Abstract:
A method is disclosed for removing a polysilicon layer from a semiconductor wafer, in which a downstream plasma source is used first to planarize the wafer, removing contours in the polysilicon layer caused by deposition over lithographic features, such as via holes. The planarizing process is followed by exposure to a plasma made by a direct, radio frequency plasma source, which may be in combination with the downstream plasma source, to perform the bulk etching of the polysilicon. The invention can produce planar surface topography after the top layer of the film is removed, in which the residual recess height of the polysilicon plug filling a via hole is less than about about 10 nm.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a process for making integrated circuit structures. More particularly, this invention relates to processes for removing deposited layers from an integrated circuit structure. 
   BACKGROUND OF THE INVENTION 
   During various integrated circuit (IC) fabrication steps, numerous layers are deposited onto lower layers which may have previously defined lithographic features such as trenches and holes formed therein. These deposited films often show a conforming surface to the topography of the underlying trenches and holes, as the deposited layers fill in these features. Therefore, a non-uniform surface with various dimple areas and depths will form as a result of the trench and hole features. A non-uniform film surface can interfere with subsequent IC fabrication steps such as photolithography, film depositions and film removals, and can cause yield loss from non-functional devices. Planarization of non-uniform surfaces is therefore a critical step in IC manufacturing. 
   For example, it is often desired to form an electrical connection to a conductive layer lying underneath an insulating dielectric layer. These connections are made by lithographically patterning a set of holes, or vias, through the dielectric layer to the conducting layer, and depositing a conductive material, for example polysilicon, in the holes. The excess conductive material can then be removed from the surface of the dielectric layer by an etch back process, leaving the conductive material only in the vias, and not on the surrounding surface. 
   It is an ongoing challenge for etch back processes to remove the conductive material thoroughly, while leaving a smooth, planar surface, with little or no surface topography in the vicinity of the vias. A figure of merit for an etch back process is the reduction of the recess height, which is the height differential between the planar surface and the top of the polysilicon plug filling the vias. In general, a recess height of less than 30 nm is specified for IC processes. 
   Achieving a planar surface after deposition is therefore a primary goal of etch back processes, processes used to remove a portion of a previously deposited material. Traditionally, a substantially planar surface is achieved by chemical mechanical polishing (CMP) tools or a high density plasma etcher. CMP is an abrasive process, in which material is removed by applying a rotating pad under pressure against the substrate, in the presence of an abrasive compound or slurry. The slurry is chosen to selectively remove one film over another. While CMP enjoys widespread use in the IC industry, it is expensive, and requires frequent operator maintenance to replace the pads or handle the slurry. The process is also slow, requiring long process times, which increases the cost of ownership of the tool. Lastly, most CMP slurry removes the polysilicon material preferentially to the oxide, and therefore leaves recesses at the sites of the polysilicon plugs. CMP typically produces wafers that have a recess height in excess of about 30 nm. 
   Alternatively, an ion etcher can be used to remove a top layer of polysilicon. However the ion etchers also tend to leave topography over the dimple areas, as the charged species are attracted to the corners of the topography because of the high fields existing there. Therefore, typical plasma etchers tend to accentuate already existing contours, and are not very effective at planarizing contoured areas remaining near lithographically patterned features. 
   The development of a high throughput advanced thin film removal or etch back process is desirable. A method for the rapid removal of the bulk polysilicon film, which leaves a flat surface topography and very little recess height, is also desirable. 
   SUMMARY 
   The present invention is directed to a process approach whereby a film is planarized using a downstream plasma source, before bulk film removal using a dual, RF and downstream plasma source. The process approach is demonstrated on the etch back of a polysilicon film. The plasma gas comprises O 2  and CF 4  from a downstream plasma source, which serves to planarize the surface of the polysilicon, by etching the film over the flat areas at a faster rate than the film over the contoured areas. Next, the bulk removal of the film is accomplished by exposing the substrate to a direct, radio frequency (RF) plasma in combination with a downstream plasma. The process is capable of removing about 1.2 μm/minute of material. 
   The invention makes use of a downstream plasma source, which produces a high density of radicals (for example, greater than about 5e13 cm −3 ) which interact chemically with the substrate surface. The distinguishing feature of a downstream plasma source is that the plasma gases are made to cross a neutralizing barrier, which neutralizes most of the ionized species in the gas through recombination at the neutralizing barrier. Therefore, the plasma generated from a downstream plasma source is rich in radicals that can react chemically with a substrate surface, while the density of ionized species is relatively low, for example, less than about 5e12 cm −3 . 
