Abstract:
A dielectric layer is deposited on a workpiece by a chemical vapor deposition method in an electron cyclotron resonance vacuum plasma processor having a plasma chamber responsive to a repetitively pulsed microwave field and gases from a plasma source. A reaction chamber responds to at least one reacting gas containing at least one element that chemically reacts in the presence of the plasma with at least one element in at least one of the gases from the plasma source to form the deposited layer on the workpiece. The turn off periods are long enough to cause electrons in the plasma on the deposited dielectric layer to be cooled sufficiently (from about 3.5 eV to a lower value having a minimum value of about 0.1 eV) to reduce the tendencies for opposite polarity charges to be established across the deposited dielectric layer and for damaging discharge current to flow across the deposited dielectric layer. The layer is deposited in a gap having an initial aspect ratio of at least about 1:1; the turn on and turn off times are such as to cause the gap to be bridged by a different deposited film each time the microwave energy is turned on. The films build up to form a layer. The turn off time is greatest during initial deposition of the layer and becomes zero as the gap is filled. The peak power per pulse and the time between pulses are controlled.

Description:
FIELD OF INVENTION 
     The present invention relates generally to electron cyclotron resonance (ECR), chemical vapor deposition (CVD) processes and apparatuses in vacuum plasma processors and more particularly to such a method and apparatus wherein layers are deposited on a workpiece by a plasma excited by a pulsed microwave source. 
     BACKGROUND ART 
     One known apparatus for depositing films, particularly dielectric films, on workpieces, such as dielectric substrates, semiconductor wafers or metal substrates includes a vacuum plasma processor enclosure wherein a plasma gas supplied to a plasma chamber of the enclosure is excited to a plasma by a continuous, i.e. non-pulsed, microwave field. Ions in the resulting plasma chemically react with ions and other particles of at least one other element introduced into a reaction chamber to form deposited layers on the workpiece. The microwave field interacts with a DC magnetic field having lines of flux generally aligned and coaxial with a longitudinal axis of the microwave field. The frequency of the microwave field and intensity of the DC magnetic field cause an electron cyclotron resonance phenomenon in the reaction chamber. The workpiece is usually mounted on a holder; if the workpiece is non-metallic the holder is typically an electrostatic chuck including an electrode usually supplied with DC chucking voltage and an r.f. bias having a frequency such as 4.0 or 13.56 MHZ. 
     The microwave field interacts with electrons spiraling in the DC magnetic field to produce a high-density plasma when the vacuum chamber is maintained at a pressure of less than about 10 milliTorr. The electron cyclotron resonance arrangement increases ion production at these low gas pressures by efficiently coupling microwave energy to the plasma gas. At electron resonance, as established by the frequency of the microwave source and the DC magnetic field intensity, electrons in the plasma orbit the DC magnetic field lines at the same frequency as the microwave field. The electrons continuously gain energy from the microwave field and are accelerated with a circular motion about the microwave field axis, which is coincident with an axis of a source of the DC magnetic field, such as a solenoid coil. The DC magnetic field inhibits plasma electrons from losing energy to walls of the processor chamber, to increase the probability of ionization of elements in the plasma. 
     In one prior art arrangement, the DC magnetic field is 875 Gauss and the microwave field has a frequency of 2.45 gigahertz, derived by a magnetron and supplied to a first end of the plasma chamber via a matching network, a wave guide and a window. Typically, the plasma gas is a mixture of oxygen and an inert gas, such as argon. At a second end of the plasma chamber, opposite the first end thereof, electrons and ions escape from the plasma chamber into the reaction chamber. The electrons escape from the plasma chamber into the reaction chamber before the ions. 
     The electrons and ions pass through the reaction chamber. Exemplary gases supplied to the reaction chamber are a silane, such as SiH 4 , or a silane mixed with phosphine (PH 3 ), tetrafluorosilane (SiF 4 ) or nitrogen (N 2 ). The chemical reaction takes place on the workpiece primarily between ions, typically O 2   − , escaping from the plasma chamber and molecules in, as well as ions dissociated from, the gases flowing into the reaction chamber, e.g. Si + , SiH + , SiH 2 , and SiH 3 . 
