Abstract:
A method of sputtering with sputtering apparatus is for depositing a layer upon a substrate. The apparatus includes a sputter target with a face exposed to the substrate and a magnetron providing a magnetic field that moves relative to the target face. The speed of movement of the field is controlled such that the uniformity of the deposition on the substrate is enhanced. A particular method includes monitoring uniformity verses speed, selecting the speed that gives the preferred uniformity and controlling the field to the selected speed. The selected speed may vary over the life of the target, with increased speeds becoming desirable as the target thins.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     A claim to priority is made to U.S. Provisional Application Ser. No. 60/619712 filed Oct. 19 2004 and British Patent Application No. 0423032.2 filed Oct. 16 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of Invention  
         [0003]     This invention relates to a method of sputtering with a sputtering apparatus for depositing a layer upon a substrate having a sputter target with a face exposed to substrate(s) and a magnetron providing a magnetic field that moves relative to the target face.  
         [0004]     2. Description of Related Art  
         [0005]     In most deposition processes that take place, for example in the formation of films on semiconductor wafers, one of the key manufacturing criteria is that there should be uniformity of deposited film thickness across the wafer. In particular for acoustic resonators, film thickness affects resonant frequency and therefore for all resonators upon a wafer to resonate at the same frequency, film thickness must be the same.  
         [0006]     One of the key elements to achieve good uniformity of deposition upon a substrate from a closely spaced magnetron sputtering target assembly is the way in which the magnetic field moves with respect to the substrate (the alternative being large source-to-substrate distances approximating to a point source, but at the cost of low sputtering efficiency).  
         [0007]     This may be achieved by moving the substrates past a static magnetron/target assembly. More recently, and particularly as substrate sizes have increased, the preference has been for the substrate to remain static and the magnetic field to move such as by physical rotation of the magnetron (or by suitable electrical switching causing the field of a fixed coil array to sweep across the face of the target). It is also known to move the field-creating element (e.g. a rotating magnet) laterally over a target and also to change target to magnetron spacing to improve uniformity of materials deposition thickness through target life (substrate to substrate repeatability). Whilst good results have been achieved using these techniques, and other techniques developed by the Applicants, lack of uniformity has still proved a significant problem in reactive sputtering processes.  
         [0008]     It is important to recognise that it is the relative motion of magnetron to substrate that improves the deposition uniformity upon the substrate (by moving the point of erosion from the target—moving the source of sputtered material) not the motion of magnetron relative to the target—which achieves improved target consumption.  
         [0009]     In the case of single wafer close coupled magnetron sputtering (as is commonplace in metallisation upon 100 mm-300 mm silicon wafers) there is generally a target little larger than the wafer (to minimise chamber size and target materials inventory) close coupled (to maximise efficiency of target utilisation) with a moving magnetron sweeping a magnetic field to gain uniformity of film deposition upon the substrate (and uniformity of target erosion).  
       SUMMARY OF THE INVENTION  
       [0010]     From one aspect the invention consists in a method of sputtering with a sputtering apparatus having a sputter target and a magnetron for providing a field which moves relative to the target including controlling the speed of movement of the field such that the uniformity of deposition on the substrate is enhanced.  
         [0011]     As will be demonstrated below, the Applicants have determined that, surprisingly, the rate at which the field passes across the target face affects the level of uniformity of deposition upon a substrate facing the target. Even more surprisingly, they have determined that a minimum in non-uniformity can be readily determined and better uniformity can be achieved by controlling the speed of rotation to that which produces a minimum. Put another way the speed can be selected to maximise the speed-enhanced uniformity.  
         [0012]     The method may include the steps of (a) moving the field over a range of speeds; (b) monitoring uniformity versus speed; (c) selecting the speed that gives the preferred uniformity; and (d) controlling the field to the selected speed.  
         [0013]     Steps (a) to (d) may be performed during the process and/or over the lifetime of the target and on single or many substrates). Preferably, as indicated above the selected speed is the one that gives the best uniformity and it may be determined by seeking a minimum in non-uniformity or a maximum uniformity.  
         [0014]     Also as indicated above a magnetron may be physically fixed, but may provided a moving field, in which case it is the speed of the movement of the field that is selected.  
