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Examination of Thin Film Uniformity at the Bottom of a Hole Structure Using a 3D Sputter Simulation Package
A. Daniels, D. Cameron
To cite this version:
A. Daniels, D. Cameron. Examination of Thin Film Uniformity at the Bottom of a Hole Structure Using a 3D Sputter Simulation Package. Journal de Physique III, EDP Sciences, 1996, 6 (9), pp.1213- 1218. �10.1051/jp3:1996180�. �jpa-00249519�
Examination of Thin Film Uniformity at the Bottom of a Hole Structure Using a 3D Sputter Simulation Package
S. Daniels (*) and D.C. Cameron
School of Electronic Engineering, Dublin City University, Dublin 9, Ireland
(Received 15 February1996, received in final form 6 May 1996, accepted 28 May 1996)
PACS.81.15.Cd Deposition by sputtering
Abstract. Sputter deposition onto substrates with large or complex surfaces is difficult as
sputtering is
a "line of sight" process. Sputtering into crevices or holes is particularly difficult if the electrical, optical or mechanical properties of the film sputtered in the "out of sight"
region needs to meet tight specifications, as in the case of biomedical applications. The throw distance of a sputter system can be optimized by adjusting process chamber geometry or process parameters. Other techniques such as substrate rotation, substrate biasing or a high substrate temperature may assist in coating an awkward substrate uniformly. In this paper we use a 3D sputter simulation package to examine the uniformity of
a film deposited at the bottom of
a hole.
We examine film thickness uniformity as a function of process pressure, racetrack geometry, and hole depth.
1. Introduction
Magnetron Sputtering is a commonly used deposition technique for depositing high quality
thin films. In the microelectronics industry, sputtering is the technology of choice for deposit- ing metallization layers, diffusion barriers and anti-reflecting coatings iii. In the mechanical
engineering industry sputtering is used to deposit hard wear resistant films such as TiN and CN onto components such as drill bits in order to increase their expected lifetime [2,3].
In any thin film application it is important to ensure that the properties of the film are consistent over the entire substrate surface of interest. This constraint may be difficult to achieve on large substrates or substrates with complex geometries. Frequently it is necessary
to ensure a uniform film thickness over a substrate surface.
We have developed a simulation package which simulates in 3D, the flux of sputtered atoms irom a target suriace [4]. For a given set of process parameters and system geometries, the
simulation will predict the intensities and energies of the fluxes of sputtered atoms to each surface of a specified 3D substrate. Details of the model "SPUTSIM" on which the simulation is based have been published elsewhere [5]. The following are the important features of the
model: Particles sputtered from the target surface are assumed to be ejected at random angles
which obey the cosine distribution law [7]. The initial energies of the ejected atoms are assumed to obey the Thompson distribution function [6]. To use the Thompson distribution function
(*) Author for correspondence Present address: Applied Materials Europe BV, Nieuwe Duken-
burgseweg 208, 6534 AD Nijmegen, The Netherlands
© Les kditions de Physique 1996
1214 JOURNAL DE PHYSIQUE III N°9
Fig. 1. Diagram of the simulated system.
we assume that the flux of argon atoms bombarding the target is a monoenergetic beam with
energy 500 eV. The mean free path of the sputtered atoms in the plasma is assumed to be independent of energy and on collision with a background gas atom the scattering process is
modelled by the hard sphere collision model [8].
In this paper we will use ~'SPUTSIM" to examine the thickness uniformity of a film at the bottom of a 'hole' structure as a function of process pressure, target racetrack geometry and hole depth.
2. Description of the Simulated System
The virtual system on which the simulation was run is a rectangular chamber 26 cm long in the ~-direction, 20 cm high in the g-direction, and 20 cm wide in the z-direction. The target is positioned on the y z plane at x
= 0. It is centered at x
= 0, y = 0, z = 0. The substrate
is an open box type structure with the open end facing the target. The open face is a square
with side length 4 cm. The racetrack geometry will be experiment specific and we assume that all particles sputtered from the target are sputtered from the racetrack region. We assume
that the target material is titanium and the process gas is argon. A diagram of the system of the system is given in Figure 1.