     Downstream of the reaction chamber, electrons and positive ions in the plasma are incident on the substrate. The silicon introduced into the reaction chamber and the oxygen ions dissociated from the plasma source gas combine on the workpiece, i.e., substrate, to form silicon dioxide layers which sometimes are doped with phosphorous, fluorine, or nitrogen, depending on the gases introduced into the reaction chamber. 
     There is a measurable nonuniform charge build up on dielectric layers being formed by the CVD process in gaps with relatively high height to width aspect ratios of at least 1:1. This charge build up is due to a disparity between the trajectories of the ions and electrons. This difference in trajectories is due to the large disparity in mass between ions and electrons, when exposed to an applied radio frequency electromagnetic field. The electrons are mobile enough to follow the applied r.f. field. The ions are not. This results in the dielectric layers being CVD formed acquiring charge separation, which then creates a net negative DC bias (also called applied bias) on the dielectric layer being formed. The net negative DC bias is with respect to the plasma, which usually has a voltage close to ground. The positive ions in the plasma see this negative bias and are accelerated to the surface of the dielectric layer being formed. The positive ion trajectories are very directional normal to the exposed layer surface because of this bias. As a result, positive ions accumulate on the bottom of a topographical structure, such as a trench or high aspect ratio dielectric layer. The electrons, on the other hand, being less directional, tend to collect at the tops of the topographical structure. 
     This separation in charge between the electrons at the top of the layer and the positive ions at the bottom of the layer can cause a destructive Fowler-Nordheim current to flow through the layer, its underlying active semiconductor device structure, and the substrate. The circuit is completed by current flowing back up to the plasma at some other location on the workpiece. This current flow through an active semiconductor device underlying the dielectric layer being found causes degradation of a gate oxide in a metal oxide semiconductor (MOS) device of the underlying structure. 
     It is, accordingly, an object of the present invention to provide a new and improved method of and apparatus for chemically vapor depositing materials in a vacuum plasma processor responsive to a plasma established by an electron cyclotron resonance mechanism. 
     Another object of the invention is to provide a new and improved method of and apparatus for electron cyclotron resonance chemical vapor depositing layers on a workpiece to provide a dielectric film or layer formed in such a manner as to substantially obviate the tendency for charge separation to occur in the formed film or layer between separated positive ions and electrons in the layer. 
     A further object of the invention is to provide a new and improved chemical vapor deposition method of and apparatus for depositing dielectric materials in a vacuum plasma processor wherein the plasma of the processor is excited by an electron cyclotron resonance mechanism energized in such a manner as to reduce damage due to separation of positively charged ions and electrons on opposite portions of the formed dielectric film. 
     An additional object of the invention is to provide a new and improved ECR, CVD method of and apparatus for forming layers in a high aspect ratio gap on a workpiece, wherein the compound in the gap has high uniformity. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention microwave energy supplied to a plasma chamber of a CVD, ECR processor is repetitively pulsed, e.g. between maximum and minimum power levels. A homogeneous layer is thereby deposited on a workpiece in the processor. The plasma chamber, in addition to being responsive to the pulsed microwave energy, is responsive to gases from a plasma source. The processor includes a reaction chamber responsive to at least one reacting gas containing at least one element that chemically reacts in the presence of the plasma with at least one element in at least one of the gases from the plasma source to form the deposited layer on the workpiece. 
     In a preferred embodiment, the layer is a dielectric and the turn off or minimum power level periods are long enough to cause electrons in the plasma on the deposited dielectric layer to be cooled sufficiently to reduce the tendency for opposite polarity charges to be established across the deposited dielectric layer and reduce the tendency for damaging discharge current to flow across the deposited dielectric layer. We have found the microwave energy preferably has a minimum power level for a time period sufficient to cause the electrons to cool from about 3.5 eV to lower level having a minimum value of about 0.1 eV, after which the microwave energy is again turned on. 
     According to another aspect, the layer is not necessarily a dielectric and is deposited in a gap having an initial aspect ratio of at least about 1:1, and the minimum and maximum power level times are such as to cause the gap to be bridged by the deposited layer each time the microwave energy is turned on and off. 
     In a preferred embodiment the microwave energy turn off or minimum power level time is greatest during initial deposition of the deposited layer. The microwave energy turn on time may gradually increase as the deposited dielectric layer increases in thickness. The microwave energy may be continuously supplied to the plasma chamber after the deposited dielectric layer has been initially deposited, to completely fill the gap at a greater deposition rate. 
