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Si films by ion assisted deposition
Vacuum/volume 39/numbers Printed in Great Britain
24lpages
Formation deposition Pan Xian-zheng
425 to 428/l
0042-207X/89$3.00+.00 Pergamon Press plc
989
of Ag/Si
and AI/Si
films
by ion assisted
and Pan Feng, Ion Beam Physics Laboratory,
and Ion Beam Laboratory,
Shanghai
Institute
of Metallurgy,
Physics Department, Academia Sinica, PRC
Wuhan University,
Wuhan
The Al and Ag films made by IVD were significantly modified in both mechanical and electrical properties. The cause lies in the deposition parameters: bombardment energy and ionfatom arrivalrate ratio. It was found that the Al film hardness, the AI-Si contact resistivity and the AI-Si adhesion were improved in the deposition range : energy 250-500 e V and arrival ratio 0.06-o. 10.
1. Introduction
Table 1. The operating
Ion beam assisted deposition or ion assisted vapour deposition (IVD) involves simultaneous ion beam bombardment in the vacof materials and uum vapour deposition’m2. In the modification the synthesis of new materials, its kinetic and charge effects play important roles in nucleation kinetics and thin film growth. The mechanisms which were used to explain the affected characteristics involve trapping, resputtering and enhanced interdiffusion between the film and the substrate3. The enhanced adhesion and the preferred orientation growth of films are the two pronounced features. This paper gives the preliminary results of an investigation on the oriented growth, interface mixing of Ag films by IVD, the properties of Al-9 adhesion and electric contact by IVD. 2. Experimental The schematic diagram of the IVD equipment is shown in Figure 1. The temperature range from liquid nitrogen up to 400°C can be obtained in the high-low temperature chamber. The substrate
n
I-
N-2
F7 4
I I
.
Ad----\I -9
G?
I-“‘-5
‘7
I sl 1
Figure 1. The schematic of the self-made IVD instrument. (1) LN chamber; (2) Kaufman ion source ; (3) rf ion source; (4) target; (5) shutter; (6) quartz oscillator; (7) evaporator; (8) vacuum system; (9) sputtered target.
conditions
of the IVD system
Deposited material Substrate temperature Deposition rate Deposition thickness Ion species
Ag Room temperature 8-30 nm s- ’ 500-2000 nm Ar+
Ion bombardment energy Ion beam current
&30 keV CUmA
Al 5-25 nm s-’ 200P1000 nm
Ar+ O-l .2 keV 3M mA
low index planes such as (100) plane appear preferentially. When the bombardment energy is larger than 1 keV, the ratio of the diffraction strength of (111) plane to that of (100) plane is saturated at 0.20-0.30. The resputtering of deposited materials can for use is fixed on a quadrilateral holder which is tightly connected to the chamber top. The sample is conveniently changed by rotating this holder. Ion gun 1 is a rf ion source which provides a beam of ions with energy 2-30 keV, beam current 0.2-I mA and beam diameter 5-10 cm. No mass analyzer is needed. Ion gun 2 is a broad-beam multi-aperture electron bombardment ion source’, in which the ion flux and energy are easily measured and controlled in the ranges l-20 mA and 0.1-2 keV, respectively. A wide choice of gas species are used as working materials. Ion guns 1 and 2 are not only used for the low-high energy bombardment, but also used as a dual ion beam deposition system. The IVD method is detailed elsewhere’,*. One can see the experiment conditions which we used in Table 1. Films were deposited in a vacuum 3 x 10e2 Pa, and substrates are p-type lO_ 20 R cm Si (111) wafers. 3. Results and discussion 3.1. Preferred orientation growth of Ag films by IVD. Ag films deposited on p-type silicon wafers were studied by X-ray diffraction. The analysis plot is drawn in Figure 2. It shows that the tendency of (111) plane growth in polycrystalline grains is restrained with increasing bombardment energy, while there is some influence on the nucleation and growth kinetics of the film. As a result, the characteristics of the film such as composition and microstructure could be changed. The evaporated materials are adsorbed and coalesce on the surface, forming many uncontinuous islands. The simultaneous ion bombardment affects 425
Pan Xian-zheng
o
Formation
and Pan Feng:
I
I
0
1000
I
of Ag/Si
I
2000
3000
I 4000
and AI/Si
films
I 5000
strength and ( I I I ) of the bombardment energy. Abscissa axis: bombarding energy, eV: longitudinal axis : 1(100)/1( I I I).
