Silicon nitride thin films in μc silicon Fab

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					Silicon nitride thin films in μc silicon                                                                                                       Fab &
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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

                                                                                                                                               Thin
  ABSTRACT
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  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
                                                                                                                                               Watch

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) [1], 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 [2]. 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
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                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       [7] 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 [8]. From
               films                                                                                              or negligible absorption.
                                                              Figure 1 it can be noted that the refractive
               It has both been calculated [6] and shown      index of an SiN layer is easily adjustable by
               experimentally [7] 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.


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               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 [10]. 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 [11].
               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

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               analyzed. The data shown in Figure 4                   3. They serve as a diffusion barrier to           [7] 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.                               [8] 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                                                                                                              [9] 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
                                                                                                                        [10] 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.                              [11] 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
  Modules
                    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
                                                                      Acknowledgements                                  Bitterfeld-Wolfen
                   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: j.frenck@q-cells.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
                                                                      NRRA measurements.
               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             [1] Meiners, L. G. 1981, Journal of
               functions.                                                 Vacuum Science & Technology, 19
                                                                          (3), 373.
               Summary
                                                                      [2] 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           [3] 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:                                              [4] 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           [5] 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              [6] Nagel, H. et al, 2nd World conference
                  ‘look and feel’ of the solar cell                       on photovoltaics, Vienna, 1998.

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