   The speed of the chemical reaction depends on the geometry of the features on the substrate, with the result that the speed of the reaction is faster on flat surfaces than on contoured surfaces. Therefore, as the substrate is exposed to the plasma, it tends to become planarized by interaction with the downstream plasma, as the flat areas are etched down until the contours are erased from the surface. After the substrate surface has been planarized, the substrate is exposed to a direct, RF plasma in combination with a downstream plasma, which performs most of the bulk film removal. 
   This invention achieves lower recess height than typical plasma etchers, because it uses a downstream source which generates little or no ionic population. Furthermore, since it is a dry plasma process, there is relatively little maintenance required, compared to, for example, CMP. The invention can also be applied to films other than polysilicon, for example, tungsten, copper, or silicon oxide. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified process flow for the application of this invention. 
       FIG. 2  shows a cross section of a portion of an integrated circuit device to which an embodiment of this invention may be applied. 
       FIG. 3  is a simplified schematic diagram of a downstream plasma tool, equipped with a microwave plasma source for the downstream plasma. 
       FIG. 4  shows a cross section of a portion of the integrated circuit device of  FIG. 1 , after planarizing the substrate according to an aspect of the invention. 
       FIG. 5  shows a cross section of a portion of an integrated circuit device, after removing the bulk polysilicon from the substrate, according to an aspect of the invention. 
       FIG. 6  shows an atomic force microscope image of the portion of the integrated circuit device shown in  FIG. 5 . 
       FIG. 7  is a simplified schematic diagram of a downstream plasma tool, equipped with an inductively coupled plasma source for the downstream plasma. 
   

   DETAILED DESCRIPTION 
   The novel process is one in which the surface of the substrate is first exposed to a downstream plasma, which serves to reduce the topography on the surface. After planarization, the substrate can be exposed to the normal etching processes, generally including a RF plasma source. By planarizing the film first, the invention avoids the problem wherein the film removal process preserves or enhances existing topography on the substrate surface. 
     FIG. 1  illustrates the novel process flow. According to the process flow, the wafer is first exposed  170  to a downstream plasma, to planarize the surface and reduce recess height. The downstream plasma source has the distinguishing feature that the plasma gases are required to cross a neutralizing barrier, which neutralizes through recombination, most of the ionized species in the gas. It has been found that among the effects of a downstream plasma on a substrate, is the general flattening of the topography of the surface. It is hypothesized that the flattening occurs because the reactive species of the downstream plasma are electrically neutral, and isotropic. They therefore interact with the surface at a rate determined, in part, by their local concentration in the plasma gas. Areas which are confined, such as dimples and trenches, may deplete the concentration of radical reactive species locally, slowing their reaction rate locally. This may slow the reaction rate of the surface in the confined areas. Therefore, the radical-rich gas produced by the downstream plasma may react more quickly with the exposed, flat areas than with the dimpled areas, with the result that more polysilicon may be removed from the flat areas than from the dimple areas by the downstream plasma components. Eventually, the fast-etching flat surface areas will reach an even level with the slow-etching dimple area, planarizing  170  the whole polysilicon surface and leaving it with good uniformity for the removal  180  of the remaining polysilicon layer. 
   The term planarizing  170 , as used here, refers to the approximate leveling of topographic features with respect to a reference surface. The topographic features are the dimples formed over the vias, and the reference surface is the surface of the dielectric layer surrounding the vias. The leveling of the topography should be such that the angle between a point of interest and a reference point on the reference surface is less than about 5 degrees. The remaining topography should have a roughness of less than or equal to about 30 nm with respect to the reference surface. The removal  180  of the polysilicon layer is achieved when about 90% or more of the material has been removed from the underlying dielectric layer, leaving polysilicon only in the vias. 
