CA1341184C - Method and apparatus for the plasma etching substrate cleaning or deposition of materials by d.c. glow discharge - Google Patents

Abstract

The invention provides new methods and apparatus for the deposition of materials on substrates by the use of a D.C.
glow discharge, sometimes also called a plasma discharge. A
precursor gas (or gases), which is a decomposable compound of the required material, is introduced at low pressure (10-500 milliTor) into an enclosure containing two spaced parallel cathode electrodes and an intermediate parallel anode electrode, preferably midway between the two cathodes. Upon the application of a suitable positive potential to the anode a deposition plasma is generated in the space on both sides of the anode, primarily it is believed by electrons that oscillate back and forth through the anode, giving an increased free path for collisions and rendering the dissociation process more efficient. The plasma of charged and neutral radicals moves to the cathodes both by charge attraction and by diffusion, giving high quality films at increased deposition rates, higher possible discharge current densities, lower gas pressures with consequent gas economy and safety, improved film adhesion, and greater independence of these parameters from one another than has been possible with the prior art processes. To avoid encroachment of the plasma on the cathodes and substrates supplementary cathode electrodes may be interposed between the first-mentioned cathodes and the anode close to the former, so that the plasma is confined between the supplementary cathodes.

Description

METHOD AND APPARATUS FOR THE PLASMA ETCHING, SUBSTRATE
CLEANING OR DEPOSITION OF MATERIALS BY D.C. GLOW DISCHARGE
Field of the Invention The present invention is concerned with new methods and apparatus for the plasma etching, substrate cleaning or deposition of materials by D.C. glow discharge.
Review of the Prior Art The deposition of materials on substrates to form thin films of the material thereon is a well established industrially important art. The processes of plasma etching and substrate cleaning will be described below. Such deposition processes in which precursor gases are decomposed to give condensable depositable radicals have been referred to generally since their first introduction as "glow discharge deposition"
processes, but are now also referred to as "plasma deposition" processes. A large number of different materials can be deposited in this manner such as silicon, carbon, phosphorus, boron and arsenic, either alone or in combination, dependent only upon the availability of suitable precursor gases. This art became very important in the development and fabrication of semiconductor devices based upon amorphous silicon upon the discovery of the beneficial effects of controlled hydrogenation thereof by deposition in the presence of hydrogen, and it is now possible to deposit amorphous silicon films with extremely small electrically active defect densities, and with electrical properties that can be controlled by substitutional doping. The hydrogenated, doped amorphous silicon layer acts as an extrinsic semiconductor and is now used in relatively large scale devices (i.e. much larger than the silicon "chip" typically employed in micro-electronics such as photovaltaic cells and xerographic type photocopiers.

1~~4! t84 There are two principal glow discharge deposition systems currently in use, namely D.C, excited and R.F. excited. In D.C. discharge processes a D.C.
field is produced in an evacuated chamber between two (usually parallel) electrodes, and the substrar_e to receive r_he deposited materials is generally mounted on the cathode, or forms the cathode. The precursor gas (or gases) is supplied to the chamber at pressures r_hat are relatively high for these discharge processes, typically 100-500 milliTorr, and dissociar_ion of r_he molecules takes place by excir_ation and ionisar_ion and electron impacr_, including impact of secondary elecr_rons produced by impact with r_he cathode of ions r_hat have been produced by the excitation, resulting in a plasma conr_aining a wide variety of ionic species. As reporr_ed in "Plasma Deposited Thin Films", edited by MORT, Joe and JANSEN, Frank, published 1986 by CRC Press Inc., Boca Baton, Florida, since molecular dissociar_ion energies are usually significantly smaller r_han ar_omic or molecular ionisar_ion energies r_he density of neur_ral dissociar_ion radicals in r_he plasma is much greater than that of r_he ionic radicals, and ir_ is r_he electrically neutral species r_har_ contribute predominantly to r_he film growth. These neutral radicals must find their way to r_he surface of r_he growing film primarily by diffusion, and consequently the processes are slow and nor_ very efficient, with the mar_erial deposir_ing also on r_he anode and r_he inr_ernal surfaces of r_he enclosure. Also, a considerable proportion is removed wir_h r_he flow of the precursor gas necessary to replenish r_he deposir_ed mar_erial while maintaining the pressure at a sufficienr_ly high value. The deposir_ion process rakes place mainly ar_ the cathode although, because of r_he nature of the plasma, considerable deposir_ion also occurrs at r_he anode.