Figure 2. The ratio ofthc (200) plane X-ray diffraction plane diffraction
strength.
as 3 function
the growing islands in which many grains are involved. Tho plants with high sputtering rates would be retarded and growth along the planes with low sputtering rates would occur. Among three main low index plants ofthe Ag films (I I I), (I IO) and (IO), there exists a relationship Y(l I I) > Y( 100) (ret” 6). According to the discussion above. (I I 1) plants would diminish in the process of deposition during ion bombardment and (100) planes would grow preferentially.
(a)
3.2. Crystallographical structure and topography of Ag films by IVD. Figure 3 shows the grain structure TEM photos of Ag films deposited at 500 eV. 0.8 mA cm ’ and no At-+ beam. The dependence of grain size on the bombardment energy is presented in Figure 4. As the bombardment energy (E < 500 eV) incrcascs. the grain sizes decrease accordingly. When the bombardment energy is larger than 500 eV, the grain sizes increase. It seems that there are two different surface diffusion mechanisms in the process of evaporation during ion bombardment. Robinson ct trlJ proposed a model in which the heat-activsated diffusion is dominant in the low ion flux and the random walk length dccreascs inversely proportional to the square root of the sputtcring yield. In the high ion flux (larger than I.5 mA cm ‘). the impact-activated surface diffusion dominates. It mukcs the grain less fine and less dense. Depending on thcsc experimental results. we considered that the surface diffusion theory was suitable hcrc. The recrystallization of Ag film grains takes place at 200 C in a vacuum. The film grains grow linearly with longer annealing time (as seen in Fipurc 3). 3.3. Interface mixing and surface damage of Ag/Si. Typical RBS and channclled backscattering spectra for Ag/Si interfaces were taken. The quantity of Ag/Si mixing is less for low bombardment energy (< 1500 eV). The 35 keV Ar’ bombardment could induce
(b)
Cd) Figure 3. TEM
0.5 h vacuum
426
structure of Ag films deposited at different conditions : (a) conventional vapour deposition ; (b) fihn structure (c) Ag tilm by IVD in 500 eV. 0.X mA em ’ : (d) film structure after 200 C. 0.5 h. vacuum annealing.
of the grain
annealing:
after 200 C’.
Pan Xian-zheng
and Pan Feng:
Formation
of Ag/Si
and AI/S
films 06
400 Figure 4. The grain size of Ag films by IVD and by the conventional vapour deposition, as a function of bombardment energy. Abscissa axis bombarding energy, eV ; longitudinal axis : grain size, nm.
:
a 10 nm interface mixing layer (Figure 5). The differences in the damage of Si substrates were compared, as shown in Figure 6. The damage layer and the total amount of damage were related to bombardment energy and arrival ratio, and the energy deposited per surface atom decided the amount of radiation damage. Although this quantitative relationship has been unclear so far, it is obvious that the damage layer is far thicker than the mixing layer.
Figure 6. Channelled backscattering spectra for Ag/Si interfaces. ~ no ion beam; -- -- 500 eV, 0.8 mA cm-‘; ........ 1300 eV, 0.8 mA cm-‘; -~.-.- 30 keV, 3 PA cm-‘. Abscissa axis : RBS channel; longitudinal axis : normalized counts.
64-
3.4. Microhardness of Al films by IVD. Figure 7 shows the microhardness values of Al films before and after annealing at 2OO”C, 4 Pa, as a function of bombardment energy. The measuring load is 50 g. The microhardness of the Al films made by IVD is larger than that of films made by vapour deposition. The grain size of IVD Ag films can reach to a much lower level (around 50-60 nm) in proper conditions. The defects and dislocation in such a dense crystal lattice are so difficult to move that the hardness and strength of the IVD films are increased’; the same explanation goes for IVD Al films as well. The microhardness decreases with the growth of Al grains in the vacuum annealing process. 3.5. AI-Si contact resistivity characteristics. According to the measuring configurations published by NBS*, the Al-Si contact resistance was measured at different deposition conditions : bom-
IO
-6
-6
-4
Figure 7. (1) AI-Si contact resistivity,
as a function of the bombardment energy. (2) Microhardness of Al films, as a function of bombardment energy. (a) Before annealing ; (b) after 2OO”C, 30 min vacuum annealing. Abscissa axis : bombarding energy ; longitudinal axis : right side, contact resistivity ( x lo-’ fi cm); left side, microhardness (kg cmm2).
