Methods and apparatus for treating a semiconductor substrate

A liquid short-chain polymer of the general formula R.sub.a Si(OH).sub.b or (R).sub.a SiH.sub.b (OH).sub.c is deposited on a substrate, where a+b=4 or a+b+c=4, respectively, a, b and c are integers, R is a carbon-containing group and a silicon to carbon bond is indicated by Fourier Transfer Infrared analysis. The short-chain polymer is then subjected to further polymerization to form an amorphous structure of the general formula (R.sub.x SiO.sub.y).sub.n, where x and y are integers, x+y=4, x.noteq.0, n equals 1 to .infin., R is a carbon-containing group and a silicon to carbon bond is indicated by Fourier Transfer Infrared analysis.

BACKGROUND OF THE INVENTION
 This invention relates to methods and apparatus for treating a
 semiconductor substrate, such as a semi-conductor wafer, and, in
 particular, but not exclusively, to methods and apparatus for providing a
 low dielectric constant (known as low k) layer in a planarisation or gap
 filling operation.
 In our earlier co-pending patent application WO94/01885, the contents of
 which are incorporated herein by reference, we describe a planarisation
 technique in which a liquid short-chain polymer is formed on a
 semiconductor wafer by reacting silane (SiH.sub.4) with hydrogen peroxide
 (H.sub.2 O.sub.2). The polymer, which initially is in a liquid state, is
 formed on to a wafer to produce planarisation either locally or globally,
 or gap filling. This technique provides a planarisation or gap filling
 layer of silicon dioxide and we have found it to be a most suitable
 material for semiconductor circuit manufacturing.
 SUMMARY OF THE INVENTION
 However, with the ever increasing demands to enhance device speed and
 reduce size, there can be problems in more advanced devices with using
 silicon dioxide as the dielectric insulator between metal lines. The RC
 time constant associated with the metal lines (or interconnects) on an
 integrated circuit structure limits the device speed and is a function of
 the resistance of the interconnections, the thickness of the insulator and
 its dielectric constant.
 Thus in order to reduce the RC time constant and enhance device speed, the
 options are to modify the characteristics of the interconnect, or the
 insulator. There are many device design constraints, and practicalities
 which restrict the designer's freedom and thus we believe it is extremely
 important to reduce the dielectric constant of the insulator, whilst
 trying to retain the other desirable properties which make silicon dioxide
 a suitable material.
 For advanced semiconductor devices, dielectric constant values of &lt;3.5 are
 required and ideally are &lt;3.0. We have found that it is possible to
 provide a dielectric layer which substantially retains the desirable
 properties of silicon dioxide but which has a significantly reduced
 dielectric constant, thereby making it suitable for use in advanced logic
 devices.
 We have also found that the dielectric constant can be reduced by applying
 a particular set of process conditions.
 Accordingly, in one aspect of this invention, there is provided a method of
 treating a semiconductor substrate, comprising forming on the substrate a
 liquid short-chain polymer of the general formula R.sub.a Si(OH).sub.b or
 R.sub.a SiH.sub.b (OH).sub.c
 where a+b=4 or a+b+c=4 respectively; a, b and c are integers, R is a
 carbon-containing group and Si--C bonding is inferred.
 The reference to the polymer being `liquid` is simply intended to indicate
 that it is neither gaseous nor solidified at the moment of formation.
 Preferably R is a methyl, ethyl, phenyl or vinyl group, with methyl
 (CH.sub.3 --) being particularly preferred.
 The further polymerisation may be enhanced by heating. It is thought that
 the liquid short chain polymer undergoes further polymerisation reactions
 to form an amorphous structure of the general formula
EQU --(R.sub.x Si O.sub.y)n--where x+y=4
 x and y are integers
 R is a carbon-containing group
 n=1 to .infin.
 Si--C bonding is inferred
 In another aspect of this invention there is provided a method of treating
 a semiconductor substrate, which comprises positioning the substrate in a
 chamber;
 introducing into the chamber in the gaseous or vapour state an organosilane
 containing compound with the general formula C.sub.x H.sub.y --Si.sub.n
 H.sub.a, and a further compound, containing peroxide bonding, and
 reacting the organosilane compound with said further compound to provide on
 said substrate a short-chain polymer.
 According to this invention a liquid short-chain polymer layer is formed on
 the substrate, the polymer being carbon doped to reduce the dielectric
 constant of the formed layer. The layer is formed by reacting a silicon
 containing compound with a compound containing peroxide bonding, and the
 dopant material may be bound to or otherwise associated with one of the
 reactants, preferably to the silicon containing gas.
