Abstract:
The invention relates to a method of pre-cleaning a semiconductor structure and to associated modular semiconductor process tools. The method includes the steps of: (i) providing a semiconductor structure having an exposed dielectric layer of an organic dielectric material, wherein the dielectric layer has one or more features formed therein which expose one or more electrically conductive structures to be pre-cleaned, in which the electrically conductive structures each include a metal layer, optionally with a barrier layer formed thereon, and the surface area of the exposed dielectric layer is greater than the surface area of the electrically conductive structures exposed by the dielectric layer; and (ii) pre-cleaning the semiconductor structure by performing an Ar/H2 sputter etch to remove material from the exposed electrically conductive structures and to remove organic dielectric material from the exposed dielectric layer.

Description:
BACKGROUND 
       [0001]    This invention relates to a method of pre-cleaning a semiconductor structure, and to associated modular semiconductor process tools. 
         [0002]    In the semiconductor industry, it is common practice to sputter etch metal contacts formed beneath an Inter Metal Dielectric (IMD) prior to depositing metal layers or other conductive layers such as titanium, titanium nitride, aluminium and copper in an integrated metallisation tool. The metal contact may be formed from aluminium or another metal. The purpose of the sputter etch is to remove native oxide from the aluminium or other metal surfaces, leaving an atomically clean surface for subsequent depositions. After the required amount of material has been removed from the metal surfaces, the plasma which is part of the sputter etch process is switched off, and the process chamber is evacuated to a pre-determined pressure to avoid contaminating other modules in the integrated process tool. The productivity of the pre-cleaning module is strongly influenced by the amount of time that it takes to recover a sufficiently low pressure in the module prior to removing the wafer from the module and moving it through the transport module into the deposition module. Typically, the sputter etch is an argon sputter etch process performed under high vacuum wherein the wafer surface is bombarded with Ar +  ions. 
         [0003]    In some application areas, for example with relatively thick IMDs (typically greater than several microns), an organic dielectric material such as polyimide (PI) is chosen on performance and cost grounds. A non limiting example of such an application area is in Under Bump Metallisation (UBM). 
         [0004]    U.S. Pat. No. 6,107,192 discloses a pre-cleaning process prior to metallisation using a variety of plasmas. The plasma pre-cleaning can include chemical reduction of native CuO without sputtering of the Cu sub layer. The IMDs disclosed in U.S. Pat. No. 6,107,192 are low k inorganic materials such as SiO 2 , SiOF, or low k CDOs (carbon doped oxides). The present invention relates to different processes using organic IMDs wherein physical removal of material is performed. U.S. Pat. No. 4,357,203 discloses the formation of multi layer metallisation systems wherein vias are formed in a polyimide dielectric insulating layer overlaying a layer of aluminium. The vias are formed through an oxygen plasma etch of the polyimide which results in a residual film (not a native oxide) being formed on the exposed, underlying layer of aluminium. The residual film is subsequently removed by a second plasma etching step. The process disclosed in U.S. Pat. No. 4,357,203 uses a second patterned layer of aluminium to form a mask prior to etching of the vias, and the aluminium mask is in place when the second plasma etch is performed. This process is incompatible with the processes addressed by the present invention. In contrast, the present invention has application to modern, commercial semiconductor fabrication technologies wherein prior to a metallisation step, the semiconductor structure is provided having a metal layer below a patterned IMD layer, but with no further layer on top of the IMD layer. It is desirable to provide a pre-cleaning step in order to pre-clean the exposed metal layer. As noted above, it is well-known to perform an Ar sputter etch to achieve this pre-cleaning. However, the present inventors have realised that the Ar sputter process has relatively low selectivity to the materials on the semiconductor wafer surface. Also, the present inventors have realised that the surface area of metal exposed to the pre-clean is in practice considerably less than the surface area of the IMD exposed to the pre-clean, with a result that the majority of material removed from the surface of the semiconductor structure is in fact removed from the surface of the IMD. Furthermore, the present inventors have realised that the sputtered IMD material is potentially a significant source of contamination of the pre-clean process chamber, and also the rest of the process tool should volatile gases exit the pre-cleaning module. Therefore, it is desirable to minimise the amount of unwanted contaminants in the pre-cleaning module following an Ar sputter etch process. The present inventors have further realised that the production of contaminants can be a particular problem when organic dielectric materials such as polyimide are used owing to the nature of the contaminant produced after Ar sputter etching. Examples of problematic contaminants are CO and CO 2 . 