   After the film has been planarized by exposure to the downstream plasma, the bulk of the film is removed by exposing  180  the substrate to an anisotropic, direct, radio frequency (RF) plasma in combination with a downstream plasma. The RF or direct plasma is responsible for most of the anisotropic etching of the polysilicon by ions accelerated in the plasma. The ions are accelerated towards the wafer surface at directions substantially perpendicular to the plane of the wafer. Consequently, the ions impinge on the inclined surfaces less frequently than on the horizontal surfaces that are parallel to the wafer plane, and when they do strike the inclined surfaces, they do so at off-normal incidence angles. Charged ions bombard the horizontal surfaces with full intensity at or near normal incidence. The interactions between the substrate surface and the RF plasma products are therefore mainly ballistic, whereas in the case of the downstream source, they are mainly chemical. The RF plasma is primarily responsible for the rapid removal of the bulk of the polysilicon layer, because of the field-induced acceleration of the ions, as well as the plasma chemistry selectivity of the polysilicon over the oxide. 
   A third procedure  190  can be included in the process, which procedure involves the exposure of the substrate to another direct RF plasma and/or a downstream plasma, to further reduce the recess height for the polysilicon plug. The third procedure makes use of a process environment in which the selectivity of the etching plasma to the dielectric compared to the polysilicon, is reversed. Therefore the dielectric is removed at a faster rate than the polysilicon. This reverses any topography generated by the etch process, and reduces the remaining recess height of the polysilicon plug. 
     FIG. 2  shows a portion of a generic integrated circuit device, upon which the invention may be practiced. The figure shows a silicon wafer  110  upon which a dielectric material  120  has been deposited. In this integrated circuit device, the dielectric material  120  is borophosphosilicate glass (BPSG), but the dielectric material can be any oxide. Vias  130  have been etched into the dielectric material  120  to allow electrical contact to a metallic underlayer such as a conducting gate (not shown). Polysilicon  140  has been deposited over the surface and into the via holes. The deposition of the polysilicon into the vias leaves a surface that conforms, to some extent, to the underlying layer, so that dimples or recessed areas  150  in the surface remain, registered over the polysilicon plugs in the via holes. It is desired to remove the top layer of the polysilicon  140 , leaving the polysilicon plugs flush with the planar surface of the dielectric layer, without creating a recess over the plugs in the via holes. 
   Various dual source plasma systems can be used in connection with the process described in  FIG. 1 . Two such products, bearing the trademarks GAMMA™ and PEP IRIDIA™, are commercially available from Novellus, Inc. of San Jose, Calif. Both tools have the advantageous feature of having dual sources, a direct RF plasma source and a downstream plasma source operatively connected to the reactor chamber. These products are convenient for the application of this invention, as the different process conditions can be applied within the same tool. 
   As a first embodiment, the invention as described is implemented on a PEP IRIDIA dual source plasma system. A schematic diagram of the plasma system is shown in  FIG. 3 . Reactor chamber  330  contains the substrate  350  supported on a platen  340 . The platen  340  is supported on a pedestal  370  which holds the substrate beneath the inlet via  380  from the downstream source  390 . The platen is coupled to a heat source (not shown) to provide heat for the substrate. The heat source can be a radiative source such as a lamp, or a hot plate, for example. The platen  340  is electrically connected to the RF source  360  via the pedestal  370  which supplies radio frequency power to the platen. 
   A plasma gas is supplied to the chamber through a gas inlet, by vacuum fittings (not shown). The oscillating fields applied by the RF source  360  to platen  340  accelerate ions in the gas, causing them to collide with neutral species, ionizing the neutral species in turn, until a plasma is formed above the substrate  350  on the platen  340 . Vacuum plumbing (not shown) is applied to the reactor chamber to allow the evacuation of the chamber to the required levels. The RF power supply is capable of delivering up to about 3000 W per wafer to the plasma. The chamber may be designed to accommodate multiple numbers of wafers, for example six, which may be processed simultaneously. In the case of multiple wafers, two or more wafers may share a downstream source. 
   The downstream source, which in this example is a microwave downstream (MWD) source  390 , ionizes a gas in a separate chamber operatively connected to the reactor chamber  330  which contains the platen  340  and substrate  350 . Alternatively, other downstream plasma sources may be used, such as an inductively coupled plasma (ICP) source. The MWD source can deliver up to about 3000 W per wafer to the downstream plasma. A microwave generator produces oscillating currents in the ions and electrons of the plasma gas. These ions and electrons transfer energy to other atoms in the gas by collisions to create a microwave plasma. The plasma is initiated by applying, for example, a UV light source  310  or other source of ionizing radiation, to the aperture of the MWD source. The UV light source  310  causes initial ionization of the gas, and the ions and electrons are then accelerated by the oscillating field. 