 The term peroxide bonding includes hydroperoxide bonds such as O--OH.
 Preferably said silicon-containing compound is of the general formula
 R--SiH.sub.3 ; R may be a methyl, ethyl, phenyl or vinyl group with methyl
 (CH.sub.3 --) being particularly preferred. Si--C bonding is inferred.
 Preferably said silicon-containing compound and said further compound may
 react in a surface reaction on the surface of the substrate. Further
 polymerisation of the polymer may take place to form an amorphous
 structure of the general Formula
EQU --(R.sub.x SiO.sub.y).sub.n
 with the constraints set out above. Further polymerisation may be enhanced
 by radiative or chemical treatment e.g. by heating.
 Preferably the dielectric constant, measured at 1 MHz, of said deposited
 material is less than 3.5 and more preferably less than 3.
 The deposition rates may be enhanced by use of a weakly ionized plasma
 within the process chamber. However, this may be at the expense of Si--C
 bonding and thus the resultant dielectric constant of the deposited layer
 may be higher than if a plasma had not been used, but it will still be
 usefully lower than an un-doped silicon dioxide layer.
 Thus, with some silicon-containing precursors, the use of a plasma enhances
 the deposition rate without significant detriment to the planarity of the
 deposited polymer.
 The method may further comprise forming or depositing an under layer or
 base layer prior to the deposition of the polymer layer. The base layer is
 preferably deposited using a Chemical Vapour Deposition (CVD) or Plasma
 Enhanced Chemical Vapour Deposition process (PECVD) before the depositing
 of the polymer layer. The PECVD or CVD process is preferably carried out
 in a separate chamber to that in which the polymer layer is deposited, but
 can be carried out in the same chamber. The under layer may be a doped or
 un-doped silicon dioxide or other silicon containing layer.
 The method may further comprise depositing or forming a capping layer on
 the surface of the formed layer. This layer is preferably applied in a
 PECVD process.
 Preferably said PECVD or CVD capping process is applied in a chamber
 separate to that in which the polymer layer is formed. The capping layer
 may be a doped or un-doped silicon dioxide or other silicon containing
 layer.
 Preferably the PECVD or CVD chamber comprises a platen for supporting the
 substrate which is maintained at a temperature in the range of from
 100.degree. C.-450.degree. C., and more preferably around 350.degree. C.
 The method may further comprise chemical or radiative treatment (e.g.
 heating) of the polymer layer and this heating preferably takes place
 after capping, as the cap provides mechanical stability for the polymer
 layer during cross-linking. The polymer layer may be heated to 350.degree.
 C.-470.degree. C. for 10 to 60 minutes. For example the heating may last
 30 minutes at 400.degree. C. The heating may be achieved using a furnace,
 heat lamps, a hot plate, or plasma heating. The heat treatment step
 removes excess water from the layer, which is a by-product of the
 cross-linking reaction. It may also remove SiOH bonds.
 In another aspect, the invention provides a method of treating a
 semiconductor substrate, which comprises positioning the substrate into a
 chamber, introducing into the chamber in the gaseous or vapour state an
 organosilane compound of the general formula (C.sub.x H.sub.y).sub.z
 Si.sub.n H.sub.a, and a further compound containing peroxide bonding and
 reacting the silicon-containing compound with said further compound.
 In another aspect, this invention provides apparatus for implementing the
 method as described above which comprises a CVD chamber and PECVD chamber,
 said CVD chamber having means for introducing therein two or more reaction
 gases or vapours, platen means for supporting a semiconductor substrate,
 and means for maintaining the temperature of the platen at a required
 level, said PECVD or CVD chamber including platen means for supporting a
 semiconductor substrate, means for introducing one or more reaction gases
 or vapours, together with means for generating a plasma if required, to
 cause the gaseous vapours to react.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Apparatus for treating semiconductor substrates, such as semi-conductor
 wafers is schematically illustrated at 10 in FIG. 1. It will be understood
 that only the features which are particularly required for the
 understanding of the invention are described and illustrated. The general
 construction of such apparatus is well known in the art.