     Control of the microwave and therefore plasma off time will allow chemical reactions to take place in the absence of bias applied to the substrate. For example, with the case of a fluorosilica glass (FSG) deposition, it was observed that chemical etching (from free F) took place during the pulse off time. This chemical etch was more anisotropic than isotropic sputtering that occurred under the applied bias. This is useful for gap fill in some structures, where the metal layer being coated by the FSG has a very high sputter yield. 
     Control of the plasma average power by varying the duty cycle, and hence average power, per pulse would appear to enable separation of gas phase and surface chemistry effects to control the material in the formed films particularly, in phosphine (PH 3 ). Experiments show that with a continuous microwave field there is a limit of how much P can be incorporated into a film growing with the SiH 4  and O 2  chemistries in the ECR reactor; 20% PH 3  in SiH 4  only yields 2% P in the SiO 2 . We believe the higher peak power per pulse available with the pulsed microwave field (with no increase in average power over a continuous microwave field) further dissociates the PH 3  molecule so it is more chemically active, and thereby is more completely incorporated into the growing film. 
     Hence another important potential application for pulsed microwave plasmas appears to involve increasing the doping of phosphosilicate glass, PSG, by the oxidation of silane, SiH 4 , and phosphine, PH 3 . Pulsing the microwave field applied to the plasma appears to enable more P to be efficiently incorporated into the PSG film. The pulsed plasma seems to permit the more efficient dissociation of PH 3  into radicals and ions relative to SiH 4 , thus increasing the P dopant in the deposited PSG. 
     Hence, it appears to be desirable to control one or more of (1) the peak microwave power per pulse, (2) the minimum microwave power between the peak pulsed power (i.e. to control the microwave power so it does not drop to zero), (3) average power per pulse, and (4) the time between microwave pulses by varying one or both of the pulse frequency and duty cycle. Controlling the average and peak microwave powers per pulse and the minimum microwave power between the peak pulsed power is a function of the materials in the layers to be formed and dependent on the gases supplied to the plasma and reaction chambers. Different species in the plasma formed from the gases supplied to the plasma and reaction chambers dissociate to a greater or lesser extent as a function of applied microwave power. Consequently layers of certain materials will be formed by the CVD process by increasing the peak microwave power per pulse to e.g. 20 kW. Some species will react at a relatively low power of e.g. 500 watts, between the peak power at the pulses. In this way, the composition of the layers can be tailored for different specifications. As the peak and minimum powers are changed, it may be necessary to change the time interval between adjacent pulses to obtain the cooling necessary to prevent damaging current flow. Further, the minimum power level can change as a function of the thickness of the deposited layer, so that, e.g. the minimum power is zero when the layer is initially being deposited and then increases as the layer deposition time increases. 
     Another potential use of applying a pulsed microwave field to the gases supplied to the plasma and reaction chambers is for nitridation of gate oxides to prevent diffusion of boron or other dopants into an active gate region of a metal oxide field effect device. Experiments have been conducted in which a continuous nitrogen ECR plasma (with no r.f. basis applied to the substrate) was used to attempt to nitride the top surface of a gate oxide to form a diffusion barrier. However, this technique was not successful due to damage of the gate oxide. However, a pulsed nitrogen ECR plasma could generate the active species for nitridation with substantially less damage of the gate oxide. 
     The workpiece is preferably on an r.f. biased electrode, while the layer is being deposited. The r.f. applied as bias to the electrode may be turned on and off simultaneously with the microwave energy having the maximum and minimum power levels. 
     Another aspect of the invention relates to an apparatus for depositing a layer on a workpiece, wherein the apparatus comprises: a source of plasma forming gases; a source of at least one gas containing at least one element that can chemically react with at least one element of the plasma forming gases; a vacuum plasma enclosure; a microwave source arranged to be repetitively pulsed, and a DC magnetic field source. The vacuum plasma enclosure includes: a plasma chamber connected to be responsive to the source of plasma forming gases, a reaction chamber connected to be responsive to the source of at least one gas that can chemically react with at least one element of the plasma to form the layer on the workpiece, and a workpiece holder. The pulsed microwave source is coupled with the plasma chamber and is arranged to supply the plasma chamber with a pulsed microwave field having sufficient power to convert the gas of the plasma source into a plasma. The DC magnetic field source is coupled with the vacuum plasma enclosure and the microwave field for establishing an electron cyclotron resonance mechanism in free ions and electrons of the plasma. The free ions and electrons chemically react to form the layer on the workpiece. 