In R.F. deposition processes r_he plasma is produced by high frequency excitation (usually ar_ r_he officially designated frequency of 13.56 MHz), the energy being applied to the interior of the container enclosing the substrate and the precursor gas eir_her by means of two internal electrodes directly capacitively coupled to the R.F. source, or by means of exr_ernal electrodes or a coil. The external electrode geometry is primarily restricted to tubular reactor shapes of small diameter and inr_ernal electrode systems are usually preferred.
The ionisation processes involved take place in r_he bulk of the plasma, the electrons oscillating in r_he high frequency field and picking up enough energy to fragment, excite and ionise r_he gas molecules. Because of the high frequency the relatively heavy ions are not displaced significantly by it, and can be regarded as immobile, and again therefore it is diffusion and drift. of the radicals that contribur_es predominantly r_o r_he growth of the deposited film. In operation, due r_o loss of electrons therefrom the plasma assumes a net. positive charge, resulting in D.C. por_enr_ials thar_ affecr_ the deposition.
R.F. sysr_ems are inherenr_ly more difficulr_ r_o conr_rol, owing to the need to maintain r_he coupling of the electrode sr_rucr_ure r_o r_he power source in r_he face of progressively changinc characterisr_ics of r_he elecr_rode structure as r_he semiconductive film builds up on r_he substrate, and as mar_erial deposits also on the electrodes.
It is of course the principal objecr_ive with all of these processes to obr_ain uniform deposir_ion of adherent films of preder_ermined characr_erisr.ics over as large an area as possiale, and r_his is primarily dependent upon r_he relative ease by which high, uniform electric fields can be crear_ed over correspondingly large areas with r_hese inr_ernal electrodes. Or_her problems encountered are that if r_he energy applied is insufficient r_he process is correspondingly slow and r_he adhesion of the resultant films is also usually found r_o be poor. On the other hand if r_he field strength is increased to speed up the process r_here is the danger of damage, usually referred to as °etching", caused by excessive ion bombardment of the film, resulting in structural damage by the generation of nucleation sites, or by heating r_o result in polycrystalline material instead of the desired amorphous structure, and/or resulting in the production of unsaturar_ed bonds which are electronically active and undesirable. The mean free path of the exciting elecr_rons in r_he gas is an imporr_anr_ parameter and is dependent upon r_he gas pressure in the enclosure; a low gas pressure slows r_he rate of deposition and can also result in excessive ion bombardment, while too high a pressure results in an inadequate mean free path and recombination reactions between the radicals. With both sysr_ems the field strengr_h and gas pressure are highly inr_erdependenr_ making the choice of r_i~e optimum parameters for operation very difficult. Despite r_he greater operating difficulties encountered with R.F. sysr_ems they generally are able to produce more uniform films, and are at present preferred in industry.
Definition of the Invention It is a principal object of r_he present invenr_ion to provide new methods for the plasma etching, substrate cleaning, or deposition of mar_erials by D.C.
glow or plasma discharge.
Ir_ is another principal objecr_ r_o provide a new apparatus for operation of such methods.
In accordance with the present invention there is provided a new mer_~:od for the plasma etching, substrate cleaning, or deposir_ion of materials on a substrate by D.C. glow or plasma discharge comprising the sr_eps of providing an enclosure having a gas inlet to ir_s interior and a gas outlet therefrom and having in its interior two spaced cathode electrodes esr_ablishing a plasma containing zone between them, and also having in its interior an intermediate anode electrode disposed in the said plasma-containing zone, the anode electrode being transparent or translucent to at least electrons of the glow or plasma discharge;
at least one of the car_hode electrodes constituting r_he said subsr_rate or having the subsr_rate attachable therer_o;
supplying to the enclosure interior at least. one precursor gas electrically decomposable to provide radicals of the required material; and providing bet.~een the anode electrode and the two cathode electrodes respecr_ive operating voltages such as to esr_ablish respective electric fields of strength sufficient r_o produce glow discharge decomposition of the precursor gas and procuction of a corresponding plasma in the said zone.