 Thus, the apparatus 10 includes a chemical vapour deposition (CVD) chamber
 11 having a duplex shower head 12 and a substrate support 13. The shower
 head 12 is connected to RF source 14 to form one electrode, whilst the
 support 13 may be earthed and may form another electrode. (The RF source
 14 is provided to allow etch back of the chamber and chamber furniture for
 cleaning purposes and/or to provide a weakly ionised plasma during
 deposition). The shower head 12 is connected by respective pipes 15 and 16
 to a source of CH.sub.3 SiH.sub.3 in N.sub.2 or other inert carrier and a
 source of H.sub.2 O.sub.2. The carrier gas is conveniently used for ease
 of operation of the equipment; it is believed that the process could be
 performed without it. The source of H.sub.2 O.sub.2 comprises a reservoir
 17, an outlet pipe 18, a pump 19 and a flash heater 20 for vaporising the
 H.sub.2 O.sub.2.
 In use the CVD chamber may be operated to form a short chain, inorganic
 polymer, which is initially a liquid, on the surface of a semi-conductor
 wafer to produce planarisation either locally or globally, or for `gap
 filling`. The polymer is formed by introducing into the chamber the methyl
 silane and the hydrogen peroxide in vapour form and reacting them either
 in a gaseous reaction or at the wafer surface spontaneously. Once the
 resultant polymer is formed on the wafer, it has been found that the rate
 of polymerisation is such that the condensate remains a liquid long enough
 to allow the polymer to flow. As a consequence the layer fills both small
 and large geometries or gaps. As the film grows thicker; surface tension
 tends to cause the film to self planerise. It is believed that effectively
 this process takes place as the polymerisation takes place. The more
 settlement which occurs prior to full polymerisation the less likelihood
 there is of cracking. Very small dimensioned gaps can be filled and
 because of the fill layer properties these gaps can even, in certain
 circumstances, be re-entrant.
 It has further been found that providing a weakly ionized plasma in the
 chamber enhances deposition rates, without being significantly detrimental
 to the properties of the layer. Thus, with some silicon containing
 precursors, the use of a plasma enhances the deposition rate without
 significant detriment to the planarity of the deposited polymer.
 The apparatus 10 also includes a Plasma Enhanced Chemical Vapour Deposition
 (PECVD) chamber 24 of generally conventional construction, comprising a
 shower head 25 and a wafer support 26. The shower head 25 is connected to
 RF source 27 to form one electrode, whilst the support 26 is earthed
 either directly on through a variable resistance and forms another
 electrode. Alternatively the shower head 25 may be earthed and the support
 may be driven. The shower head 25 is connected by respective pipes 28 and
 29 to a source of silane (SiH.sub.4) in N.sub.2 or other inert carrier and
 a source of N.sub.2 O
 In use, the PECVD chamber may be operated to deposit a base layer or under
 layer on a semiconductor wafer or other semiconductor substrate prior to
 deposition of the doped polymer layer discussed above. Likewise, after
 deposition of the polymer layer in the CVD chamber 11, the semiconductor
 wafer may be returned to the PECVD chamber for plasma deposition of a
 capping layer. Both the under layer or base layer and the capping layer
 have a similar chemistry of silicon dioxide.
 A pump 22 is provided for reducing chamber pressure.
 EXAMPLE
 A wafer is loaded into the machine and transferred to the PECVD chamber. A
 1000 .ANG. base layer of silicon dioxide is deposited at a temperature of
 350.degree. C. (The base layer could be between 100 .ANG. and 3000 .ANG.
 thick). Whilst still in the PECVD chamber, the wafer may be subjected to
 pre-treatment with a plasma, for example using a gas such as N.sub.2 O,
 O.sub.2 or N.sub.2 O. The wafer is then transferred to the CVD chamber 11
 where the polymer layer is formed at a platen temperature of 0.degree. C.,
 to a thickness of 8000 .ANG.. The pressure in the CVD chamber 11 during
 formation of the polymer layer is typically around 850 mT. For good
 quality films and to reduce the dielectric constant, it is desirable to
 remove as much water and OH from the film at an early stage. The layer is
 therefore exposed to a reduced pressure (typical 1-2 mT) for a period of
 thirty seconds.
 The wafer is then transferred to the PECVD chamber and a capping layer of
 1000 .ANG.-6000 .ANG. is deposited at a temperature of 350.degree. C. The
 wafer is then unloaded from the machine and furnace heat treated at a
 temperature of 400.degree. C. for thirty minutes to remove the residual
 moisture and OH from the film, the inclusion of which would cause the
 dielectric constant to be higher.
