Silicon nitride thin films in μc silicon Fab &
solar cell production Materials
Hubert-Joachim Frenck, Q-Cells AG, Bitterfeld-Wolfen, Germany Cell
This article first appeared in Photovoltaics International journal’s first edition in August 2008. Processing
Ever since the introduction of attractive feed-in tariffs for photovoltaic electricity generation, there has been a huge
surge in all kinds of photovoltaic applications. Products based on multicrystalline wafers still have the largest market PV
share with thin-film products picking up in recent times. In the course of this process, production technology for wafer- Modules
based solar cells has been improved. With the second generation of tools, a trend towards standardization is apparent.
Deposition of silicon nitride is one of the key processes of solar cell production. While its technological significance is Power
often underestimated, it is the only process step that serves a multitude of purposes. In this contribution we will present Generation
aspects of the deposition of silicon nitride thin films and discuss open questions with respect to the physics of the
deposition process and its implication on machine technology. Market
Introduction to be fulfilled at the same time often Silicon nitride has been investigated
The deposition of thin films is a key causes difficulties in assessing the effect intensively for a long time for a variety of
technology for a large variety of technical of a single process parameter, let alone purposes like passivation of InP (indium
and scientific applications. Among them the task of optimizing the SiN film in all phosphide) , while its application to
is the deposition of silicon nitride (SiNx) required aspects at the same time. The MIS solar cells has been known for quite
to passivate the surface of silicon solar aforementioned technical features of some time . It is therefore beyond
cells. The SiN film serves several purposes. the SiN film provide the very property the scope of this paper to discuss all
It is a broadband anti-reflection layer, it that largely determines the aesthetically aspects of various features of the SiN
serves to saturate dangling bonds and/or pleasing appearance of a cell, and hence a deposition, nor is it the goal of this
other surface states of the silicon, and last PV module, as the colour of the module article to discuss the machinery used
but not least, it is a protection layer to is determined by the cell composition. In and its implications on film properties.
prevent alkali ions and other impurities order to complicate things further, there While to the best of our knowledge
from diffusing into the silicon causing are numerous deposition techniques being there is no comprehensive re v ie w
perturbations of the performance of the applied both on a scientific level as well as article in existence, major aspects of the
solar cell. This multitude of properties in production environments. process have been covered elsewhere
Figure 1. Calculated refractive index and absorption coefficient of a SiNx film as a function of incident wavelength. The effect of
a change in stoichiometry of the SiNx film is indicated with red and blue arrows, respectively.
Photovolt aic s Inter national 53
Figure 2. Relative efficiency and short circuit current as a function of SiN thickness. Data are presented in comparison with
calculations using PC1D.
[3,4,5]. In this contribution, we will efficiency of light conversion for the Si/ the gas ratio between the Si-containing
focus on assorted, globally measurable SiN/air stack is obtained at n633nm~1.9 at precursor (SiH 4 ) and the nitrogen-
bulk properties of the SiN film and their a thickness of d~85nm, assuming zero or containing precursor (N 2 or NH 3 ).
impact on solar cell performance. We negligible absorption. Currently, most ARC
will concentrate mainly on the effect coatings on solar cells are deposited using a
of a change in film thickness – at first slightly higher refractive index accounting The maximum efficiency
glance a very simple parameter that
appears to be easily controlled. Its effect
for the encapsulation in a module. In most of light conversion for the
cases, a refractive index of n 633nm~2.05
can easily be demonstrated; nevertheless  is used today. Considering the low Si/SiN/air stack is obtained
the underlying physics are a little more refractive indices of glass and EVA, the
complicated. at n633nm~1.9 at a thickness
optimum refractive index is even higher,
which would also be desirable from a point of d~85nm, assuming zero
Optical properties of thin SiNx of view of electrical passivation . From
films or negligible absorption.
Figure 1 it can be noted that the refractive
It has both been calculated  and shown index of an SiN layer is easily adjustable by
experimentally  that for a single layer changing the silicon content of the film, In Figure 1, the spectral behaviour of the
of antireflection coating, the maximum which in turn is achieved by changing absorption value k is also given. Films
F 3. Model showing the effect of hydrogen diffusing to the interface of silicon to SiN and into the bulk of the silicon substrate.