         [0005]    In summary, the present inventors have realised that a specific, but commercially important, combination of process conditions give rise to the specific problems stated above. Accordingly, the present inventors have devised the present invention which, at least in some of its embodiments, addresses the above described problems. 
       SUMMARY 
       [0006]    According to a first aspect of the invention there is provided a method of pre-cleaning a semiconductor structure including the steps of: 
         [0007]    i) providing a semiconductor structure having an exposed dielectric layer of an organic dielectric material, wherein the dielectric layer has one or more features formed therein which expose one or more electrically conductive structures to be pre-cleaned, in which the electrically conductive structures each include a metal layer, optionally with a barrier layer formed thereon, and the surface area of the exposed dielectric layer is greater than the surface area of the electrically conductive structures exposed by the dielectric layer; and 
         [0008]    ii) pre-cleaning the semiconductor structure by performing an Ar/H 2  sputter etch to remove material from the exposed electrically conductive structures and to remove organic dielectric material from the exposed dielectric layer wherein step ii) is performed using Ar and H 2  present in a partial pressure ratio Ar:H 2  of 1.0:1 or less, more preferably less than 0.5:1 and most preferably 0.4:1 or less. 
         [0009]    In this way, it is possible to reduce the amount of unwanted contaminants produced during the sputter etch and reduce the pumping time required to achieve a desired pressure, typically corresponding to a high vacuum. A further advantage is that contamination of the semiconductor structure, in particular the metal layer, is reduced. 
         [0010]    The skilled reader will appreciate that CDOs are not ‘organic dielectric materials’ if the bulk oxide is not itself an organic material, ie, doping with carbon does not render an inorganic material organic. 
         [0011]    Contaminants such as CO and CO 2  may be reduced as a result of practising the present invention. 
         [0012]    The term ‘metal’ is understood to include alloys within its scope. For the avoidance of doubt, the term ‘alloy’ can encompass combinations of metals, as well as metals having an amount of a non-metal additive. For example, aluminium-silicon alloys are within the scope of the invention. Silicon is typically present as a minor component, for example in the range 0.5-2.5 At %. 
         [0013]    Step ii) may be performed using Ar and H 2  present in a partial pressure ratio Ar:H 2  of 0.1:1 or greater. 
         [0014]    It is understood that all possible partial pressure ratio ranges between any two of the figures provided above are within the scope of the invention. For example, the invention includes within its scope Ar:H 2  partial pressure ratios in the range 1.0:1-0.1:1, less than 0.5:1-0.1:1, and 0.4:1-0.1:1. 
         [0015]    The organic dielectric material may contain carbon and oxygen. 
         [0016]    The organic dielectric material may be an organic polymer. Preferably the organic dielectric material is polyimide. 
         [0017]    The electrically conductive structures may each have a native oxide layer thereon, and step ii) may include pre-cleaning the semiconductor structure by removing native oxide from the exposed electrically conductive structures. 
         [0018]    The ratio of the surface area of the exposed dielectric layer to the surface area of the electrically conductive structures exposed by the dielectric layer may be greater than 25:1, and preferably it is greater than 50:1. In some embodiments, the ratio of the surface area of the exposed dielectric layer to the surface area of the electrically conductive structures exposed by the dielectric layer is greater still, and may be greater than 75:1, or greater than 100:1. 
         [0019]    Step ii) may be performed to remove organic dielectric material from the exposed dielectric layer to a depth of at least 10 nm. 
         [0020]    The dielectric layer of the organic dielectric material may have a thickness of at least one micron. 
         [0021]    The metal layer may be aluminium or copper. 