3. Results of Simulations
For each of the simulations, the total number of particles simulated was 60,000. It was assumed that the sticking coefficients of particles impinging on any surface was unity. In order to
calculate the film uniformity the following formula was used:
L(%) = (L/Mean) x 100
Where: L is the non-uniformity and x is the number of measurement points on the surface.
In these simulations, 9 surface sites were "measured". The substrate surface was divided into 9 squares of1.33 cm side length. The centre of each square corresponded to a measurement
site.
84 83 82 81 z~ 80
j 59
( 58
Ic 57
58 55 54 53
0.5 0.8 3 8
Pressure 10~ mbor
Fig. 2. Graph of film non-uniformity us. pressure.
3.I. FILM UNIFORMITY As A FUNCTION OF PRocEss PRESSURE. In this experiment, the
process pressure was varied between 0.8 x 10~3 mbar and 8.0 x 10~~ mbar. All other parameters
were kept constant. The hole depth was 4 cm. The racetrack configuration was set to be a
circular ring, with inner diameter 4 cm and an outer diameter of 4.5 cm. All the particles from the target surface were assumed to be ejected from the racetrack region. The results of the
simulations are displayed in Figure 2.
It can be seen that the non-uniformity generally decreases as the system pressure is increased
(the increase at a pressure of1 x 10~~ mbar is an artefact due to the limited number of particles
in the experiment). The only parameter in the model which changes in these simulations is the mean free path of the sputtered particles. As the pressure is increased, I-e- the mean free
path is reduced, the sputtered atoms undergo more collisions before reaching the substrate and hence there is more randomness in their direction and more even distribution. It should be pointed out, however, that the thickness of the film also decreases with increasing pressure
since more atoms are scattered away from the substrate on to the chamber walls.
3.2. FILM UNIFORMITY As A FUNCTION OF RACETRACK GEOMETRY. The purpose of
this experiment is to examine how the film uniformity at the bottom of the hole varies with the racetrack geometry. For this experiment the pressure was kept constant at 3 x 10~~ mbar.
The racetrack is a 0.5 cm thick circular ring. By varying the inner and outer diameters we vary the size of the racetrack. The following geometries were simulated: #1 ri
= 2, r2 " 2.5;
#2 ri
= 2.5, r2
" 3; #3 ri
= 3, r2 " 3.5; #4 ri = 3.5, r2 = 4; #5 ri
= 4, r2
" 4.5;
#6 ri
" 4.5, r2 = 5; where ri is the inner diameter and r2 is the outer diameter. It has
been assumed that the flux of sputtered atoms is emitted uniformly from all parts of the racetrack region. In reality, the flux from a magnetron target will show a peak at the diameter of the racetrack and will decrease gradually at greater or lesser diameters, the exact shape of
the distribution depending on the magnet configuration in the magnetron. It is intended to
1216 JOURNAL DE PHYSIQUE III N°9
64
82
80 Zl
f 58
(
56 O
54
52
50
2 3 4 5 8
RocehockGeometry#
Fig. 3. Graph of film non-uniformity us. racetrack geometry.
incorporate such a variation in the flux model, however, at present assuming a uniform flux distribution gives a reasonable approximation to this. It is likely that the non-uniformity may
be rather overstated in the existing model since it will give too abrupt a change between the racetrack and the rest of the target. The results of the simulations are shown in Figure 3.
From the results we see that the optimum uniformity is achieved at racetrack geometry #5.
Here the racetrack outer diameter is slightly greater than the hole orifice. It can be seen that the best uniformity, unsurprisingly, occurs when the racetrack region is centrally placed above the hole. This highlights the difficulty which will be achieved in coating inside an aperture which is not in line of sight with the emitting area of the magnetron target.