54 w w w. p v - te ch. o rg
Figure 4. Relative open circuit voltage as a function of SiN thickness.
with low Si content (i.e. low refractive Bearing these thoughts in mind, it interface between silicon and nitride,
index) can be correctly assumed to is clear that the main effect given in we performed NRRA measurements
exhibit zero or negligible absorption. On Figure 2 stems from increasingly perfect [9,10] to evaluate a hydrogen depth
increasing the silicon content in the SiN matching of the optical film thickness profile on different parts of a wafer. It
film, the absorption of the film is also to the optimum required to yield a was expected that the depth profile of
increased. broadband antireflection layer. Thus, the samples in question would reveal
Figure 2 shows the relative efficiency it is also clear that from a standpoint of differences in the depth distribution of
of solar cells and their respective short a cell manufacturer, utmost care needs the hydrogen. Interestingly, there was
circuit currents. The data given in to be taken to achieve and control good no clear correlation, a fact that was also
Figure 2 has been obtained by preparing reproduction of data on a day-to-day basis observed by Hofmann et al . This
solar cells using different metallization in both the homogeneity of deposition on result indicates that the bulk properties
procedures taking into account varying a cell as well as in run-to-run. Fortunately, of the passivating film may not correlate
sintering properties of the films yielding this matches with the need to produce as clearly to properties of the silicon
well-contacted cells. Care was also cells and modules with an aesthetically solar cells. On the other hand, this does
taken in comparing films with the same pleasing appearance. not rule out a contribution of bulk SiN
refractive indices. Both the relative properties on the passivation of silicon
efficiency and the I sc increase with surfaces. More investigation is needed
increasing SiN film thickness to reach a Thus, it is also clear that to clarify the role of the SiN layer in
maximum at the desired film thickness. passivating silicon solar cells.
The data obtained by experiments from a standpoint of a cell The quality of a surface passivation
correlate well to calculations using PC1D, is usually assessed by determining the
the solar cell modelling program.
manufacturer, utmost care emitter saturation current joe. The lower
In order to capture as many photons needs to be taken to achieve the j oe , the lower the recombination
from the solar spectrum as possible, it rate of electrons. Assuming that the
is necessary to coat the silicon substrate
and control good reproduction recombination of electrons is evenly
with a broadband antireflection layer. of data on a day-to-day basis distributed in the solar cell, so the
The basic data for this AR coating can recombination rate is by and large
be derived from first principles. For
in both the homogeneity determined by the surface properties.
economic reasons, it needs to be a single of deposition on a cell as The j oe gives a good estimation of the
layer, thus exhibiting a single, broad quality of a surface passivation, which
minimum. Since photovoltaic devices are well as in run-to-run. in turn has a direct impact on the open
essentially photon counting systems, it circuit voltage Voc. As it is more difficult
is desirable for the AR coating to have a to correctly determine joe than Voc, which
minimum in the spectral reflectivity at Silicon nitride is also supposed to be is routinely measured together as part
an optical thickness of approximately a source of hydrogen to saturate defects of the electrical characteristics of a solar
650nm. Once the refractive index of and micro fissures in the bulk of the cell, we took this value as associated with
the coating is known, the necessary silicon wafer. This process is schematically the quality of a surface passivation .
geometrical thickness can be calculated. shown in Figure 3. The silicon nitride This assumption is especially justified in
Assuming no absorption and a refractive deposited on the wafer typically contains experiments in which all other factors
index of n~2.05, it is concluded that a approximately 10-15 at. % hydrogen. It (e.g. sheet resistance) determining V oc
geometrical thickness of d~80nm should is shown by FT-IR that this hydrogen is are kept constant.
be deposited. The case is somewhat both bonded to the silicon as Si-H as well In an attempt to assess the influence
different in case of a layer exhibiting a as to the nitrogen as N-H. of the SiN thickness on the passivation
gradient of the refractive index with layer In order to confirm correlations quality of the interface between SiN and
thickness, although basic data can be between the bulk hydrogen content of Si, the Voc data from the same set already
derived in the same way. silicon nitride and the properties of the discussed in reference to Figure 2 was
56 w w w. p v - te ch. o rg
analyzed. The data shown in Figure 4 3. They serve as a diffusion barrier to  Schmidt, J. et al, 17th European PV
Fab & indicates that while a minimum thickness impurities like alkali ions and other Solar conference, Munich, 2001.
Facilities of around 25-35nm is required for surface ambient defects.  Schmidt, J. et al, 19th European PV
passivation, little or no changes in Voc are 4. The hydrogen in the SiN films is Solar conference, Paris, 2004.
noted with increasing film thickness as assumed to ser ve as a means of
Materials  Chow, R. et al 1982, Journal of Applied
required by the demands of optical light saturating dangling bonds at the surface Physics, 53 (8), 5630.
coupling as discussed previously. of the silicon, and also to passivate defect
 Hofmann, M. et al, 22nd European PV
Cell states stemming from microfissures
Processing Solar conference, Milan, 2007.
and other mechanical faults deep in the
substrate itself.  Britnar, B., Ph.D. thesis, Konstanz, July
While it appears to be 1998.