   The plasma gases are routed downstream under vacuum flow, and cross a neutralizing barrier  320  at the entrance inlet via  380  to the reactor chamber  330 . The neutralizing barrier  320  is a metallic grid which presents a large surface area to the incoming gas. Collisions between the ionized gas species and the neutralizing barrier neutralize the ions, through recombination, such that the flow into the reactor chamber comprises mainly neutral species and reactive radicals. 
   In this first exemplary embodiment, the first plasma gas is composed of oxygen in combination with a fluorine-containing gas and optionally, a diluent gas. The diluent gas can be argon, nitrogen, hydrogen, forming gas (96% nitrogen and 4% hydrogen), helium, or a combination thereof. The amount of diluent gas is chosen to be sufficient to maintain a stable plasma, and generally the proportion of diluent gas in the plasma is less than about 20% of the total gas volume. 
   The fluorine-containing gas can be SF 6 , CHF 3 , CH 2 F 2 , F 2 , CF 4 , or NF 3 . In this embodiment, the fluorine-containing gas is chosen to be CF 4 . The CF 4  gas is highly reactive, and reacts with the surface layer of the polysilicon. The concentration of the fluorine gas is chosen to provide an adequate etch rate, and is generally in the range of about 5–40% of the total gas volume, or preferably in the range of about 9–11%. Higher concentrations of the fluorine-containing gas may lead to excessive formation of fluoropolymer residues, which can interfere with the etching of the polysilicon material. Typical process parameters for the planarizing process may be about 1 Torr total chamber pressure, about 720 sccm (standard cubic centimeters per minute) O 2  and about 80 sccm CF 4 . The flow rates quoted are per wafer. 
   The power applied to the downstream microwave source may be up to about 3000 W per wafer, and is preferably about 1500 W to about 1700 W per wafer. The substrate temperature is maintained at between about 20° C. and about 100° C., and preferably about 40° C. to about 90° C., and more preferably about 70° C. The total process time for the first process is about 20–120 seconds, preferably about 50–70 seconds, and more preferably about 60 seconds. 
   The planarizing process  170  involves the downstream plasma only. A flow rate of about 500 sccm to about 3000 sccm is sufficient for maintaining about 500 mTorr to about 3 Torr of gas pressure in the downstream plasma source. The total flow rate for the first gas in this embodiment is about 800 sccm per wafer, and the pressure is maintained at about 1 Torr. 
     FIG. 4  shows the device of  FIG. 2 , alter de application of the downstream plasma under the conditions described above. After about 60 seconds, the surface shows that the dimple topography which was located over the via holes in  FIG. 2 , has been removed, and the remaining polysilicon has a planar surface. After the film has been planarized, the substrate is ready for the bulk removal of the remaining polysilicon. 
   The bulk removal  180  of the polysilicon film is accomplished by the RF directional plasma in combination with the downstream microwave plasma. The RF directional plasma enhances the polysilicon removal rate. The plasma gases may again be a combination of O 2  and a fluorine-containing gas. The fluorine-containing gas may be one selected from the group consisting of SF 6 , CHF 3 , F 2 , CF 4  and NF 3 . In this embodiment, the fluorine gas is chosen to be CF 4 . In some embodiments, the oxygen gas is mixed with a diluent gas which may be argon, nitrogen, hydrogen, forming gas, helium, or a combination thereof. The total amount of diluent gas is in an amount sufficient to provide a stable plasma, and the proportion of diluent gas to the total gas pressure may be in general less than about 20%. The proportion of CF 4  to the total gas volume in the removal process is somewhat higher than in the planarizing process, about 5%–50%, for example about 40%, and can be selected to adjust the etch rate of the polysilicon. The total gas flow supplied to the chamber can be in the range of about 300 sccm to about 3000 sccm. However, typical process parameters in this exemplary embodiment, for the polysilicon removal process are about 700 mTorr total chamber pressure, and about 300 sccm O 2  and about 180 sccm CF 4 . The total power to the microwave source is about 500 to about 1500 W per wafer, and preferably about 800 W per wafer, and to the RF source is about 50 W to about 600 W per wafer, and preferably about 450 W per wafer. The substrate temperature is maintained at about 20° C. to about 100° C., and preferably about 70° C. until the endpoint of the process is reached. Using the conditions described here, a polysilicon removal rate of about 1.2 μm/min is achieved. 
