Method of treating a semiconductor wafer in a chamber using hydrogen peroxide and silicon containing gas or vapor

A semiconductor wafer is treated in a chamber by introducing into the chamber a silicon-containing gas or vapor and hydrogen peroxide in vapor form. The silicon-containing gas or vapor is reacted with the hydrogen peroxide to form a short chain, inorganic fluid polymer on the wafer, which thus forms a generally planar layer.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to a method for treating a semiconductor wafer and
 in particular, but not exclusively, to what is known as planarisation.
 2. Description of the Related Art
 It is common practice in the semi-conductor industry to lay down layers of
 insulating material between conducting layers in order to prevent short
 circuits. If a layer of insulating material is simply deposited in the
 normal way undulations begin to build up as the layers pass over the
 metallic conductors which they are designed to insulate. Various
 techniques have been developed to try to overcome this problem by filling
 the trenches or valleys between the conductors to a height above the top
 of the conductors so that after treatment a generally planar layer exists
 on the top of the wafer. One example of such a technique is to spin on
 layers of polyimide to smooth out the surfaces. However, in practice,
 narrow trenches tend to be incompletely filled whilst wide valleys are not
 fully leveled. As the 2-D dimensions of devices are reduced, these
 problems are accentuated.
 SUMMARY OF THE INVENTION
 One aspect the invention resides in a method of treating a semi-conductor
 wafer comprising, depositing a liquid short-chain polymer having the
 general formula Si.sub.x (OH).sub.y or Si.sub.x H.sub.y (OH).sub.z on the
 wafer to form a generally planar layer.
 The reference to the polymer being liquid is simply intended to indicate
 that it is neither gaseous nor solidified at the moment of deposition.
 Another aspect the invention resides in a method of treating a
 semi-conductor wafer in a chamber including, introducing into the chamber
 a silicon-containing gas or vapour and a compound, containing peroxide
 bonding, in vapour form, reacting the silicon-containing gas or vapour
 with the compound to form a short-chain polymers on the wafer to form a
 generally planar layer.
 The silicon-containing gas or vapour may be inorganic and preferably is
 silane or a higher silane, which may be introduced into the chamber with
 an inert carrier gas, for example nitrogen. The compound may be, for
 example, hydrogen peroxide or ethandiol.
 The method may further comprise removing water and/or OH from the layer.
 For example the layer may be exposed to a reduced pressure and/or exposed
 to a low power density plasma, which may heat the layer to 40 to
 120.degree. C.
 The method may further comprise forming or depositing an under layer prior
 to the deposition of the polymer. This under layer may be silicon dioxide
 and may have a thickness of between 1000 and 3000 .ANG.. It may for
 example be 2000 .ANG. thick. The under layer may conveniently be deposited
 by plasma enhanced chemically vapour deposition. Either the under layer
 and/or the wafer may be pretreated by, for example a plasma, to remove
 contaminants. In that case it may be pretreated with a plasma, for example
 using oxygen as a reactive gas.
 Similarly the surface of the deposited polymer layer may be treated in a
 plasma using a reactive oxygen gas in order to enhance chain lengthening
 and cross-linking within the polymer. This gas could be, for example,
 oxygen, nitrogen or hydrogen peroxide vapour and other gases may be
 appropriate. The plasma has a heating effect which enhances crosslinking,
 but there may also be a radiation effect from the various gases. This
 chain linking may alternatively be catalysed by exposing the polymer layer
 to UV light, x-rays or ion bombardment. However, in many applications
 acceleration of chain linking may not be desirable; instead it may be
 desirable for the polymer molecule particles to settle before significant
 chain linking occurs.
 The method may further comprise depositing or forming a capping layer on
 the surface of the deposited layer. This capping layer may be silicon
 dioxide. The capping layer is deposited after a proportion of the
 condensation reactions have occurred and water has been removed from the
 layer.
 The method may further comprise heating the polymer layer and this heating
 preferably takes place after capping. The polymer layer may be heated to
 between 180-220.degree. C. for between 50-70 minutes. For example it may
 be heated to 220.degree. C. for 60 minutes. The layer may subsequently be
 allowed to cool to an ambient temperature and then reheated to
 430-470.degree. C. for 30-50 minutes. For example the second heating may
 last 40 minutes at 450.degree. C. Indeed this second heating may suffice
 and may be achieved using a furnace, heat lamps, a hotplate or plasma
 heating.