Also in accordance wir_h r_he invention there is provided new apparar_us for r_he plasma etching, substrate cleaning, or deposition of mar_erials on a subsr_rate by D.C. glow or plasma discharge comprising:
an enclosure having a gas inlet r_o ir_s interior and a gas outler_ there~rom and having in ir_s interior r_wo spaced car_hode electrodes establishing a plasma containing zone between r_hem, and also having in ir_s interior an intermediate anode elecr_rode disposed in r_he said plasma-containinc zone, the anode electrode being r_ransparenr_ or translucenr_ to at leasr_ elecr_rons of the glow or plasma discharge;
at least one of the car_hode elecr_rodes constituting the said substrate or having r_he substrar_e attachable r_herer_o;
means for sub plying r_o r_he gas inler_ at least one precursor gas electrically decomposable to provide radicals of the required material; and means for providing between the anode electrode and the r_wo cathode elecr_rodes respective operating voltages such as to establish respective electric fields of sr_rength sufficient to produce glow discharge decomposir_ion of the precursor gas and production of a corresponding plasma in the said zone.
Description of the Drawings Methods and apparatus which are particular preferred embodiments of the invenr_ion will now be described, by way of example, wir_h reference to the accompanying drawings, wherein:-Figures 1 through 3 are respective schematic and longitudinal diagrammatic cross-sections of firsr_, second and third embodiments;
Figure 4 is a graph of the current flow at different gas pressures through an inr_ermediar_e anode electrode with r_he vol rage applied ber_ween r_har_ elecr_rode and a cathode electrode; and Figure 5 is a graph of the rar_io ISH~IME of the current flows, also at different gas pressures, through a supplementary car_hode elecr_rode (ISH) and r_he intermediar_e anode electrode (IME) wir_h r_he voltage as in Figure 4.
Descripr_ion of the Preferred Embodiments The invenr_ion will be described inir_ially in ir_s application to methods of deposition of suir_able materials, and ir_s application r_o plasma etching and substrar_e cleaning will be described below. Referring now to Figure 1, a deposition chamber as employed for relatively small substrates consists of an open-ended outer metal cylinder i0 provided with removable end covers 12, each of which in this embodimenr_ also consr_ir_utes one of a pair of parallel cathode ~3~~ ~a~~
electrodes. The covers can be clamped r_ighr_ly in place by any suitable means which are nor_ illusr_rated, and in this embodiment have electrically insulating gasker_s 14 sandwiched between r_hem and the cylinder 10 for a purpose explained below. In this embodiment r_he cathode end plates do not r_hemselves constitute r_he substrate for deposition, and accordingly at least one of r_hem has a substrate element 16 mounted on its inside surface; in this embodiment both of the end plates are provided with such substrates. The central portions of r_he end plar_es receiving the substrates are depressed inwardly towards one another so r_hat they are more closely spaced than the remainder of the plates. The resultanr_ recesses at the outer surfaces are provided with respective elecr_ric hearers 18 supplied from a power source 20 r_o maintain the substrates ar_ a r_emperar_ure suitable for deposir_ion (usually abour_ 200°-400° in r_he case of amorphous hydrogenar_ed silicon). A second open-ended replaceable cylinder 22 of any suitable mar_erial is mounted coaxially within r_ha outer cylinder 10 r_o more closely define a plasma-containing zone 24 between r_he r_wo more-closely spaced portions of the two car_hodes; in use the inr_erior surface of this cylinder also becomes coar_ed wir_h deposir_ed material and can be replaced when r_he deposit becomes too thick, reducing very considerably r_he deposition on the inner wall of the our_er cylinder 10 and the need to clean or replace it.