         [0015]     The speed of rotation may lie between 20 and 500 rpm [approximately 0.33 to 8.33 Hz].  
         [0016]     From another aspect the invention consists in a method of sputtering with sputtering apparatus having a sputter target and a magnetron for providing a field which moves relative to the target, including determining the rate of erosion of parts of the face of the target and scanning the magnetic field across the face of the target so that the erosion from all parts of the face of the target is at a substantially constant average rate.  
         [0017]     In that case both the rate of erosion may be monitored (directly or indirectly e.g. by reference to film deposition uniformity) and the speed may be controlled in real time during the sputter process or a deposition process or from deposition to deposition through a target life.  
         [0018]     In a particularly preferred embodiment the target is aluminium and the sputtering process is reactive sputtering and the deposited layer is aluminium nitride.  
         [0019]     From a still further aspect the invention consists in sputtering apparatus having a sputter target and a magnetron for providing a field which moves relative to the target and a control for operating the apparatus in accordance with any of the methods defined above.  
         [0020]     The target may be aluminium and the deposited layer may be aluminium nitride.  
         [0021]     Although the invention has been defined above, it will be understood that it includes any inventive combination of the features set out above or in the following description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     The invention may be performed in various ways and specific embodiments will now be described with reference to the accompanying drawings in which:  
         [0023]      FIG. 1  is a plot of an indication of non-uniformity of deposition against magnetron rotating motor speed,  
         [0024]     FIGS.  2  to  6  are plots of a cross wafer deposition characteristics for the various rotation speeds shown on the graph of  FIG. 1 ,  
         [0025]      FIG. 7  is a schematic view of a basic speed control for sputter apparatus;  
         [0026]      FIG. 8  shows a more sophisticated version of  FIG. 7 ;  
         [0027]      FIG. 9  is a plot of an indication of non-uniformity of deposition of molybdenum against magnetron speed;  
         [0028]      FIG. 10  is another plot of an indication of non-uniformity of deposition molybdenum against magnetron speed; and  
         [0029]      FIG. 11  is a plot of magnetron motor speed and uniformity against target utilisation.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0030]     Referring first to  FIG. 7 , as is well-known, sputtering apparatus may include a target  10  having a rotatable magnetron  11  on its non-sputtering side. The magnetron may be rotated by a motor  12  or by other means such as a flow of cooling wafer impinging on an impeller.  
         [0031]     In an embodiment of the inventive apparatus, an inverter  13  is supplied between the motor  12  and the supply  14  to allow control of the rotational speed of the motor  12 .  
         [0032]     Using such apparatus the Applicants have processed a wafer under the following process conditions: 
        200 mm Wafer     target—aluminium     45 mm target to wafer spacing     Ar Gas Flow: 10 sccm     N2 Gas Flow: 50 sccm     Process pressure 2.2 millitorr     Platen Temperature: 400° C., no wafer clamping     Pulsed DC Target Power: 6 kW, Pulse frequency−100 kHz, Pulse width+4 μs     Platen Bias Power: 130 W of 13.56 MHz        
 
         [0042]     By using the inverter  13  to change the frequency of the supply to the magnetron motor  12 , the speed of rotation of the magnetron  11  was varied and a plot, as shown in  FIG. 1 , was made of so-called thickness uniformity (%) against magnetron motor drive frequency where magnetron rpm is ten times inverter frequency such that 30 Hz equals 300 rpm of the magnetron. The thickness ‘uniformity’ figure is an industry standard expression (widely known as “1 sigma uniformity” in the semiconductor industry), though actually a measurement of non-uniformity being a standard deviation shown as a percentage of the mean. In essence the lower the value the greater the uniformity. As will be seen very clearly, not only did the thickness uniformity (%) vary with speed it reached a clear and discernable minimum at inverter frequency 32.5 Hz (which equates to 325 rpm or approximately 5.4 Hz of the magnetron) when the magnetron motor inverter was varied between 25 to 35 Hz.  
         [0043]     The physical equivalence of these results can be clearly seen in FIGS.  2  to  6  and  FIG. 5  shows the optimum or “minimum” value.  
         [0044]     Using the  FIG. 7  apparatus, it would be possible to preset the rotational speed or to vary it over time, as target erosion takes places, for example in accordance with a look-up table or algorithm as it has been found that the optimal rotational speed changes as the target is consumed.  