         [0022]    The electrically conductive structures may each consist of a metal layer. Alternatively, the electrically conductive structures may comprise a metal layer having a barrier layer formed thereon. The barrier layer may be a metal layer such as titanium, or a layer of a conductive compound such as TiN. The barrier layer may be present as an ARC (anti-reflective coating) layer, as is well understood by the skilled reader. 
         [0023]    The method of the invention may be performed in a pre-cleaning process chamber. The method may include the further step of: 
         [0024]    iii) evacuating the pre-cleaning process chamber to a pre-determined pressure or below. Preferably, the pre-determined pressure is 1×10 −6  Torr. The method may include the further step of: 
         [0025]    iv) transferring the semiconductor structure to a further process chamber so that a further process step can be performed, wherein the semiconductor structure is transferred after the pre-determined pressure in the pre-cleaning process chamber is achieved. 
         [0026]    The pre-cleaning performed in step ii) may produce CO, and the evacuation of the process chamber to a pre-determined pressure performed in step iii) may include obtaining a partial pressure of CO of 1×10 −7  Torr or less. 
         [0027]    The pre-cleaning performed in step ii) may produce CO 2 , and the evacuation of the process chamber to a pre-determined pressure performed in step iii) may include obtaining a partial pressure of CO 2  of 1×10 −7  Torr or less. 
         [0028]    Typically, the semiconductor structure includes a semiconductor wafer having the electrically conductive structures and the exposed dielectric layer formed thereon. 
         [0029]    According to a second aspect in the invention there is provided a modular semiconductor process tool including: 
         [0030]    one or more processing modules for providing a semiconductor structure having an exposed dielectric layer of an organic dielectric material, wherein the dielectric layer has one or more features formed therein which expose one or more electrically conductive structures to be pre-cleaned, in which the electrically conductive structures each include a metal layer, optionally with a barrier layer formed thereon, and wherein the surface area of the exposed dielectric layer is greater than the surface area of the electrically conductive structures exposed by the dielectric layer; 
         [0031]    a pre-cleaning process chamber; 
         [0032]    a further process chamber for performing a further process step on the semiconductor structure; 
         [0033]    means for transferring the semiconductor structure from a processing module to the pre-cleaning process chamber; and 
         [0034]    means for transferring the semiconductor structure from the pre-cleaning process chamber to the further process chamber after pre-cleaning; 
         [0035]    in which: 
         [0036]    the pre-cleaning process chamber includes sputter etching apparatus for performing an Ar/H 2  sputter etch using Ar and H 2  present in a partial pressure ratio Ar:H 2  of 1.0:1 or less, more preferably less than 0.5:1, and most preferably 0.4:1 or less to remove material from the electrically conductive structures and to remove organic dielectric material from the exposed dielectric layer. 
         [0037]    In some embodiments, the further process chamber is a metallisation tool for depositing one or more layers of a metal on the semiconductor structure. A metal layer or other conductive layer such as titanium, titanium nitride, aluminium or copper may be deposited using the metallisation tool. 
         [0038]    The means for transferring the semiconductor structure from the pre-cleaning process chamber to the further process chamber may be operable on the detection of a process condition associated with the pre-cleaning process chamber. The process condition may be the attainment of a pre-determined pressure in the pre-cleaning process chamber after pre-cleaning. Typically, the process tool includes a control system for detecting the process condition and controlling operation of the means for transferring the semiconductor structure from the pre-cleaning process chamber to the further process chamber. Typically, the control system monitors other conditions of the process tool and/or process parameters, and controls other operations of the process tool. Transport modules and transport equipment for transferring the semiconductor structure between modules of the process tool are well-known to the skilled reader. 