bardment energy from 0 to 1200 eV at 2OO”C, vacuum annealing 30 min. The electric contact shows betterlinear Z-V characteristics. At about 250 eV, its resistivity falls to a minimum and goes up with the increment of bombardment energy (Figure 7). It seems that the formation mechanism of such low-resistivity contacts by IVD is due to the elimination of the oxides and other contaminants on the substrate surface by the cleaning effects of energetic particles. E > 250 eV, the increment of the contact resistivity results from the more intensive damage layer on the Si substrates. References Figure 5. A typical RBS of Ag films by IVD, at deposition keV, 1.4 PA cm-‘, Ar+ bombardment. longitudinal axis : RBS counts.
Abscissa
axis:
conditions 35 RBS channel;
‘J M Harper, J J Cuomo, R J Gambino and H R Kaufman, in Ion Bombardment ModiJication of Surfaces: Fundamentals and Application (Edited by 0 Auciello and R Kelly). Elsevier, New York (1984). 427
Pan Xian-zheng
and Pan Feng:
Formation
of Ag/Si
and AI/S1 films
B D Sartwell. I L Singer and R G Vardiman, Nucl furrum Me/h, B7/8,915 (1985). ‘T Takagi, Proc Int Ion Engineering Congrrx-ISIAT ‘83, IPA T ‘83, Kyoto, p 785 (1983). “R S Robinson and S M Rossnagel, in Ion Bomhardmen~ Modification (!/ Su[@es: Fundamentals and Application (Edited by R Kelly and 0 Auclello). Elsevier. New York (1984). ’ R A Kant,
428
* H R Kaufman, J ‘H E Roosendaal. R Behrisch), p 230. ‘R W Armstrong, and Volker Weiss). ‘F R Kelly, R L 400-I. Washington
Vuc Sci Techno/. 15,272 (1978). Sputreriny hi, Particle Bomhrrrdmcw I (Edited by Springer-Verlag, New York (I 98 I). in Ultrajinr-Gruin Met& (Edited by John J Burke Syracuse University Press. New York (1970). Mattis and M G Buhler, NBS Special Publication DC’ (1974).
24lpages
Formation deposition Pan Xian-zheng
425 to 428/l
0042-207X/89$3.00+.00 Pergamon Press plc
989
of Ag/Si
and AI/Si
films
by ion assisted
and Pan Feng, Ion Beam Physics Laboratory,
and Ion Beam Laboratory,
Shanghai
Institute
of Metallurgy,
Physics Department, Academia Sinica, PRC
Wuhan University,
Wuhan
The Al and Ag films made by IVD were significantly modified in both mechanical and electrical properties. The cause lies in the deposition parameters: bombardment energy and ionfatom arrivalrate ratio. It was found that the Al film hardness, the AI-Si contact resistivity and the AI-Si adhesion were improved in the deposition range : energy 250-500 e V and arrival ratio 0.06-o. 10.
1. Introduction
Table 1. The operating
Ion beam assisted deposition or ion assisted vapour deposition (IVD) involves simultaneous ion beam bombardment in the vacof materials and uum vapour deposition’m2. In the modification the synthesis of new materials, its kinetic and charge effects play important roles in nucleation kinetics and thin film growth. The mechanisms which were used to explain the affected characteristics involve trapping, resputtering and enhanced interdiffusion between the film and the substrate3. The enhanced adhesion and the preferred orientation growth of films are the two pronounced features. This paper gives the preliminary results of an investigation on the oriented growth, interface mixing of Ag films by IVD, the properties of Al-9 adhesion and electric contact by IVD. 2. Experimental The schematic diagram of the IVD equipment is shown in Figure 1. The temperature range from liquid nitrogen up to 400°C can be obtained in the high-low temperature chamber. The substrate
n
I-
N-2
F7 4
I I
.
Ad----\I -9
G?