 In one preferred arrangement the polymer layer may be heated to between
 200-450.degree. C., prior to capping, in order that the cap can be
 deposited at elevated temperatures. Although the capping layer could be
 deposited in one or more steps e.g. a `cold` capping layer deposited at
 the temperature of the planarising layer followed by a hot capping layer;
 the polymer layer having first been heated to 200-450.degree. C. as
 described above.
 The density of the hydrogen peroxide may be in the range of 1.20-1.35
 gms/cc and a density of 1.25 gms/cc may be particularly preferred. The
 hydrogen peroxide is preferably at 50% concentration when introduced into
 the chamber.
 The ambient temperature within the chamber may be within the range of
 0-80.degree. C. during the deposition of the polymer layer, but the wafer
 platten is preferably at 0.degree. C. or at the dew point of the polymer
 when in vapour form. Low pressure is also desirable but requires low
 temperatures (eg 400 mT, -10.degree. C.).
 In order to avoid heating the platten, the wafer is preferably lifted from
 the platten for each processing step which involves heating.
 The method can be used to achieve planarisation or gap filling. In the
 latter case the ambient chamber temperature may conveniently be even
 higher.
 The invention also includes wafers treated by any of the methods set out
 above and semi-conductor devices including polymer layers formed by the
 method above.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 An apparatus for treating 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 chamber 11 having a duplex shower head 12
 and a wafer support 13. The showerhead 12 is connected to RF source 14 to
 form one electrode, whilst the support 13 is earthed and forms another
 electrode. Alternatively the R.F. source 14 could be connected to the
 support 13 and the shower head 12 earthed. The shower head 12 is connected
 by respective pipes 15 and 16 to a source of SiH.sub.4 in N.sub.2 or other
 inert carrier and a source 16 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 16 comprises a reservoir 17 of H.sub.2 O.sub.2, an outlet pipe
 18, a pump 19 and a flash heater 20 for vaporising the H.sub.2 O.sub.2.
 In use the apparatus is arranged to deposit a short chain, inorganic
 polymer, which is initially a liquid, between the interconnects on a
 semi-conductor chip to produce planarisation either locally or globally,
 or for `gap filling`. The polymer is formed by introducing into the
 chamber the silane and the hydrogen peroxide in vapour form and reacting
 them within the chamber spontaneously. Once the resultant polymer is
 deposited on the wafer, it has been found that its viscosity is such that
 it fills both small and large geometries or gaps and is generally self
 levelling. It is believed that effectively there is a settlement process
 taking place as the polymerization takes place. The more settlement which
 occurs prior to full polymerization 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.
 As has been mentioned, if left, the chains within the polymer will slowly
 extend and cross link. In some circumstances it may be desirable to
 accelerate this process by plasma treatment. This treatment produces UV
 radiation and it is believed that it is this radiation which is
 responsible for increasing the speed of chain extension and cross linking.
 Other forms of radiation treatment may therefore be equally applicable. A
 variety of gases may be appropriate for use at this stage, for example any
 inert gas or hydrogen, nitrogen or oxygen containing gases.
 For good quality films it is desirable to remove as much water and OH from
 the film at an early stage. This can be done by exposing the layer to a
 reduced pressure causing the layer to pump water out and then subsequently
 heating the layer to between say 40.degree. C. and 120.degree. C. A pump
 22 is provided for reducing chamber pressure.
 However in order to solidify fully the polymer layer, it has been found
 that it is generally necessary to subject the layer to more intense heat
 treatment. In many instances it is necessary or desirable first to deposit
 a capping layer over the polymer. It is believed that this assists in
 providing mechanical stability for the polymer layer cross linking. It may
 also help to control the rate at which the layer looses water during
 heating and so have a controlling affect on shrinkage and cracking.
 A suitable capping layer would be silicon dioxide.
 The heat treatment stage after the capping involves removing excess water
 from the layer which is a by-product of the cross-linking reaction. The
 bake also removes SiOH bonds. The speed at which the water is removed may
 be important and several ways of removing water have been successful. One
 suitable sequence comprises baking the layer for 60 minutes at 200.degree.