         [0039]    Whilst the invention has been described above, it extends to any inventive combination of the features set out above, or in the following description, drawings or claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]    Embodiments of methods and apparatus in accordance with the invention will now be described with reference to the accompanying drawings, in which: 
           [0041]      FIG. 1  is a cross-sectional view through a pre-clean module of the invention; 
           [0042]      FIG. 2  is a simplified, semi-schematic cross-section through a semiconductor structure to be pre-cleaned; 
           [0043]      FIG. 3  is a Residual Gas Analyser (RGA) trace of a PI/aluminium wafer etched using an Ar sputter etch pre-clean; 
           [0044]      FIG. 4  is a RGA trace of a PI/aluminium wafer etched using an Ar/H 2  sputter etch pre-clean of the invention; and 
           [0045]      FIG. 5  shows time to reach a partial CO pressure of 1×10 −7  Torr for an Ar sputter etch pre-clean and Ar/H 2  sputter etch pre-cleans of a wafer having aluminium and PI layers formed thereon. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0046]      FIG. 1  shows a pre-clean process tool  10  which can comprise a pre-clean module forming part of an integrated semiconductor processing tool (not shown) such as a metallisation tool. When the process tool  10  is provided as a pre-cleaning module in an integrated metallisation tool, the pressure tool  10  is used to clean a semiconductor wafer  12  prior to a metal deposition step by PVD. The process tool  10  comprises a chamber  14  having a gas inlet  16 , a wafer loading slot  18 , and a pumping port  20  for connection to a suitable pump, such as a turbo pump (not shown). The process tool  10  further comprises plasma production means for producing and sustaining a plasma within the process tool  10 . In the embodiment shown in  FIG. 1 , two RF sources are used to produce the plasma in chamber  14 . An upper portion of the chamber  14  has a ceramic wall  14   a  which can be formed from any suitable ceramic material such as Al 2 O 3 . Disposed around the ceramic wall  14   a  is a band or strap  22  which forms part of an ICP source. The ICP source and indeed the plasma production device in general is of a type well-known to the skilled reader, and therefore further elements of the ICP source are not shown in  FIG. 1 . An RF bias source  24  is applied to a platen assembly  26 . The platen assembly  26  comprises an upper face  26   a  on which the wafer  12  is mounted. In use, the RF bias source  24  applies an RF bias to the wafer  12  causing ions from the plasma to be accelerated onto the wafer surface. The RF bias can be of any convenient frequency, and typically 13.56 MHz is used. The platen can be heated in a manner which is well-known to the skilled reader. The chamber  14  is earthed. The platen assembly  26  is shown in both solid and dotted lines in  FIG. 1 . The solid lines denote the platen assembly in its retracted position adjacent to wafer loading slot  18 . In use, a wafer is introduced via the wafer loading slot  18  onto the surface  26   a  of the platen assembly. The platen assembly  26  is then raised to the position shown by dotted lines in  FIG. 1 . This is the configuration adopted for pre-cleaning by sputter etching which is achieved by forming a plasma  28  using the plasma production device. 
         [0047]      FIG. 2  shows in semi-schematic form a semiconductor structure, depicted generally at  30 , of a general type which can be pre-cleaned in accordance with the invention. For simplicity this schematic representation does not show the interconnect layers below the dielectric  38  and the transistors that are present on the surface of the substrate. The semiconductor structure  30  comprises a semiconductor substrate  32  having aluminium layers  34  formed thereon. A patterned PI layer  36  is formed on layers  38  of a dielectric material. The PI layer  36  has previously been patterned to leave a number of features  40  such as trenches or vias. The features  40  expose the underlying aluminium layers  34 . However, as a result of the processing conditions, the semiconductor structure  30  is supplied to the pre-cleaning module in a condition wherein aluminium oxide layers  42  are present over the portions of the aluminium layers  34  which are exposed by the features  38 . It is necessary to remove the aluminium oxide layers  42  by pre-cleaning, and the present invention enables this objective to be achieved. It is noted that the semiconductor structure  30  is provided for pre-cleaning with an exposed PI layer  36 , i.e., there are no further layers on top of the PI layer  36 . In practice, the topography of the semiconductor structure  30  is such that the PI layer  36  forms the majority of the surface area of the semiconductor device which is exposed to the sputter etch during pre-cleaning. It is noted that the layers  38  of a dielectric material may also be formed from PI, but this is not necessarily the case, and the layers  38  may consist of a different dielectric material. 
         [0048]    In accordance with the invention, pre-cleaning was performed using a sputter etch process which employs a mixture of argon and hydrogen as process gases. For comparison purposes, and to demonstrate the advantages of the present invention, experiments were also performed using a pure argon sputter etch pre-clean. 