I-“‘-5
‘7
I sl 1
Figure 1. The schematic of the self-made IVD instrument. (1) LN chamber; (2) Kaufman ion source ; (3) rf ion source; (4) target; (5) shutter; (6) quartz oscillator; (7) evaporator; (8) vacuum system; (9) sputtered target.
conditions
of the IVD system
Deposited material Substrate temperature Deposition rate Deposition thickness Ion species
Ag Room temperature 8-30 nm s- ’ 500-2000 nm Ar+
Ion bombardment energy Ion beam current
&30 keV CUmA
Al 5-25 nm s-’ 200P1000 nm
Ar+ O-l .2 keV 3M mA
low index planes such as (100) plane appear preferentially. When the bombardment energy is larger than 1 keV, the ratio of the diffraction strength of (111) plane to that of (100) plane is saturated at 0.20-0.30. The resputtering of deposited materials can for use is fixed on a quadrilateral holder which is tightly connected to the chamber top. The sample is conveniently changed by rotating this holder. Ion gun 1 is a rf ion source which provides a beam of ions with energy 2-30 keV, beam current 0.2-I mA and beam diameter 5-10 cm. No mass analyzer is needed. Ion gun 2 is a broad-beam multi-aperture electron bombardment ion source’, in which the ion flux and energy are easily measured and controlled in the ranges l-20 mA and 0.1-2 keV, respectively. A wide choice of gas species are used as working materials. Ion guns 1 and 2 are not only used for the low-high energy bombardment, but also used as a dual ion beam deposition system. The IVD method is detailed elsewhere’,*. One can see the experiment conditions which we used in Table 1. Films were deposited in a vacuum 3 x 10e2 Pa, and substrates are p-type lO_ 20 R cm Si (111) wafers. 3. Results and discussion 3.1. Preferred orientation growth of Ag films by IVD. Ag films deposited on p-type silicon wafers were studied by X-ray diffraction. The analysis plot is drawn in Figure 2. It shows that the tendency of (111) plane growth in polycrystalline grains is restrained with increasing bombardment energy, while there is some influence on the nucleation and growth kinetics of the film. As a result, the characteristics of the film such as composition and microstructure could be changed. The evaporated materials are adsorbed and coalesce on the surface, forming many uncontinuous islands. The simultaneous ion bombardment affects 425
Pan Xian-zheng
o
Formation
and Pan Feng:
I
I
0
1000
I
of Ag/Si
I
2000
3000
I 4000
and AI/Si
films
I 5000
strength and ( I I I ) of the bombardment energy. Abscissa axis: bombarding energy, eV: longitudinal axis : 1(100)/1( I I I).
Figure 2. The ratio ofthc (200) plane X-ray diffraction plane diffraction
strength.
as 3 function
the growing islands in which many grains are involved. Tho plants with high sputtering rates would be retarded and growth along the planes with low sputtering rates would occur. Among three main low index plants ofthe Ag films (I I I), (I IO) and (IO), there exists a relationship Y(l I I) > Y( 100) (ret” 6). According to the discussion above. (I I 1) plants would diminish in the process of deposition during ion bombardment and (100) planes would grow preferentially.
(a)
3.2. Crystallographical structure and topography of Ag films by IVD. Figure 3 shows the grain structure TEM photos of Ag films deposited at 500 eV. 0.8 mA cm ’ and no At-+ beam. The dependence of grain size on the bombardment energy is presented in Figure 4. As the bombardment energy (E < 500 eV) incrcascs. the grain sizes decrease accordingly. When the bombardment energy is larger than 500 eV, the grain sizes increase. It seems that there are two different surface diffusion mechanisms in the process of evaporation during ion bombardment. Robinson ct trlJ proposed a model in which the heat-activsated diffusion is dominant in the low ion flux and the random walk length dccreascs inversely proportional to the square root of the sputtcring yield. In the high ion flux (larger than I.5 mA cm ‘). the impact-activated surface diffusion dominates. It mukcs the grain less fine and less dense. Depending on thcsc experimental results. we considered that the surface diffusion theory was suitable hcrc. The recrystallization of Ag film grains takes place at 200 C in a vacuum. The film grains grow linearly with longer annealing time (as seen in Fipurc 3). 3.3. Interface mixing and surface damage of Ag/Si. Typical RBS and channclled backscattering spectra for Ag/Si interfaces were taken. The quantity of Ag/Si mixing is less for low bombardment energy (< 1500 eV). The 35 keV Ar’ bombardment could induce
(b)
Cd) Figure 3. TEM
0.5 h vacuum
426
structure of Ag films deposited at different conditions : (a) conventional vapour deposition ; (b) fihn structure (c) Ag tilm by IVD in 500 eV. 0.X mA em ’ : (d) film structure after 200 C. 0.5 h. vacuum annealing.
of the grain
annealing:
after 200 C’.
