was observed for a film deposited at 10-4 torr on a glass substrate; this value was four times as much as the resistivity of a similar film deposited at 10 - 6 torr. None of the films described above, exhibited a preferred orientation when examined by transmission electron diffraction. These observations differed from reflection electron diffraction investigations o f Preece, Wilman and Stoddard 3 who reported preferred orientations of tin films deposited on glass substrates at room temperature. The present work shows that the grain-size of tin films is influenced by the presence of oxygen during evaporation. However, unlike the observations of Caswell and Budo 2 the admission of oxygen after UHV-evaporation, but before heating to room temperature is not sufficient to avoid agglomeration in tin films < 50 nm. Better results are obtained i f a thin layer of A1 or EuS is evaporated. In this case the activation energy for diffusion of tin atoms at the interface between Sn and the layer seems to be increased compared with that at the Sn-vacuum or Sn-SiO interface. REFERENCES 1 H . L . CASWELL,J. Appl. Phys., 32 (1961) 105. 2 H . L . CAS~/ELLAND Y. BUt:O, J. Appl. Phys., 35 (1964) 644. 3 J.B. PREECE, H. WILraAr~AND C. T. H. STODDARD, Phil. Mag., 16 (1967) 447.
Thin Solid Films, 7 (1971) R43-R47