     To provide greater uniformity in the deposited film and eliminate the need for a matching network in the pulsed microwave source, i.e. between a pulsed magnetron and waveguide feeding a window in the plasma chamber, the source is arranged to supply the plasma chamber with microwave energy that is circularly polarized in only one direction. Right hand polarization has been found to substantially reduce microwave reflections in the chamber. 
     We are aware of prior art in “Time-Modulated Electron Cyclotron Resonance Plasma Discharge for Controlling the Polymerization in SiO 2  Etching,”  Japanese Journal of Applied Physics,  December 1993, Vol. 32, No. 12T, pages 6080-6087 (Samukawa) and “Pulse-time Modulated Electron Cyclotron Resonance Plasma Etching for Highly Selective, Highly Anisotropic, and Less-Charging Polycrystalline Silicon Patterning,”  J.-Vac. Sci. Technol.  Serial No. 08/885.864  B  12(6), November-December 1994, pages 3300-3305 (Samukawa et al.) wherein microwave sources have been pulsed on and off in electron cyclotron resonance vacuum plasma etching processes. This prior art reports etching of polysilicon and how an on and off pulsed microwave source increases the selectivity of polysilicon to silicon dioxide, with suppressed “charge buildup damage.” Kofuji et al., “Sub-Quarter Micron Poly-Si Etching with Positive Pulse Biasing Technique,” 1 st International Symposium on Plasma Process - Induced Damage , May 1996, pages 234-236 discloses an electron cyclotron resonance plasma etching system wherein on and off pulsed r.f. bias is supplied to a substrate. 
     The prior art of which we are aware deals with etching dielectrics and is not concerned with forming films, particularly dielectric films, and the problems associated therewith; nor is it concerned with forming uniform composition films in high aspect ratio gaps. Frequently, it is necessary to fill relatively high aspect ratio gaps or grooves, i.e., the height of a gap to be filled between a pair of adjacent walls exceeds the width of the gap. We have information showing such gaps can be substantially completely uniformly filled by the pulsed microwave ECR, CVD process and apparatus of the present invention, and that the invention overcomes the tendency for dielectric layers and the structures underlying such layers to be damaged by the charge separation mechanism. 
     The use of a pulsed microwave source in electron cyclotron resonance deposition appears to be advantageous because Langmuir data of an argon plasma shows that the average ion current density is higher in a pulsed microwave system than in a continuously operated microwave system for a given average power. Therefore, it appears that the same deposition rates can be achieved with a lower pulsed microwave average power level than for a higher continuous microwave average power level. Further, because there is a modified high density plasma produced by the pulsed microwave source, the pulsed plasma appears to aid in the deposition of fluorosilica glasses (FSG), relative to the application of a continuous wave microwave field to the plasma. The pulsed microwaves appear to reduce the amount of argon incorporated into a deposited film. Pulsing appears to provide a more uniform film composition in depositing layers in high aspect ratio grooves or gaps than is attained by applying a continuous microwave source to the plasma. 
     The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of a specific embodiment thereof, especially when taken in conjunction with the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The sole figure is a schematic diagram of a preferred embodiment of an ECR, CVD plasma processor responsive to a pulsed microwave source for performing the method of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference is now made to the figure, a partially schematic and partially cross-sectional view of an electron cyclotron resonance (ECR) chemical vapor deposition (CVD) vacuum processor including cylindrical electron resonance plasma chamber  10 , frustoconical reaction chamber  12  and workpiece holder  13 , all forming or located in a single non-magnetic sealed enclosure  15 , pumped to a vacuum by turbo-pump  17 . Plasma gas source  18  supplies a mixture of oxygen and argon to plasma chamber  10  via conduit  20  and port  22  in enclosure  15 . The gaseous mixture flowing through port  22  combines with a pulsed microwave (preferably 2.45 gigahertz) electromagnetic field derived by source  24  and a DC magnetic field (preferably 875 Gauss) derived by cylindrical, solenoidal coil  26 . The lines of DC magnetic flux derived by coil  26  (located outside of and coaxial with plasma chamber  10 ) extend axially through plasma chamber  10 . The DC magnetic field magnitude and the orientation and frequency of the microwave field produced by source  24  in chamber  10  are such that charge carriers (electrons and ions) plasma source  30  supplies to chamber  10  rotate about the axis of the plasma chamber. The angular frequency of electrons in plasma chamber  10  due to the magnetic field established by coil  26  matches the microwave frequency of pulse source  24 , to establish the electron cyclotron resonance phenomenon. Electrons gain energy from the microwave field and are accelerated with a circular motion. 