   The RF plasma source is responsible for the uniformity of the process, and for generating detectable signals for the endpoint detection system. The endpoint system comprises a detector for detecting emissions from the by-products of the etching process. A sudden appearance or disappearance of an emission signal from a species indicates that the end of the process has arrived. The endpoint detection system in this embodiment detects photons emitted in the recombination of ions and electrons from the SiF 4  by-product of the polysilicon etching. The total process time required for the second process  180  is generally about 30–60 seconds. 
     FIG. 5  shows the devices of  FIGS. 2 and 4 , following the bulk polysilicon removal  180 . The image in the figure was taken after exposure of the film to the direct plasma and microwave plasma for about 35 seconds, under the conditions described above. As can be seen in the figure, the polysilicon layer has been removed and the residual polysilicon recess area  150  is below the resolution limit of the imaging system. 
   In order to gain sufficient resolution to measure the residual polysilicon recess height, the devices shown in  FIG. 5  were imaged using an atomic force microscope (AFM). The resulting image is shown in  FIG. 6 . The AFM image shows that the recessed areas  150  of the polysilicon plugs in the dielectric vias is on the order of about 7 nm, well below the specification target of about 30 nm. 
   The optional procedure  190  can be included in the process flow of  FIG. 1 , which is designed to further improve recess height control, to achieve a recess height of less than about 10 nm. The procedure involves using the RF plasma source with or without the downstream plasma source for higher selectivity to the dielectric underlayer. This procedure is used to remove the last traces of the polysilicon layer, and it is designed to etch preferentially the dielectric material. Therefore, after application of this procedure, any recess height differential between the polysilicon plugs and the planar surface may be reversed, leaving a topographically flat surface. The duration of this optional procedure is generally quite short, between about 5 and 10 seconds, and preferably about 5 seconds. Typical process parameters for the optional procedure are about 500 sccm per wafer of O 2  and about 120 sccm of CF 4 . Using this process, the recess height for the polysilicon plug may be less than about 10 nm. The use of the optional procedure can also allow the process to over-etch the polysilicon target layer, and avoid the use of an endpoint detection system. 
   Other downstream plasma sources can be used to practice this invention, for example an inductively coupled plasma (ICP) source. Such a tool is shown schematically in  FIG. 7 . The tool is functionally similar to that shown in  FIG. 3 , except that the downstream plasma source  260  is a chamber  270  encircled by an induction coil  280  of wire. The coil  280  carries a RF current, which induces a current in the gas in the interior of the chamber. The induced current causes collisions between the ionized species in the gas, and the neutral species in the gas, until a plasma is formed of predominantly ionized species. The ionized plasma gas is then forced to pass by a neutralizing barrier  240 , which neutralizes most of the ionized species through recombination. The plasma gas is then introduced into the reactor chamber  210  through the inlet via  250 , and applied to the exposed substrate surface  220 . The plasma gas interacts with the exposed surface of the substrate  220 , held on a platen  230 . The platen is connected to a RF power source  290 , which causes an additional RF plasma to form above the surface of the substrate  220 . The process that can be applied to practice the invention using the tool shown in  FIG. 7 , is the same as that shown in  FIG. 1 , and produces similar results. 
   The invention described herein can be used to generate flat, planar surfaces which are preferred for further IC processing. The invention as described can yield surfaces with a residual recess height of less than about 10 nm, well below the specification target of about 30 nm residual recess height. The process can be implemented on tools designed to provide a downstream plasma in addition to a direct, RF plasma. The downstream plasma provides most of the planarization effect, and the direct RF plasma performs the bulk etching of the film from the surface. 
   While this invention has been described in the context of the etching of a polysilicon film, it is clear that the invention can be applied to other materials having undesirable surface topography, such as for example, tungsten, SiO 2 , or copper. 
   The invention is therefore not limited to the embodiments described above. For instance, the invention is not limited to the particular materials or process parameters, layer thicknesses, or other details. Likewise, the invention is not limited to the particular shape of the vias or their positioning relative to each other, and the invention is not limited to particular tool designs. Process conditions can be altered, and additional steps added, depending on the nature of the polysilicon films and the structure of film stacks. Various aspects and features of the present invention have been explained or described in relation to beliefs or theories, although it will be understood that the invention is not bound to any particular belief or theory. Other embodiments and variations within the scope of the invention, as defined by the appended claims, will occur to practitioners in view of the disclosure herein.