         [0049]    A Residual Gas Analyser (RGA) was attached to the process chamber  14  to monitor the gases present during and after various pre-cleaning sputter etches. PI/aluminium semiconductor wafers of the type generally shown in  FIG. 2  were subjected to various pre-cleaning steps, and the results are shown in  FIGS. 3 to 5 . In both  FIG. 3  and  FIG. 5 , the traces corresponding to CO, CO 2 , CH 3 , OH and H 2 O are denoted by reference numerals  50 ,  52 ,  54 ,  56 , and  58 , respectively.  FIG. 3  shows a typical graphical output from the RGA following an Ar ion pre-clean step of a conventional type. It has been found using RGA analysis that during ion bombardment of PI IMD films, fragments of different mass are ejected from the PI surface. This can be seen in  FIG. 3 , which shows partial pressures of various mass fragments as a function of time during and after the pre-clean step. The dominant fragments observed are CO and CO 2 , along with other gas phase hydrocarbon molecules. The levels of these species can be monitored using RGA. CO is a particular problem because it can take a long time to pump away from the chamber, and it is a very reactive species which can promote facile oxidation of a cleaned aluminium or other metal surface to provide a metal oxide layer. This is extremely undesirable, because the purpose of the pre-clean step is to remove metal oxides from the upper surface of the metal layer. The existence of a metal oxide layer is extremely undesirable when the objective is to produce metal to metal contacts to the metal layer, because oxidation of the interface leads to high contact resistance and poor metal to metal adhesion. It is believed that the poor contact resistance which is often observed by prior art processes is the result of residual contaminants, particularly CO, generated by sputtering of PI, which land on the aluminium contacts. More generally, gaseous phase species produced by sputter etching of the PI film can react with the wafer surface, be pumped away, absorb onto the chamber walls, or reside in the chamber as contamination. After sputter etching, the PI surface is also known to be more volatile since the protective ‘skin’ on the surface is broken, allowing further and on-going contamination of the chamber. It can be seen in  FIG. 3  that undesirable contaminants such as CO and CO 2  are readily produced following pre-cleaning using a conventional Ar sputter etch, and that these species can take a long time to pump away from the chamber. In the pre-clean resulting in the trace shown in  FIG. 3 , material was removed from the wafer to a depth of 200 Å (Angstroms). 
         [0050]    The present invention physically removes native oxide from metal contacts on semiconductor wafers using an organic dielectric material such as PI as an IMD. The present invention employs an Ar/H 2  sputter etch to remove material from the surface of the semiconductor wafer. It has been found that this can reduce the levels of organic contamination within the chamber, and provide productivity benefits, since residence time in the process module can be reduced.  FIG. 4  shows an RGA trace for an Ar/H 2  plasma sputter etch of a PI and aluminium coated wafer. The typical but non-limiting process conditions for this pre-clean step are Ar flow rate 18 sccm, hydrogen flow rate 1-100 sccm, ICP coil power 500 W, ICP coil frequency 13.56 MHz, platen bias power 1200 W, platen bias frequency 13.56 MHz. Material was removed to a depth of about 200 Å. It can be seen that the use of the argon/hydrogen plasma reduces the levels of CO contamination considerably when compared to the sputter etch using argon only. It can be seen that within 12 seconds, the partial CO pressure drops to 1×10 −7  Torr, and this is a substantial improvement over the results shown in  FIG. 3 . 