Pan Xian-zheng
and Pan Feng:
Formation
of Ag/Si
and AI/S
films 06
400 Figure 4. The grain size of Ag films by IVD and by the conventional vapour deposition, as a function of bombardment energy. Abscissa axis bombarding energy, eV ; longitudinal axis : grain size, nm.
:
a 10 nm interface mixing layer (Figure 5). The differences in the damage of Si substrates were compared, as shown in Figure 6. The damage layer and the total amount of damage were related to bombardment energy and arrival ratio, and the energy deposited per surface atom decided the amount of radiation damage. Although this quantitative relationship has been unclear so far, it is obvious that the damage layer is far thicker than the mixing layer.
Figure 6. Channelled backscattering spectra for Ag/Si interfaces. ~ no ion beam; -- -- 500 eV, 0.8 mA cm-‘; ........ 1300 eV, 0.8 mA cm-‘; -~.-.- 30 keV, 3 PA cm-‘. Abscissa axis : RBS channel; longitudinal axis : normalized counts.
64-
3.4. Microhardness of Al films by IVD. Figure 7 shows the microhardness values of Al films before and after annealing at 2OO”C, 4 Pa, as a function of bombardment energy. The measuring load is 50 g. The microhardness of the Al films made by IVD is larger than that of films made by vapour deposition. The grain size of IVD Ag films can reach to a much lower level (around 50-60 nm) in proper conditions. The defects and dislocation in such a dense crystal lattice are so difficult to move that the hardness and strength of the IVD films are increased’; the same explanation goes for IVD Al films as well. The microhardness decreases with the growth of Al grains in the vacuum annealing process. 3.5. AI-Si contact resistivity characteristics. According to the measuring configurations published by NBS*, the Al-Si contact resistance was measured at different deposition conditions : bom-
IO
-6
-6
-4
Figure 7. (1) AI-Si contact resistivity,
as a function of the bombardment energy. (2) Microhardness of Al films, as a function of bombardment energy. (a) Before annealing ; (b) after 2OO”C, 30 min vacuum annealing. Abscissa axis : bombarding energy ; longitudinal axis : right side, contact resistivity ( x lo-’ fi cm); left side, microhardness (kg cmm2).
bardment energy from 0 to 1200 eV at 2OO”C, vacuum annealing 30 min. The electric contact shows betterlinear Z-V characteristics. At about 250 eV, its resistivity falls to a minimum and goes up with the increment of bombardment energy (Figure 7). It seems that the formation mechanism of such low-resistivity contacts by IVD is due to the elimination of the oxides and other contaminants on the substrate surface by the cleaning effects of energetic particles. E > 250 eV, the increment of the contact resistivity results from the more intensive damage layer on the Si substrates. References Figure 5. A typical RBS of Ag films by IVD, at deposition keV, 1.4 PA cm-‘, Ar+ bombardment. longitudinal axis : RBS counts.
Abscissa
axis:
conditions 35 RBS channel;
‘J M Harper, J J Cuomo, R J Gambino and H R Kaufman, in Ion Bombardment ModiJication of Surfaces: Fundamentals and Application (Edited by 0 Auciello and R Kelly). Elsevier, New York (1984). 427
Pan Xian-zheng
and Pan Feng:
Formation
of Ag/Si
and AI/S1 films
B D Sartwell. I L Singer and R G Vardiman, Nucl furrum Me/h, B7/8,915 (1985). ‘T Takagi, Proc Int Ion Engineering Congrrx-ISIAT ‘83, IPA T ‘83, Kyoto, p 785 (1983). “R S Robinson and S M Rossnagel, in Ion Bomhardmen~ Modification (!/ Su[@es: Fundamentals and Application (Edited by R Kelly and 0 Auclello). Elsevier. New York (1984). ’ R A Kant,
428
* H R Kaufman, J ‘H E Roosendaal. R Behrisch), p 230. ‘R W Armstrong, and Volker Weiss). ‘F R Kelly, R L 400-I. Washington
Vuc Sci Techno/. 15,272 (1978). Sputreriny hi, Particle Bomhrrrdmcw I (Edited by Springer-Verlag, New York (I 98 I). in Ultrajinr-Gruin Met& (Edited by John J Burke Syracuse University Press. New York (1970). Mattis and M G Buhler, NBS Special Publication DC’ (1974).