 Where the PECVD and CVD process are to be carried out in the same chamber,
 a wafer loading device 21 can be used to lift the wafer to an intermediate
 position 23 during heating of the wafer, to avoid unnecessary heating of
 the support 13.
 EXPERIMENT
 Analysis of the basic chemistry involved suggested that the Si--H bonds in
 the SiH.sub.3 component of CH.sub.3 --SiH.sub.3 would react with H.sub.2
 O.sub.2 in a similar manner to the Si--H bonds in SiH.sub.4, leaving the
 Si--CH.sub.3 bond intact. The resultant film was therefore expected to
 contain a basic SiO.sub.2 structure with a CH.sub.3 group attached to each
 silicon atom. An example of such a structure is shown in FIG. 7.
 Initial observations confirmed that the doped polymer layer was indeed
 formed using the new process. The resultant film was then evaluated to
 confirm that Si--CH.sub.3 was present in the film.
 In order to confirm that Si--CH.sub.3 and C--H bonds were present in the
 layer, we subjected the layer to Fourier Transform Infra-red (FTIR)
 analysis and the results are shown in FIG. 2, which indicate that both the
 C--H and Si--C bonds are present.
 It is known that the refractive index of a material is related to the
 dielectric constant. Refractive index measurements confirmed a lower
 refractive index was obtained for the doped polymer layer following thirty
 minute furnace heat treating in nitrogen ambient. As discussed above, the
 heat treating removes residual moisture from the layer. In FIG. 3 the
 results are plotted, for increasing heating temperature, for a polymer
 layer of this invention "DOPED", for a silane/hydrogen peroxide layer as
 described in WO94/01885 "ET2" and a hybrid of these two layers "50%/50%".
 A marked decrease in refractive index was observed for the doped polymer
 layer in comparison with the layer according to WO94/01885. The hybrid
 (50%/50%) layer showed a smaller reduction in refractive index which was
 consistent given the smaller proportion of methyl silane in the source
 gas.
 Scanning Electron Microscope (SEM) observation confirmed that the polymer
 layer exhibited good flow properties, as seen in the SEM views given in
 FIG. 4.
 Dielectric constant measurements were taken over an average of five wafers
 produced in accordance with the above example, with 25 capacitors per
 wafer. The wafers were furnace heat treated prior to measurement at a
 temperature of 400.degree. C. in nitrogen for thirty minutes. The doped
 polymer layer averaged a dielectric constant of 3.24 at 1 MHz.
 An important property of any doped oxide layer is its temperature
 stability. Theory suggests that the Si--C bond should be generally stable
 to temperatures up to about 400.degree. C., and we had predicted that the
 same would be true for Si--C bonds within the doped polymer layer. To
 confirm the temperature stability of the doped polymer layer, a wafer was
 furnace heat treated in nitrogen ambient at sequentially higher
 temperatures whilst monitoring both the Si--C and C--H peaks using FTIR.
 FIG. 5 shows a plot of the integrated Si--C and C--H peak areas versus
 heating temperature. The curves indicate that the Si--C and C--H bonds are
 stable within the hardened doped polymer layer up to temperatures of at
 least 400.degree. C. after which the areas of the peaks and hence the
 number of bonds are seen to reduce.
 As has been indicated above the methyl silane may be substituted by ethyl,
 phenyl, vinyl silane or other organic silane and sources of peroxide
 bonding other than hydrogen peroxide may be used. Further precusors could
 be employed that provide an Si--C bond that was maintained from the
 gaseous phase adjacent to the wafer to the resultant hardened polymeric
 layer upon the semiconductor substrate.
 When forming films from the reaction of hydrogen peroxide with hydrocarbon
 and silicon containing gases and in particular methyl e.g. methyl silane
 or dimethyl silane containing gases it has been discovered that a lower
 density film is formed than films formed under similar conditions by the
 reaction of silane and hydrogen peroxide. Some of these reactions will
 take place at suitable temperatures and pressures without any additional
 energy input to yield useful semiconductor dielectric layers. Other source
 materials may require energy input in the form of RF or microwave plasma
 or thermal energy. Thus, an organosilane of general formula (C.sub.x
 H.sub.y).sub.z Si.sub.n H.sub.a can be used in the invention, where x, y,
 z, n, and a are any suitable integers.
 The absence of a plasma or heating may usefully slow or modify the thin
 film formation process to enable flowing liquid intermediate states that
 have gap filling characteristics.
 Where gap filling is not required a wider choice of source materials and
 process conditions is possible to include those materials or process
 conditions requiring a plasma or heating to be used.