     The DC magnetic field established by coil  26  extracts ions out of plasma chamber  10  into reaction chamber  12  and onto a workpiece, that can be a dielectric (e.g. a flat panel display), a semiconductor (e.g. a wafer) or a metal (e.g. a plate), mounted on holder  13 . Because the ends of coil  26  and the intersection of chambers  10  and  12  are approximately aligned the magnetic field established by coil  26  begins to diverge at the intersection so the plasma flowing from plasma chamber  10  into reaction chamber  12  has a tendency to be cone shaped. The ions in the plasma produced in chamber  10  follow the diverging magnetic flux liner, established by the magnetic field as the ions follow the diverging flux lines and spiral from chamber  10  into chamber  12  and onto the substrate on holder  13 . The divergent magnetic field established by coil  26  also creates a force pulling on electrons in the plasma, so the electrons are pulled out of the magnetic field. The resulting potential established by the electrons attracts ions to form a variable directional ion beam in reaction chamber  12  and on the workpiece on holder  13 . The magnetic field and plasma flow direction are shaped by cylindrical mirror and cusp coils  30 , outside enclosure  15  below substrate holder  13  and coaxial with coil  26 . 
     Vacuum enclosure  15  is operated at pressures in the relatively low range of 0.5 to 10 milliTorr to establish a high density plasma in chambers  10  and  12  and on the workpiece on holder  13 . The density of ions to neutral particles on the workpiece is as high as 1:100 with in excess of 1×10 12  ions per cubic centimeter. 
     Reaction chamber  12  includes multiple ports  32 , located about the periphery of the reaction chamber, in proximity to the reaction chamber open end, fairly close to substrate holder  13 , i.e., remote from the end of the reaction chamber intersecting plasma chamber  10 . Ports  32  are supplied via conduit  33  and a suitable manifold (not shown) with one or more deposition gases from one or more sources, shown collectively as source  34 . Ions in some of the gases of source  34  chemically react on the workpiece with ions from plasma source  30  to form dielectric or other films on the workpiece. To form dielectric films of silicon dioxide, source  34  is preferably silane (SiH 4 ) or silicon tetrafluoride (SiF 4 ). 
     In response to the plasma in chamber  12  the hydrogen or fluorine dissociates from the silicon. The dissociated fluorine forms positive and negative ions (F +  and F − ) to dope the SiO 2  film formed by the chemical reaction on the substrate of O 2  molecules and O 2   −  ions in the plasma with silicon dissociated from the SiF 4 . To form other types of dopant layers on the substrate carried by holder  13 , a gas including a dopant, such as phosphine, is included in source  34  and supplied to chamber  12 . 
     The gases of source  34  are, in certain embodiments, gases which enable formation of dielectrics having dielectric constants of less than 3.5 (frequently referred to in the art as ultra low K dielectrics) on the substrate carried by substrate holder  13 . Under these conditions, source  34  includes carbon fluoride gaseous compounds, such as the compounds which react with at least one of the plasma gases to form fluorocarbons. The carbon fluoride compounds chemically react on the substrate with at least one of the plasma gases to form amorphous films including carbon. 
     Because microwave source  24  is pulsed the gases of source  34  chemically react on the substrate with the oxygen supplied by source  18  to reaction chamber  10  to form multiple films which are built up to form a layer. For high aspect ratio gaps, it is believed that the space between adjacent walls of the gap is filled with a very thin film of the desired composition each time the microwave source is pulsed on and then off or pulsed to a maximum power and to a minimum, non-zero power level. The compound in the deposited film spreads between the gap walls during the interval while the microwave source is off. Hence, pulsing microwave source  24  is advantageous to substantially completely and uniformly fill a desired composition into high aspect ratio gaps or grooves between adjacent walls of metal, semiconductor or dielectric layers on the substrate. In other words, pulsing microwave source  24  improves the topography of an integrated circuit on the substrate. 