The precursor gas or gases are inr_roduced inr_o the inr_erior of r_he enclosure via an inlet 26 and the depler_ed gas or gases removed therefrom via an our_ler_ 28. The procedures and apparar_us for r_he safe supply and conr_rol of these gases, many of which are pyrophoric or highly toxic, are well documenr_ed in r_he art and need nor.
be derailed in r_his application.
An anode electrode 30, which in r_his embodiment has the form of a flat wire open grid, is mounr_ed in the interiors of the two cylinders 10 and 22 by means of an elecr_rically insulating sr_ructure 32 passing r_hrough the cylinder walls so as to be parallel to r_he two parallel cathodes 12. A stabilised D.C. power supply 34 is connected to the anode 30 and r_he two cathodes 12 to apply suitable constant potential differences ber_ween them, and produce a corresponding electric field in the zone 24, which we refer to colloquially as a "saddle"
field owing to its characteristic profile between the r_wo cathodes.
The production of thin films using r_he processes and apparatus of the invention shows r_hat such glow discharge or plasma formation can be made to take place over a much wider range of gas pressures and at higher current densities than has been possible using prior arr_ D.C. systems. It is oar present hypor_hesis, by which we do not intend r_o be bound, r_har_ r_hese improved results follow from the facr_ that this parr_icular electrode configuration causes the available electrons r_o oscillate in the central zone 2~, as indicar_ed by the broken arrows 36, passing freely through r_he electrode 30 which is transparent or at least translucenr_ to them, promoting an ionisation r_har_ is reyatively insensir_ive r_o geometry and gas pressure. This saddle field configuration promotes ionisar_ion of the gas close to the anode electrode, while the oscillating trajectory of the electrons increases the effective path lengr_h for ionising collisions, facilitating the formation of high currenr_ discharges at relatively low pressures. The saddle field also causes positively ionised racicals to be accelerar_ed towards the our_side electrodes, a:.d a greater number of charged radicals are produced, so r_har_ the speed of deposir_ion is greater and is not so dependent as r_he prior art D.C.
process on r_he diffusion and drifr_ of radicals; there is therefore much more e~~icienr_ ur.ilisar_ion of the precursor gas.
_ g _ Since the methods and apparatus of r_he invention are D.C. operated the sensitive r_uning of the prior art R.F. apparatus is not required. If bor_h car_hodes are employed as substrate supports then r_he anode 30 preferably is centrally disposed and the respective portions of the saddle field are symmetrical about the anode. If only one substrar_e is provided then the field need not be symmetric, bur_ instead can be asymmetric with the stronger portion toward the substrate supporting car_hode, r_he asymmer_ry being produced either mechanically by placing the anode closer r_o the respective cathode, or electrically by increasing the potential difference between the two electrodes. The two cathodes 12 are insulated from the enclosure 10 and from one anor_her by the gaskets 14 to permit this electrically-produced asymmetry.
It is undesirable for r_he plasma containing zone 24 to extend so far as to include r_he subsr_rate since the ionised radicals then impinge on r_he growing film with substantial velocities and may damage it, as described above. This can be avoided while employing relar_ively high fields by r_he embodiment of Figure 2, in which two subsidiary flay. open grid cathode electrodes 38 are disposed close and parallel to the respective substrates 16. The negative potenr_ials are applied to r_hese subsidiary electrodes and the two end plates 12 are grounded, and may be at a small negative or positive por_ential relative to the subsidiary electrodes. The plasma is now conLined to r_he zone ber_ween the r_wo subsidiary electrodes, which are however transparent or ar_ least r_ranslucenr_ r_o bor_h r_he charged and neur_ral raaicals, so that r_hey can pass freely therethrough r_o the substrate. A small posir_ive por_enr_ial on the end plate 12 will result in a decelerating field ber_ween each electrode 38 and its end plar_e, reducing r_he impact of the ions thereon. The two electrodes are mounr_ed on the _ g _ cylinder 22 by respective electrically insulating structures 40 so that their potentials can be adjusted individually; in the embodiment illustrated they are strapped r_ogether to be at r_he same potenr_ial.