         [0051]      FIG. 5  shows the time taken to achieve a partial pressure of CO of 1×10 −7  Torr after sputter etch pre-cleaning with argon only, and with various mixtures of argon and hydrogen. The numerals provided directly adjacent the data points in  FIG. 5  are the times in seconds to achieve this partial pressure of CO. The numerical values given in parenthesis against data points corresponding to mixtures of argon and hydrogen are the partial pressure ratios of argon to hydrogen. It can be seen from  FIG. 5  that using an argon/hydrogen plasma to perform sputter etch pre-cleaning causes the partial pressure of CO to reach the low level of 1×10 −7  Torr much more quickly than when no hydrogen is present. Consequently, there is a reduced chance of contamination occurring at the sputter cleaned aluminium surface. Furthermore, the productivity of a processing tool which comprises a pre-clean module which utilises the argon/hydrogen sputter etch is improved since the high vacuum conditions necessary before removal of the semiconductor wafer from the pre-clean module can be effected are achieved more quickly. It is noted that this recovery process to achieve a desired high vacuum condition such as the partial pressure of CO of 1×10 −7  Torr is substantially improved by increasing the relative contribution of hydrogen in the pre-clean process. Note that the data points in  FIG. 5  at 0, 50 and 100 sccm of hydrogen all relate to a process in which 18 sccm of argon is used, whereas the 8:5 argon/hydrogen partial pressure ratio data point corresponds to an argon flow of 50 sccm. 
         [0052]    Without wishing to be limited by any particular theory or conjecture, a possible explanation for the CO partial pressure traces is a chemical reaction involving CO and H 2  which can take place in the gas phase in the presence of heat from the plasma: 
         [0000]      CO+2H 2 →CH 3 OH
 
         [0053]    It is then probable that this molecule will be broken up rapidly into both CH 3  (mass 15) and OH (mass 17) fragments by the plasma. This theory agrees well with the RGA trace shown in  FIG. 4 . The rate of recovery for CO (i.e., the CO removal rate) is far greater than it is for CH 3  and OH when the argon/hydrogen pre-cleaning is used. In contrast, as can be seen in  FIG. 3 , with the argon only pre-clean, the opposite is true, and CO levels take far longer to decrease than CH 3  and OH. This suggests that the argon/hydrogen plasma is promoting the formation of CH 3  and OH by using up more CO, thereby removing CO which would otherwise be present as residual contamination. By reacting with CO, hydrogen is helping to reduce the contamination of the metal surface by producing species that are less reactive and can be pumped rapidly. In that regard, we note that  FIG. 4  shows the partial pressure of CH 3  is very low and rapidly achieves a level of 1×10 −7  Torr or lower. OH tracks the curve for H 2 O (mass 18), and takes some additional time to pump away. This can be explained by the presence of an additional reaction, namely the hydrogenation of CO 2 . This again agrees with the RGA data shown in  FIGS. 3 and 4 , since the CO 2  peak is reduced when an argon/hydrogen plasma is used. The reaction could be as follows: 
         [0000]      CO 2 +4H 2 →CH 4 +2H 2 O
 
         [0054]    This reaction could explain why levels of mass 16 (CH 4 ) and mass 18 (H 2 O) are higher when an argon/hydrogen plasma is used in comparison to an argon only plasma. It is noted that higher levels of H 2 O are not considered to be a concern, as H 2 O is far less reactive and easier to pump (for example with a cryo trap) than CO. Therefore, H 2 O is considered to be a much less concerning contaminant than CO. These mechanisms are provided as conjecture only. The experimental results presented herein may be explained by other mechanisms, or the explanation for the results may be a combination of the mechanisms discussed and other mechanisms. For example, it is possible that the presence of ionised hydrogen is resulting in a reaction with CO and/or a reaction with the PI surface itself to lower the levels of CO produced. 
         [0055]    The invention is not limited to the specific examples provided above, and the skilled reader will appreciate that many variations are possible. For example, instead of using PI, it is possible to use other organic dielectric materials. The invention provides the teaching that advantageous results can be achieved using an argon/hydrogen sputter etch pre-cleaning. In general, lower partial pressure ratios are preferred when the partial pressure ratio is expressed as the ratio of the partial pressure of argon to the partial pressure of hydrogen. However, the invention is not limited in this regard. By using the invention, it is possible to effectively pre-clean metal surfaces of native oxides whilst lowering the levels of gaseous contaminant species, in particular carbonaceous contaminants such as CO and CO 2 . This improves the vacuum environment for semiconductor wafers when compared to the industry standard argon only sputter etch pre-cleaning methods. Furthermore, it leads to productivity gains when processing semiconductor wafers in integrated process tools because base pressures can be achieved more quickly.