     Preferably, source  14  is pulsed at the beginning of formation of dielectric films on the substrate, for example, during the first 10 to 20 percent of the film formation. During the remainder of the dielectric film formation process, the microwave source can be operated as a continuous wave source or there can be a gradual transition and increase in duty cycle of the microwave source as the deposition process proceeds beyond the 10 to 20 percent initial processing interval. For example, the microwave duty cycle can be initially 50% and increase gradually or in steps and become 100% when the gap is about half filled. Increasing the duty cycle results in successive films of the layer having greater thicknesses. As the gap becomes filled the gap aspect ratio decreases and the likelihood of having a void in the formed film decreases, so thicker films can be laid down. Increasing the microwave duty cycle and then continuously filling the gap by operating microwave source  24  continuously after the gap has been filled to a certain percentage enables more rapid filling than is achieved by only operating microwave source  24  in the pulsed mode. 
     In a preferred embodiment, microwave source  24  includes pulsed magnetron  36 , having a maximum continuous wave output power of approximately 2.5 kW. The 2.45 gigahertz electromagnetic energy derived by magnetron  36  has a rectangular propagation mode and is supplied by rectangular waveguide  37  to rectangular to circular polarization converter  38  and mode separator  39 , which together convert the rectangularly polarized output of the magnetron into left and right hand circularly polarized waves. The left hand polarized wave is supplied to dummy load  40  while the right hand polarized wave is supplied to circular wave guide  42 , configured to operate in the TE 11  mode. The right hand circularly polarized microwave energy propagating through wave guide  42  is supplied through microwave window  44  to the top of plasma chamber  10 , i.e., the end of chamber  10  remote from the intersection of the plasma chamber and reaction chamber  12 . Converting the rectangularly polarized output of magnetron  36  into a circularly polarized wave having only right hand polarization avoids the need for a matching network between the magnetron and plasma chamber  10  and improves the uniformity of films deposited on the workpiece on holder  13 . Despite the lack of a matching network, there is a low level of microwave reflection in plasma chamber  10  and reaction chamber  12 , resulting in a relatively low voltage standing wave ratio (VSWR). 
     The dielectric and semiconductor workpieces are preferably held on holder  13  by an electrostatic chuck structure (not shown) energized by a DC source (not shown). The substrate holder includes metal plate  50  forming an electrode to which the workpiece is chucked. An AC bias voltage, preferably having a frequency of 13.56 MHZ, is applied by source  52  to electrode plate  50 . A suitable power level for source  52  is 2 kW. The substrate is cooled by helium and a cooling liquid, derived from suitable sources (not shown) and supplied in a conventional manner by suitable conduits (not shown) to holder  13 . 
     In a preferred embodiment, r.f. source  52  is pulsed on and off synchronously with pulsing of magnetron  36  under the control of programmed variable duty cycle, variable frequency pulse oscillator  54  so the r.f. and microwave sources are simultaneously on and off. Duty cycle controller  56  controls the duty cycle of oscillator  54  and therefore sources  24  and  54  so the microwave field and the r.f. applied to electrode  50  initially have 50% duty cycles. Controller  56  preferably increases the duty cycle to 100%, gradually or in steps. Controller  56  controls the duty cycle of oscillator  541  to achieve any of the different duty cycles mentioned previously. The frequency of oscillator  54  is controlled by an operator for the various duty cycles to assure adequate cooling of the deposited layer from about 3.5 eV to a lower value, no greater than about 0.1 eV, to substantially prevent charge separation and flow of destructive Fowler-Nordheim tunneling current, as discussed supra. 