3.3. FILM UNIFORMITY As A FUNCTION OF HOLE DEPTH. In Figure 4 the uniformity
of the film on the bottom surface of the hole is examined as a function of the hole depth at
a constant pressure of 3 x 10~3 mbar. The racetrack geometry, as in Section 3.1., was set to
inner and outer diameters of 4.0 and 4.5 cm. Simulations were carried out for hole depths of 2 to 10 cm at 2 cm intervals.
The results are plotted in Figure 4. It is evident that the non-uniformity of the film thickness decreases as the hole depth increases. This can be understood as follows. As the hole depth increases, the flux of atoms arriving at the bottom surface will become more perpendicular to it since oblique-angled atoms will tend to collide with the hole walls and thus become filtered out, I-e- it behaves like a collimator. However, as shown in Figure 5, the film thickness at
the centre of the bottom surface also decreases markedly as a function of hole depth due to
increased scattering of the sputtered atoms and their subsequent collision with the hole walls.
At a point in the centre of the bottom surface, the simulation predicts that at a distance of 10 cm the film thickness is only 12% of that at a distance of 2 cm. Moreover, at a hole depth of 10 cm, only 2% oi the sputtered atoms reach the bottom surface. Under these circumstances it is difficult to obtain an accurate description oi the topology or uniformity from the simulation
because of the relatively few atoms arriving on the surface.
88
88
84
~ 82
j 80
58
I
~ 58
54
52
50
2 4 8 8 lo
Hole Depth (cm)
Fig. 4. Graph of film non-uniformity us. hole depth.
300
jf 250
j
4 200
<
)w
150
j
~~~
g
~ 50
0
2 3 4 5
Hole Depth(cm)
Fig. 5. Graph of film centre thickness us. hole depth.
4. Conclusions
The above simulations illustrate how predictions can be obtained of the substrate coverage
on substrates with complex geometry. The process was applied to a "hole" structure but the simulation package could just as easily have been applied to a substrate of any particular shape or size. Section 3.2 demonstrates the importance ofconfiguring the system such that the
emitting area of the target is adjacent to the surface,in particular a re-entrant surface, to obtain
1218 JOURNAL DE PHYSIQUE III N°9
good coverage. The importance of rotation is evident for this type of substrate. The model also predicts the improvement in uniformity which can be obtained by operating at increased
pressure, where possible. The simulation model correctly displays the features which occur in real sputtering systems. However, to ensure its validity and accuracy, the simulations require
to be correlated with experimental measurements to redefine the model. Further extensions of the model are planned to take into account the effect on the stoichiometry of the films of
substrate topology during reactive sputtering.
References
iii Dixit G-A-, l&'ei C-C-, Liou F-T- and Zhan H., Reactively Sputtered titanium nitride films for submicron contact barrier metallization, Appi. Phys. Lett. 62 (1993) 357-359.
[2] Al-Jaroudi M-Y-, Hentzell H-T-G- and Homstrom S-E-, Deposition of Titanium Nitride on Surface-Hardened Structural Steel by Reactive Magnetron Sputtering, Thin Solid Films 182 (1989) 153-166.
[3] Teer D.G., Technical Note: a Magnetron Sputter Ion-Plating System, Surf. Coatings Techn.
39/40 (1989) 565-572.
[4] Daniels S., SPUTSIM: A 3-D computer simulation of sputtered atom transport in a sputter discharge using monte-carlo techniques. Unpublished.
[5] Daniels S. and Cameron D.C., Monte Carlo Simulation of Particle Fluxes in the Magnetron Sputtering Process, Proc. of the International Conference of Advances in Materials and
Processing Technologies (Dublin, Ireland, August 1995) pp. 953-958.
[6] Thompson M-W-, Phiios. Mag. 18 (1968) 377.
[7] Maissel L-I- and Glang R., Handbook of Thin Film Technology (McGraw-Hill, 1983).
[8] 3/Iotohiro, Applications of Monte Carlo Simulation in the analysis of a sputter deposition
process, J. Vac. Sci. Technoi. A 4 (1986) 189-195.