         [0045]     However, as indicated in  FIG. 8 , it would also be possible to vary this speed in real time using a process modular controller  15  to control the speed in accordance with a monitored parameter e.g. the thickness of deposition.  
         [0046]     When depositing Aluminium Nitride in a reactive sputtering process using an aluminium target, an improvement in uniformity of deposition through target life is achieved by increasing the magnetron speed as the target is consumed. An example of this is illustrated in  FIG. 11 . This plots Aluminium Nitride (AlN) non-uniformity of deposition on a substrate (expressed as “uniformity” as is conventional in the semiconductor and related fields) and magnetron speed in revolutions per minute (r.p.m.) through the usage of an aluminium target expressed in accumulated power consumption expressed in kilowatt hours (kWH) of usage.  
         [0047]     As can be seen, as the target is consumed the uniformity worsens. Increasing the magnetron speed then brings the uniformity percentage down (improving uniformity). This example used a planar target with a total thickness (including backing plate) of 24 mm, with 1 mm spacing to a moving magnetron and 300 gauss field strength at the target face.  
         [0048]     The uniformity of deposition through target life was further optimised by combining changes in magnetron offset as described in GB2386128B with changes in magnetron rotation speed. Improved uniformity for aluminium nitride deposition was achieved with a magnetron rotation speed of about 300 rpm for a fresh target and incrementally increasing the magnetron speed to 380 rpm at 250 kWH and about 420 rpm at about 350 kWH to about 500 rpm at accumulated power of 500 kWH. These particular values will be dependent on the thickness of the target and the rate of its consumption (material dependent).  
         [0049]     The magnetron speed at any given target usage/thickness for best deposition uniformity can be established experimentally and the results used to establish a look-up table or an algorithm such as a 4 th  order polynomial that can be used by a control program to adjust the magnetron speed throughout the target life.  
         [0050]     Further experiments have been performed for non-reactive sputtering (noble gas ambient) of molybdenum. As can be seen in  FIGS. 9 and 10  minima are reached as magnetron rotation speeds were selected from 50 to 200 rpm (0.83 to 3.33 Hz) with the best uniformity at 175 rpm. As can be seen there are significant improvements. The change from 1.1% to 0.7% is a 36% improvement.  
         [0051]     It should be understood that the improvements in uniformity from controlling the magnetron speed, whilst very significant for resonators and significant for gate, capacitor or barrier layers, are very small in absolute levels and may be masked by larger effects where uniformity is not already high.  
         [0052]     Whilst molybdenum and aluminium nitride examples have been shown here there are many sputtering and reactive sputtering processes that may benefit from this invention, particularly where extremely good film thickness uniformity is requires, such as for resonators such as Film Bulk Acoustic Resonators (FBARs). Other example electrode, seed and resonant films are: Au, Ti, ZnO, Cr, NiCr, In, MoC, Zn, W, WSi, Cu, Pt and Si.  
         [0053]     It is not yet fully understood why uniformity is improved by changing the frequency of magnetic sweep across the target face, though it provides an economical and easily implemented method of improving uniformity. It should be remembered that magnetron speed is in general fixed by mains supply frequency, water flow speed or other drive mechanism. It is not intuitively obvious to vary this speed and without the addition of a means of varying speed it is impossible to select a uniformity maximising speed. And, as has been demonstrated here, different processes and/or materials require different speeds for maximum uniformity—so no one fixed speed will give the best uniformity to a range of materials or processes.  
         [0054]     Magnetron scanning speed may not necessarily be constant and the magnetron may dwell longer in some places than others, depending on their physical relationship, for example, to a gas inlet or pumping port.  
         [0055]     Where the field is being moved around by selective switching of a fixed magnetic coil arrangement, the field may not need to pass continuously around the target face but may be move from sector to sector in accordance with reaction product creation rate.  
         [0056]     Eddy currents will be set up in certain sputter targets by a moving magnetic field passing through it. These currents may offer an explanation as to why speed of movement of the magnetic field affects uniformity of target erosion. This also suggests that the optimum magnetic sweep speed may be a function of the magnetron field strength and shape at the target.