Germanium surface passivation using ozone gaseous phase
Virginie Loup1,a, Pascal Besson2,b, Olivier Pollet3, Eugénie Martinez1,
Emmanuelle Richard4, Sandrine Lhostis2.
1
CEA-DRT - LETI/DPTS - CEA/GRE – 17 avenue des Martyrs, 38 054 Grenoble Cedex 9, France
2
STMicroelectronics, rue Jean Monet, F-38920 Crolles, France
3
SEMITOOL, 655 W. Reserve drive, 59901 Kalispell MT, USA
4
LTM/CNRS – 17 avenue des Martyrs, 38 054 Grenoble Cedex 9, France
a
virginie.loup@cea.fr, b pascal.besson@cea.fr
Keywords: germanium, surface passivation, ozone.
Introduction
As silicon tends towards ultimate physical limitations for high-performance devices, germanium is
extensively investigated for its high electron and hole mobility advantages [1]. As for silicon,
advanced integration node implies the use of very efficient germanium cleaning sequences to gain
yield. However, germanium surface sensitivity against most of the conventional wet solutions is a
major issue [2]. Specific wet chemical processes also need to be developed to clean and prepare
germanium surfaces before being integrated.
In this study, the influence of various chemical treatments on germanium surfaces state is
investigated using AR-XPS analysis. Then, a new method to efficiently passivate a germanium
surface is described. Using gaseous ozone phase on a spin-on single wafer cleaning tool, we
effectively managed to grow a GeO2 layer which is, according to AR-XPS analysis, stable in time.
Experimental details
In this work, 2.5 µm thick germanium layers epitaxially grown on Si (001) wafers by Chemical
Vapor Deposition were used [3]. Cleaning tests were performed on three different tools. The Akrion
Gama One automated wet bench was mainly used to evaluate the compatibility of typical silicon
wet treatments (CARO, SC1, SC2…) with germanium surfaces cleaning. A 200 mm spin-on single
wafer tool from SEZ was also used for HF-last processes. Finally, both wet and vapor phase
cleanings were experimented on the 200/300 mm Raider SP spin-on single wafer tool from
Semitool.
Post cleaning, germanium surfaces were analyzed by AR-XPS at various incidence angles, with a
0.7 eV energy resolution. The angle scanning enables to enhance the sensibility either to the
extreme surface or to the interface with the substrate. Both Ge 3d and Ge 2p (respectively centered
at 29 and 1217 eV) were also used to vary the scanned thickness according to the photoelectrons
kinetic energy. For each measurement condition, analyzed thicknesses (d) can be estimated with the
following formula: d = 3×L×sinθ, where θ represents the scanning angle and L, the electrons mean
free path (in GeO2, L is around 2.35 nm and 0.6 nm respectively for Ge 3d and Ge 2p).
Results and discussion
As germanium is a potential candidate for the 45 nm node and below, its consumption control
during cleaning steps is a key point. However, because of the high sensitivity of germanium to
oxidation combined with the high kinetic of dissolution of GeO2 in water, standard H2O2-based
chemistries (like CARO, SC1 and SC2) lead to excessive etch rates (see Figure 1 and Table 1).
Consequently, standard H2O2-based wet chemical treatments are totally prohibited for germanium
cleaning. Among the wet solutions classically used for silicon cleaning, only the HF/HCl solutions
can potentially candidate for germanium treatments. However, even the so-called “DDC” cleaning
process developed in LETI [4] leads to a 200 Å germanium consumption which is due to the acidic
ozonated DIW rinse step necessary to guarantee a high particle and metal removal efficiency.
Adapting the “DDC” process, a new sequence (called “GecleanP”) has been optimized for
germanium surfaces cleaning applications. First, samples are des-oxidized in an acidic (HF 0.2% +
HCl 1%) solution for 120 s to dissolve the native oxide. Next, germanium wafers are rinsed in a
two-step sequence: first, in an acidic de-ionized water (DIW) solution (with a pH near to 2) for 180
s; and then in an acidic ozonated DIW solution (strict control of the O3 injection) for 120s. This
two-step rinse sequence enables to combine cleaning efficiency (particle and metal removal > 98%)
with a low Ge consumption (~35 Å).
As supported by the XPS spectrum presented in Figure 2, following the “GecleanP” rinse step, the
germanium surface is covered by a thin oxide (GeOx) layer. Compared with the GeO2 peak energy
position, the GeOx peak is shifted slightly toward the lower binding energy values, indicating the
under stoechiometry of this germanium oxide (x<2). Moreover, XPS spectra of germanium wafers
rinsed in pure DIW (not shown here) show a similar GeOx peak signature. These results totally
contrast with what happens on silicon surfaces: indeed, the XPS spectrum of a “DDC” cleaned Si
substrate clearly differs from a “HF-last” Si surface in the presence or not of the Si-O peak
signature. Consequently, a stable germanium interface cannot be obtained after a DIW rinse in
presence of diluted ozone or not.
In order to bypass a potential re-oxidation due to the DIW rinse, the rinse step was removed from
the germanium cleaning sequence. XPS spectra are given in Figure 3. Even when XPS analysis
immediately follows the HF/no rinse treatment, a shoulder is observed on the elemental germanium
peak pointing out that the germanium first monolayers are instantaneously re-oxidized by air
exposure only. After 48 hours storage in clean room, XPS analysis clearly demonstrates the
increase and the shift of the GeOx-attributed peak indicating the re-growth of the GeOx layer.
According to XPS quantifications (given in Table 2), wet processes (with or without rinse)
inevitably leave a non stabilized sub-oxides–covered germanium surface which gradually oxidizes,
tending toward the GeO2 stoechiometry without reaching it.