 Typically a film formed from the reaction of silane and hydrogen peroxide
 results in a film of density 2.2 g/cc when fully hardened. Films have been
 formed from the reaction of methyl silane and hydrogen peroxide with
 densities of 1.39 g/cc and 1.53 g/cc and other similar values. These lower
 density films have also been noted to have lower k values and the k values
 correlate linearly to the density as shown in FIG. 8.
 Low k values are particularly useful for the production of insulators in
 semiconductor devices. By reducing the k value of the dielectric film the
 RC time delay in metal conductors is reduced without increasing line
 spacing thus allowing faster transmission of data along the conductors.
 It has been discovered that the carbon and in particular the hydrocarbon in
 the source material is also contained within the deposited film. The FTIR
 spectra (FIG. 2) of a fully formed film deposited from methyl silane and
 hydrogen peroxide shows a characteristic peak associated with
 silicon-carbon bonds as well as the presence of carbon-hydrogen bonding.
 It has also been discovered that there is a close correlation between
 carbon content and k value. The higher the carbon content (over a useful
 range) the lower the k value. The correlation between carbon content and k
 (dielectric constant) is shown in FIG. 9. This chart was plotted using
 Rutherford BackScattering (RBS) to measure carbon content. So far attempts
 to obtain useable semiconductor dielectric layers with more than
 approximately 12 at. % of carbon from a spontaneous reaction of methyl
 silane and hydrogen peroxide have been unsuccessful. Problems have
 included "haze" in the film and whilst the film might be electrically and
 mechanically useful its visual appearance is such that such a film would
 probably be rejected by potential users.
 It is now supposed that the empirical formula of the completely formed thin
 film is predominately CH.sub.3 SiO.sub.3/2 where the methyl (CH.sub.3) is
 contained within the Si--O by carbon bonding to silicon. There will
 however be some residual Si--H bonds.
 What is not as yet fully understood is why this correlation between carbon
 content and k value occurs. What is now supposed is that methyl (CH.sub.3)
 in the source material remains attached in some way to silicon and is
 contained within the fully hardened film in such a way as to suppress
 further cross linking within the film during its formation and/or
 solidification. Thus the presence of methyl suppresses a regularly ordered
 silicon dioxide polymeric film perhaps forming a cyclorandom polymeric
 structure. This disordering of the silicon dioxide polymer results in
 lower density films and thus a low k value. A diagrammatic representation
 of the fully hardened polymer film is given in FIG. 10 where
 R=predominately CH.sub.3 with some H. There may also be ladder like
 structures present in the fully hardened polymeric film again with further
 crosslinking terminated by methyl.
 Evidence for this supposition is in density vs. peroxide flow correlation
 that is evident in FIG. 11. With a fixed flow rate of 87 sccm of methyl
 silane various different peroxide flows were experimented with. Thus the
 ratio of methyl in the total source material was varied. By weighing and
 measuring the volume of a semiconductor wafer before and after deposition
 the density of the finally hardened deposited film was calculated. It was
 found that as the carbon containing methyl proportion of the source
 material was increased the density of the fully formed deposited film
 decreased and the k value also decreased.
 It is thought that the fully formed material by this process has voids
 substantially only at a molecular level and is not characterised as
 nanoporous in the sense of an aerogel or xerogel structure where the
 completed film is typically half the density of a fully formed film (see
 FIG. 12 appears as FIG. 1 in Nanoporous Silica for low k dielectrics,
 Teresa Ramos et al Mat. Res. Soc. Symp, Proc. Vol 443 1997). Thus the
 process is not characterised by a gelation where there is formation of a
 gel intermediate. Microscopy of the fully formed film by this invention
 supports this view as the material appears to be a dense polymeric film;
 the reduction in density coming from the disordered linking of molecules
 due to lattice disruption caused by the hydrocarbon presence in the film.
 These low k films had the following desirable characteristics; stable up to
 480.degree. C., low outgassing, low moisture uptake, capable of self
 planarisation, chemical and thermal stability and a resistance to cracking
 when annealed at 450.degree. C. for 30 minutes.
 Thus a film of a density of between 2.2 g/cc and 1 g/cc and more
 particularly between 1.5 g/cc and 1 g/cc may be produced by the reaction
 of a methyl containing silicon containing gas with a peroxide containing
 vapour resulting in a k value of less than 3.0 and more particularly
 approximately 2.5 to 3.0.