Thin 5. SiN films must allow for sufficient
Film necessary to supply a sintering through of the metallic pastes
minimum amount of to ensure a reasonable ohmic contact. About the Author
PV D r. H u b e r t - J o a c h i m
thickness passivation, We showed in this contribution that Frenck studied physics at
there is no clear-cut with all of the above properties, it is the University of Münster
vital for the overall quality of the solar a n d to o k th e ch a i r o f
Power correlation to solar cell cell that the geometrical and optical
Generation Prof. Kassing at Kassel
film thickness is controlled within strict
properties with increasing limits. Any deviation has direct impact
to conclude his Ph.D. on
‘Molecular engineering in PE-MOCVD
Market film thickness other than on the efficiency of light conversion of thin film deposition’ in 1988. The work
Watch the cell. While this may not be a problem
optical optimization. on laboratory scale, the sheer volume of
included the tailoring of silicon nitride
precursor molecules to best match the
production puts forward high demands needs of a PECVD process. He has been
on the achievable reproducibility of the working in thin-film technology ever
The data in Figure 4 also explains in deposition machines, a feature that is since, albeit in various specialist fields. In
a straightforward way the difference very often underestimated. It is the firm 1999 he joined Ikarus Solar, where he was
between the relative efficiency and the belief of the author that any potentially responsible for the division fabricating
I sc with decreasing film thickness as successful machine concept needs to solar selective films for thermal collectors
noted in Figure 2. Thus, we conclude take reproducibility into account as a key and for developing a range of solar
that while it appears to be necessary parameter. thermal products. He continued this
to supply a minimum amount of The data given in this article further work at Viessmann SA from 2005-07.
thickness passivation, there is no clear- affirms that while application of SiN in Dr. Frenck has been with Q-Cells since
cut correlation to solar cell properties μc silicon solar cell production is used Februar y 2007 and currently leads
with increasing film thickness other very successfully, a lot more work needs the vacuum division of the process
than optical optimization. Hence, the to be conducted to clarify the role of the development department. Dr. Frenck is
passivating role of hydrogen stemming Si/SiN interface, which to a large extent married and has two children.
from the bulk of the SiN film needs to determines the efficiency of the solar cell.
be investigated more closely. Moreover, In particular, the role of the hydrogen
there is no clear correlation of the from the SiN film and its effect on the Enquiries
amount of hydrogen in the SiN film Si surface remains to be investigated Hubert-Joachim Frenck
to the minority carrier lifetimes as in detail. Q-Cells AG
measured by μ-PCD. Our data suggests Sonnenallee 9-21
that the properties of D-06766
the SiN/Si interface with respect to
Any work relies on the contribution Germany
density of surface states’ impurities
of several co-workers. To this end, the
imposed by dangling bonds or crystal
author is extremely grateful for the Email: email@example.com
mismatch and their saturation by the
work of Martin Hanke and Carsten
passivating layer may play a more
Swiatkowski of Q-Cells, as well as Dieter
important role than is commonly
Grambole from FZ Rossendorf for the
assumed. We realize, however, that
much more and detailed work needs to
be done to clarify the physical principles
underlying the interface properties and References
their respective correlation to solar cell  Meiners, L. G. 1981, Journal of
functions. Vacuum Science & Technology, 19
 Hezel, R. & Schörner, R. 1981, Journal
S i l i co n n i t r i d e th i n f i l m s o f th e
of Applied Physics, 52 (4), 3076.
stoichiometry Si xNyHz (SiN) are widely
used in μc silicon solar cell production. As  Bragnolo, J. A. et al, 12th NREL
with most other thin-film applications in Workshop on crystalline solar cells,
any industry, the SiN films serve multiple August 2002.
purposes:  Dekkers, H. F. W. et al, Crystal Clear
1. They serve as an antireflection layer in Workshop, IMEC, November 2005.
order to increase light absorption in the  Duerinckx, F. & Szlufcik, J. 2002, Solar
wavelength range where a silicon solar Energy Materials and Solar Cells,
cell can convert light to electricity. 231-246.
2. The colour of the layer determines the  Nagel, H. et al, 2nd World conference
‘look and feel’ of the solar cell on photovoltaics, Vienna, 1998.
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