As stable and passivated germanium surfaces cannot be achieved by standard wet cleaning
sequences, gas phase treatments, especially treatment including ozone, were investigated on the
Raider SP tool. Among the nine process chambers available on this cleaning equipment, the specific
configuration of the so-called “Spray” chamber was exploited. Initially developed for stripping
using the FluorozoneTM process [5], HF liquid solution and ozone vapor can be simultaneously
injected in the chamber. For specific germanium surface treatments, the liquid phase was
suppressed from the recipe and concentrated gaseous phase ozone was directly injected on
germanium wafers.
XPS spectra corresponding to ozone-treated germanium substrates are presented in Figure 5. We
can observe that the ozone-formed germanium oxide and the Ge fundamental peaks are separated
by a 3.5 eV binding energy gap. In other words, using an ozone gaseous phase treatment enables to
grow a stoechiometric GeO2 passivation layer. According to ellipsometry measurements, the GeO2
layer thickness can be estimated to about 20 Å. This value is corroborated by XPS data.
Varying the XPS scanning angle and so the scanned depth, we can distinguish the germanium states
respectively present near the oxide surface and near the germanium substrate interface. For this
study, the Ge 2p line is more adapted than the Ge 3d one. Indeed, thanks to a lower photoelectrons
kinetic energy (Ec = 269 eV compared with 1457 eV for Ge 3d), the Ge 2p line is more sensitive to
the sample’s extreme surface. The XPS extracted data (Figure 6) highlight the presence of both
GeOx sub-oxides and GeO2 components. AR-XPS also shows that the GeO2 component is located
on the surface (GeO2 contribution systematically higher at 10°) whereas the germanium sub-oxides
are more localized near the germanium interface. To conclude, XPS analysis tend to prove that the
ozone-formed germanium oxide is in fact constituted by a Ge substrate/GeOx/GeO2 stack, in
accordance with what is well-known for silicon oxide.
The stability of the gaseous phase passivation was also checked. Ozone-treated germanium wafers
were stored in clean room during one month. Totally oxidized, this germanium oxide is stable after
one month ambient air exposure but, not surprisingly, very unstable in water (see Figure 7). Indeed,
upon water immersion, only GeO2 disappears confirming that the stoechiometric GeO2 is more
soluble than the GeOx sub-oxides [2].
Conclusion
Using an ozone gas-based process, we managed to passivate germanium surfaces with a stable,
smooth and stoechiometric GeO2 layer. Unlike “native” re-oxidation by air exposure following wet
cleaning, the proposed method for germanium passivation is well-controlled, reproducible,
industrially viable and compatible with the criteria of minimal germanium consumption.
References
[1] C. Chui et al., IEDM Dig. (2002), p 437-438.
[2] B. Onsia, Proceeding of the Int. Symp., UCPSS, Belgium (2000).
[3] J.M. Hartmann , J. Appl. Phys. 95, (2004), p 5905.
[4] F. Tardif, Proceeding of the Int. Symp., UCPSS, Belgium (1996), p 175.
[5]
10000
SC10.02/1/20
@65°C
EDI @ 60°C
Etch Rate [Å/min]
1000
EDI @ RT + O3 +
HCl
SC2 1/2/20 @
50°C
CA RO @ 110°C
100
10
1
0,1
0
20
40
60
80
100
[%Ge]
Figure 1: SiGe alloys (0< [%Ge] <100) etch rates in standard cleaning solutions.
Cleaning process
Ge etch rate
HF 0.5% @ RT
1.5 Å/min
DIW @ RT (+N2)
0.5 Å/min
O3 solution @ RT
70 Å/min
Table 1 : Germanium etch rates in diluted HF chemistry, DIW and Ozone solution.
GeOx
GeO2
Ge-Ge
Figure 2: XPS spectrum of a germanium surface after a DIW rinse.
Ge po st DIW rinse
Ge 3d - 10°
Ge po st HF
witho ut rinse (t0)
Ge po st HF
witho ut rinse (t1)
36
34
32
30
28
26
Binding energy (eV)
Figure 3: Compared XPS spectra (at grazing incidence) for a DIW rinsed Ge sample and two “HF- last” Ge surfaces
respectively immediately (t0) and one week post treatment (t1)
Ge0
%
Sample
Ge post DIW
i rinse
Ge post HF (t1)
Ge post HF (t0)
GeOx %
1+
2+
3+
20,7
1,8
2,7
7,1
GeOx
total
%
11,6
17,8
26,4
2,8
4,1
1,2
2,9
8,0
4,7
12,0
11,7
GeO2
%
4,0
-
1,6
Table 2: Atomic concentrations (%) of GeOx and GeO2 extracted from Ge 3d spectra (Analysis angle = 10°).
Comparison of Ge oxidations states between a rinsed only Ge and Ge surfaces just after (t0) or one week after (t1) HF
treatment (without rinse).
GeOx
Ge- Ge
GeO2
Figure 5: XPS spectrum of a germanium surface passivated by the ozone vapor process.
1,2
Relative intensities
GeO2
GeOx
Ge-Ge
1
0,8
0,6
0,4
0,2
0
2p (10°)
2p (35°)
3d (10°)
3d (35°)
Figure 6: Ge0, GeOx and GeO2 relative intensities for a ozone passivated Ge surface (scanning angles = 10° and 35°).
120
Atomic concentration (%)
Ge
O
C
100
80
60
40
20
0
P2-t0
P2-t1
P3-t0
P3-t1
Figure 7: XPS extracted concentrations respectively at t0 (immediately post treatment) and t1 (one month‘s wait).
P2 corresponds to a ozone passivated Ge surface and P3 to a ozone passivated Ge surface after DIW rinse.