Figure 3 is an example of another apparatus characterized in r_har_ only a single substrate 16 is provided. A movable shutter 42 is provided to permit the substrate r_o be screened for selective deposir_ion of r_he material thereon. The car_hode electrode 12 not associated with a subsr_rate is of ring formar_ion, and a removable crucible 44, the contents of which are heated by an induction heater 46, is provided to permit coevaporation of r_he material in the crucible during the deposition.
As described above one of r_he mosr_ commercially interesting processes involving plasma deposir_ion is r_he direct formar_ion or r_::in films of hydrogenar_ed amorphous silicon, and the methods and apparatus of the invenr_ion have been used very successfully in the production of such films. As is usual r_hese films have been produced by the dissociation o_ silane gas (SiH4), and ir_ has been found possible r_o operate effecr_ively wir_h gas pressures as low as 10 milliTorr, as compared r_o about 100 milliTorr found r_o be the minimum wir_h convenr_ional D.C. mer_hods. Relar_ively high growth rates of about 5-10 Angstroms/sec. were obtained, as compared with about 2-3 Angstroms/sec, with t::e prior art. The hydrogen incorporation was controlled by coevaporar_ion of silicon, and films were obr_ained exhibiting high photoconductive gain exceeding 104 uncer Air I~lass 1 illuminar_ion wir_h a wide range of hydrogen content from about 5 to about 25 ar_omic percent. The films obtained exhibited good adhesion and activates conduction over a wide r_emperature range from abour_ 200°K r_o abour_ 450°K.
Figure 4 is a graph showing wir_h the producr_ion of such silicon films the current flow r_hrough r_he anode 1341 18~~
electrode 22 with the volr_age applied between r_he anode and the cathodes ar_ different silane gas pressures. The individual test result values are plotted and are not connected since they converge so much in the range 600-1000 volts as to possibly be confusing. This shows that r_he results obtained at r_he low pressure of 10 milliTorr were strictly comparable wir_h those obtained at the higher pressures; the highest pressure usable with this apparatus was below 500 milliTorr, since at this pressure an apparent arc-type discharge was esr_ablished to give excessively high current. flow and r_he consequent high possibility of deleterious er_ching. The mosr_ important characr_eristic established by r_hese test results is the relative independence of potential applied (and hence field strength) and r_he gas pressure r_hat could be used, permir_ting a much wider range of individual adjustmenr_ of these different parameters than has been possible hitherto. Ir_ may be nor_ed r_har_ this was a laboratory-type apparatus employing electrodes of about 13 cm diamer_er and abour_ 4-6.5 cm spacing between each car_hode and r_he anode. As is indicar_ed by the graph, voltages of up to about 1500 volts can be employed to give field gradienr_s of about 300 volts per cm., although the more usual value for silane is abour_ 800 volts, or about 160 volts per cm. With alr_ernar_ive strucr_ures it is possiole r_o operate r_he processes of r_he invention with gas pressures of about 10 r_o about 300 milliTorr, preferably in the range 30-50 milliTorr, with an absolute maximum at 500 milliTorr. These lower pressures have the advantage of considerable economy in the use of the expensive precursor gases, which is of course of particular commercial inr_eresr_, They also result in increases in safer_y and in r_he handling and disposal of the gases. The application of the invenr_ion also permits higher discharge current densities produced by the application of higher fields ber_ween r_he electrodes. For example, in the prior art it has been customary to operate with discharge current densities of from about 10 to about 50 microamperes per cm, sq., corresponding to fields of about 150 to about 350 volts per cm. With the methods and apparatus of the invention it is possible to operate with current densir_ies in the range of about 10 and 500 microamperes per cm. sq., and with corresponding field strengths in the range of about 100 to 300 volts per cm.