 C., cooling it to ambient temperature and then rebaking it for 40 minutes
 at 450.degree. C. Microwave heating has also been successful. A simple
 bake at 450.degree. will often also suffice, or the bakes may be replaced
 by the following steps:
 1. 2000 .ANG. `cold` cap deposited at between 20-40.degree. C.
 2. Plasma heat treatment in N.sub.2 O which raises the temperature to
 300-400.degree. C.
 3. 4000-6000 .ANG. `hot` cap is deposited.
 Alternatively, in some cases, a single stage `hot cap` deposited at
 300-400.degree. C. will suffice.
 It has been found that the adhesion of the polymer layer to the underlying
 substrate material can be enhanced by depositing an under layer, for
 example of silicon dioxide. Typically this should be of the order of 2000
 .ANG. thickness and it may be laid down by plasma-enhanced chemical vapour
 deposition.
 Examples of actual deposited layers are illustrated in the photographs of
 FIGS. 2A and 2B. It will be seen that the upper surface of the layers 21
 are generally planar despite the huge magnification involved.
 Although SiH.sub.4 has proved to be particularly successful, it is believed
 that the method will be applicable with most silicon-containing gases or
 vapours. It has been found that to some extent a suitable polymer can be
 obtained with any concentration or density of H.sub.2 O.sub.2, but a
 density range 1.20-1.35 gms/cc has been particularly successful. The most
 preferred H.sub.2 O.sub.2 density is 1.25 gms/cc. An H.sub.2 O.sub.2
 concentration of 50% is very effective but it is believed that the
 preferred concentration may vary depending on whether the object is to
 achieve planarisation or gap filling. It is preferred that more H.sub.2
 O.sub.2 is supplied than SiH.sub.4 and it is particularly preferred that
 the H.sub.2 O.sub.2 :SiH.sub.4 ratio is of the order of 3:1.
 In the event that the wafer needs to be removed from the chamber between
 processing stages, it may be desirable to pre-treat the exposed surface,
 when the wafer is placed back in the chamber, in order to remove any
 organics or other contaminants from the exposed surface.
 FIGS. 3A to E illustrate the preferred processing sequence schematically.
 FIG. 3A shows formation of the underlayer 302 (adhesion enhancer) by PECVD
 at 300 Deg. C., with a probably chemistry of SiH.sub.4 +2N.sub.2
 O.fwdarw.SiO.sub.2.dwnarw.+2H.sub.2 +2N.sub.2. FIG. 3B shows formation of
 the planarising layer 304 (planarises features), with reference numeral
 306 denoting surface tension forces, by CVD at approx. 0 deg. C., with a
 probably chemistry of SiH.sub.4 +3H.sub.2
 O.sub.2.fwdarw.Si(OH).sub.4.dwnarw.+2H.sub.2 O+H.sub.2. FIG. 3C shows a
 treatment stage, i.e., a first post treatment (promotion of polymerisation
 and removal of water), by pumpout at approx. 0 deg. C., and pumpout at
 approx. 150 deg. C., with a probable chemistry of
 Si(OH).sub.4.fwdarw.SiO.sub.2.fwdarw.+2H.sub.2 O.uparw.. FIG. 3D shows
 formation of the capping layer 308 (provides mechanical stability during
 densification step) by PECVD at 300 deg. C., with a probably chemistry of
 SiH.sub.4 +2N.sub.2 O.fwdarw.SiO.sub.2.dwnarw.+2H.sub.2 +2N.sub.2. FIG. 3E
 shows a second treatment stage, i.e., a second post treatment
 (densification of film, where reference numeral 310 denotes shrinkage), by
 anneal at 450 deg. C. It may be advantageous to wash the chamber with
 H.sub.2 O.sub.2 between at least some of the processing stages.
 As it is desirable to keep the platten or support 13 at around 0.degree.
 C., the wafer may be lifted above the support 13 for each heating process
 so that the heat of the wafer is not significantly transmitted to the
 support 13. This can be achieved by arranging an intermediate position 23
 for a wafer loading device 21.