     An acceptable range for the frequency of oscillator  54 , and therefore the pulsing frequency of microwave source  24  and r.f. source  52 , has been found to be between 5 kHz and 20 kHz; tests were conducted at 5 kHz, with a 50% duty cycle, 8 kHz with 60%, 70%, 80% and 90% duty cycles, 10 kHz with a 50% duty cycle and 20 kHz with a 50% duty cycle. In some of these tests r.f. bias was applied to substrate electrode plate  50  while no r.f. was applied in some other tests. At an 8 kHz pulse rate, the lowest duty cycle for the r.f. bias was found to be 50%; adjustment of an r.f. matching network (not shown) between source  52  and electrode  50  is unstable at pulse frequencies less than 8 kHz and duty cycles less than 50%. At 8 kHz for 70%, 80% and 90% duty cycles of magnetron  36 , with 900 watts being continuously applied to electrode  50  (i.e. with no connection from oscillator  54  to source  52 ), and an average microwave power of 1000 watts being supplied to plasma chamber  10  while a semiconductor wafer workpiece was maintained at a temperature of 330° C., a SiO 2  layer was deposited with a 1-1.5% uniformity across the wafer. The SiO 2  film had a dielectric constant of about 3.6; the layer was formed by feeding SiH 4  from source  34  to openings  32  while O 2  and Ar were supplied by source  18 . The experimental data show the pulsed microwave source can provide a much higher height to depth aspect ratio without significant voids than can be obtained with a continuous microwave source. 
     Pulsing the microwave source has the disadvantage of considerably reducing the deposition rate for the same peak microwave power. Average applied power for the pulsed microwave source is typically about 40% less than that of the continuous microwave power. The decrease in deposition rate for the pulsed microwave source makes increasing the duty cycle important as the gap is being filled during the deposition process. 
     Pulsing the microwave source, in addition to attaining improved topography, significantly reduces damage to deposited dielectric layers and the structure underlying the layers. If the microwave source were continuously operated, positive ions would be accumulated at the bottom face of a topographical structure. While ions are accumulated at the bottom face of the topographical structure, electrons are accumulated at the top face of the topographical structure during continuous application of the microwave field. The accumulation of oppositely charged particles on the top and bottom of the topographical structure results in a Fowler-Nordheim current through the layer and a structure underlying the layer, causing damage to the layer and the underlying structure. 
     Pulsing the microwave source appears to significantly reduce damage to the formed layer and the structure underlying the layer. The off time of the microwave energy as given, e.g. by the foregoing examples, allows the electrons to cool from about 3.5 eV to a lower value, no lower than about 0.1 eV. We believe this cooling enables the electron attachment cross-section to increase sufficiently to increase the number of negative ions in the plasma. Since the negative ions accumulate at the bottom of the film, the plasma charge non-uniformity across the dielectric decreases to substantially reduce or prevent destructive Fowler-Nordheim tunneling current flow across the layer thickness. By pulsing the microwave source and the r.f. bias voltage applied to electrode  50 , the plasma sheath on the workpiece surface may collapse to allow for charge redistribution on the workpiece exposed surface. The charge redistribution seems to eliminate the accumulation of opposite polarity charges on the opposite sides, i.e. faces, of the deposited film and prevent the flow of destructive Fowler-Nordheim tunneling current through the layer and device structure underlying the layer. 
     In one embodiment, the maximum or peak power of the microwave pulses is adjustable by an operator, as is the minimum power of the pulses, i.e., here can be a controllable non-zero power level between the maximum microwave power levels during each complete cycle of oscillator  54 . Such adjustments control the average microwave power applied to the plasma. To this end, magnetron  36  is coupled with minimum output power controller  58  and peak output power controller  60 , both driven by oscillator  54 , as indicated by the dash lines between the oscillator and controllers. During the low level portion of each cycle of oscillator  54 , controller  58  sets the input and therefore output powers of magnetron  36  to an appropriate minimum level set by an operator. During the high level portion of each cycle of oscillator  54 , controller  60  sets the input and therefore the output powers of magnetron  36  to an appropriate maximum level set by the operator. The operator sets the minimum and peak powers of controllers  58  and  60  as a function of the materials to be deposited in the films forming the layer to be deposited on the workpiece. The operator also sets limit values for the durations of the minimum and peak power levels by adjusting one or both of the duty cycle and frequency of oscillator  54  so there is adequate cooling of the formed films between the peak output power periods of magnetron  36 , bearing in mind that the amount of required cooling is determined, inter alia, by the peak and minimum powers. 
     While there has been described and illustrated one specific embodiment of the invention, it will be clear that variations in the details of the embodiment specifically illustrated and described may be made without departing from the true spirit and scope of the invention as defined in the appended claims. For example, the frequency of r.f. bias source  52  could be suitable values, e.g. 4.0 MHZ or 700 kHz.