Figure 5 is a graph, again produced in r.he formation of thin amorphous silicon films from silane, showing the ratio of the currents obtained at the subsidiary cathode electrodes 38 (ISH) and at the middle anode electrode 22 (IME) with the voltage applied to the electrode 22 (VME) at various gas pressures. All three plots obtained show a peak in these rar_ios ar_ a voltage t::at increases with incrzasing pressure and show than high deposition efficiencies are available ar_ the low c'_scharge pressures and low voltages made possible by the ==esent invention.
The desc~ipt:on of the invention has been in general terms since i~ is applicable to a wide range of deposition materials, even though .specific examples have been given only for t'.~.e production of amorphous silicon layers employing sila::e. Examples of other processes are plasma etching, subst_ate cleaning and the deposition of carbon, germanium anc silicon nitride. Plasma etching is a chemical-type process in which surface atoms are removed by reaction w'_~h r_he active radicals of r.he plasma; it is useful _~r example in producing grooves in integrated circuit deign. Substrar_e cleaning is a mechanical-type process employed to remove unwanr_ed surface ar_oms by traps°erring kiner.ic energy of r_he plasma radicals to t::ese atoms. Reference may also be made to the next book =eferred to above:

The drawings show the apparatus in a particular attitude, but the processes employed are of course not attitude-dependent and the apparatus can be employed in any other appropriate atr_ir_ude and configurar_ion. The electrode 30 and the electrodes 38 are described and illustrated herein as open grids; in the laboratory-type apparar_us employed such a grid is for example of thin wires of 0.3 mm diameter spaced approximately 1-2 cm apart, and are adequate to apply r_he required field while being translucenr_, i.e. sufficienr_ly r_ransparent to the plasma electrons. A plain open ring electrode can also be used provided the resultant non-uniformity of the field can be tolerated. In either laboratory or commercial pracr_ice it will usually be difficult and inconvenient to make the apparatus as illustrar_ed sufficiently gas-tight to operate readily at r_he low operating pressures involved, and it will therefore usually be compler_ely enclosed in an evacuable chamber whose structure and arrangement does not consr_ir_ute part of r_he present invenr_ion. Appropriate sr_rucr_ures will be readily apparenr_ to r_hose skilled in r_his particular art, and therefore need not be specifically described and illustrated herein.

Claims (3)

1. WE CLAIM
   A method for the plasma etching, substrate cleaning or deposition of materials on a substrate by D.C. glow or plasma discharge comprising:
   providing an enclosure having a gas inlet to its interior for precursor gas ionizable by the D.C. glow or plasma discharge to provide radicals of material that will perform said plasma etching, substrate cleaning or material deposition, and having a gas outlet therefrom, the enclosure having in its interior;
   an anode electrode permeable to electrons;
   two spaced first cathode electrodes on opposite sides of the anode electrode, at least one of these first cathode electrodes constituting said substrate or having the substrate attachable thereto; and a supplementary cathode electrode which is permeable to ions and which is disposed between the anode electrode and a respective one of the first cathode electrodes and relatively close to the respective first cathode electrode;
   the two cathode electrodes on opposite sides of the anode electrode establishing a plasma-containing zone between them, supplying at least one precursor gas to the enclosure interior; and providing between the two cathode electrodes and the anode electrode respective operating D.C. voltages such as to establish between them a saddle configuration electric field of strength sufficient to produce the ionization of the precursor gas, the saddle configuration field attracting electrons toward both sides of the anode electrode which because of its permeability permits electrons to pass therethrough, whereby electrons oscillate back and forth through the anode electrode and produce ionization of the precursor gas and generation at least in the immediate vicinity of the anode electrode of corresponding glow or plasma discharge that is a source of radicals moving to the substrate to perform said plasma etching, substrate cleaning or material disposition.
2. 2. A method as claimed in claim 1, including supplying an additional material for deposition to the enclosure interior by evaporation thereof within the enclosure interior.
3. 3. A method as claimed in claim 1, wherein an operating voltage is provided between the anode electrode and at least one supplementary cathode electrode cooperating with a first cathode electrode that constitutes a substrate or has a substrate attachable thereto such as to produce a current density in the range of about 10 to 500 microamperes per cm.
   sq.