Plasma processes under vacuum conditions

Vacuum/volume 41/numbers 7-9/pages 1957 to 1961/1990 0042-207X/90S3.00 + .00 Printed in Great Britain © 1990 Pergamon Press plc

Plasma processes under vacuum condi t ions A Bubenzer, Phototronics Solartechnik GmbH, D-8011 Putzbrunn, FRG

and

J P M Schmi t t , Solems, F-91120 Palaiseau, France

In the first part the basics of low pressure plasma processes are briefly reviewed considering excitations by gas phase collisions and electrical potential near surfaces in contact with the plasma (sheath formation). In the second part we discuss the special situation given by the combination of vacuum technology and low pressure plasma technology. The impact of plasma technology on pumps and vacuum vessels is discussed taking into account in particular degassing, impurity situation in the gas phase and gas flow. This is done with a special emphasis on large area industrial machines. In the third part we present a new type of design for a machine for plasma enhanced CVD. This so-called plasma box type reactor is a good example for vacuum technology adapted to a plasma process. The plasma box reactor is designed for large area industrial applications, ensures high thin film uniformity and a uhv-type gas phase purity. The plasma box reactor was tested by deposition of amorphous silicon (a-SO (for solar cells or thin film transistors), first results of these a-Si depositions are presented.

1. Introduction nm

The topics of this paper overlap on two quite extended fields, i.e. vacuum technology and plasma chemistry. Both fields com- bine into the still very large topic of low pressure plasma processing. We will further confine the scope of our discussions to solid material processing excluding synthesis of gas phase compounds such as hydrazyne of polysilanes. Most low pressure plasma processes are performed in the range of 1 to 100 Pa. The plasma generator electromagnetic energy is fed into the free electrons; the electrons pick up kinetic energy up to energy of 10 eV or more. Then by inelastic collisions with the background gas the fastest electrons generate ionization, many more electrons can induce into the gas molecules high levels of chemical activation. In a low pressure discharge, the diffusion coefficient is large and a large part of the activated species can reach the exposed surfaces where their extreme reactivity can be used for etching, modification or growth of the solid phase.

Figure 1 shows a wide variety of such processes. Depending on the nature of the feedstock gases and of the plasma reactor walls, on the plasma energy density, on the pressure, on the substrate polarization and the electrode geometry, the processes will lead to deposition or removal of an unbalance between the two cohabiting mechanisms.

The purpose of this paper is to draw attention of the vacuum specialist towards a new set of interesting problems associated to plasma processing such as measuring and stabilizing the flow, pressure and composition of the feedstock gases and the actual substrate temperature, such as pumping toxic and/or corrosive effluent gases, such as keeping a low level of contamination by particulates and/or chemical impurities despite the high reactivity of the plasma.

• lO

t~ 0.1

o.i Z o

0 1 Lu

0 10 0 Z Z 100

~S ('AI ~.SPUTTERING " ~ ~ / ' ~(R EACT IVE) .~ '~

/ \ 1 T ~ . ) O DE) <~'-. (~ " ~ ~ PLASMA ENHANCED. -~"

HARD C \ \ ~ . = f CVD <J ~',,~Si3 N4 ~ ) aS, ;H " ~ r . - - - . PLASMA """.,

1 \ / j . , T . , O , . G f

/JT- EL SU. ACe:" ~ A L L T R ~ E R ~ T RE AT MEN T "" > i

~ A L O G E N E E TC<R4 NG U

I ~ ALL "~h"" .OXYGEN . ' " IRC SHING ~- . . ~Lu

Z~

[ / MAGNETRON\ / "'"-~ ~ ,/~CATHODE \ / I " " X""

~ _ J EROSION ~ j <.- ~O. I I I I ~ I ] " - . "T-

0.1 1 10 100 1000 104 Pascal TOTAL PRESSURE

Figure 1. Typical parametric range for low pressure plasma processing of solids.

2. Basics of low pressure plasmas ~

Terrestrial plasmas are basically out of equilibrium. In low temperature plasma a high energy content is maintained in the free electron kinetic energy (ranging from 0.1 to 10-20 eV) via the driving electric field ac or dc injected into the plasma by antennas or electrodes. Free electron density is ranging from l0 s to 10 =° cm 3, the positive ion density is equal since the bulk of plasma is electrically neutral. The background gas pressure

1957

A Bubenzer and J P M Schmitt: Plasma processes under vacuum conditions

range being about 1 to 500 Pa, the degree of ionization remains generally well below 10 -3 . Electrons are transferring their energy to molecules, the external orbital electrons being excited to higher energy levels, a process leading for a molecule like methane CH 4 to various events 2 - - ionization CH4 ~ + e - - dissociative ionization CH2 + 2H + e, etc. - - dissociative excitation CH 3 + H* (photon) - - dissociation CH + 3H, etc. - - rovibrational excitation CH~.

In molecular plasmas molecular fragments (radicals) and highly activated molecules (metastables) are produced at a large rate, however the density fraction of chemically unstable vs stable neutrals is generally below one percent. Activated species, ions or radicals are generated in the plasma phase and diffuse out towards the walls. Depending on their reactivity they experience on the way secondary reactions with the background gas. Ions have very large reaction cross sections and they generally go through many secondary reactions 3. For radicals the situation varies depending on species and processes, however one can state that a very large flow of reactive species, mostly neutral, are reaching the wall. Chemical activation is not the only mechanism affecting the surface. Photon irradiation 4 can also activate the surface and most important ion impact 5.

Indeed, due to the formation of a sheath at the transition between the plasma and the solid, ions are accelerated up to kinetic energies which can reach several keV in sputtering processes and several 10eV in reactive processes. The sheath formation as shown in Figure 2 is due to the much larger mobility of free electrons compared to positive ions. A voltage barrier spontaneously forms at the plasma edge reflecting most electrons and accelerating out ions. The sheath potential drop adjusts in such a way that both positive and negative current compensate 6. In RF excited plasma the average sheath drop can be close to half the RF peak to peak voltage 7.

As shown in Figure 3, ion impact on the surface can, by momentum transfer, induce expulsion or sputtering of surface atoms or molecules 5. It can also induce lattice damage enhanc- ing the surface reactivity s. In more complex processes like in RIE (Reactive Ion Etching) it can remove a passivation layer inhibiting reactive etching 9 (impurities transported from a photoresist for example).

Three examples of plasma processes are sketched in Figure 4. In Figure 4b the sputtering of a solid target by argon ion is a purely physical mechanism. This technique is widely used for metal. It can be made reactive by adding to argon a reactive gas ]° (02 for oxides, N 2 or NH3 for nitrides, HES for sulfide etc.). The reactive ion etching described in Figure 4c is able in many cases to achieve anisotropic etching of semiconductor grade materials It. The softer plasma etching process is done without ion bombardment causing less surface damage of the semiconductor devices. Both etching techniques are generally

~ E C T R O N S

,o s

Figure 2. Mechanism of formation of a sheath (shaded zone) at the interface between a solid and a plasma.

/ A ~ % Etching

~ ° ~ R A D I C A L ~" ~Deposi t ion ~-= _ ~,~v'v,v'v PHOTON

~- Surface damage

[] Sputtering

Figure 3. Mechanism of interaction between a solid surface and a reactive plasma.

Sill4 Ar,02, Sn ~ i O 2

H 2 Ar CO2,SiF 4

Figure 4. Principles of (a) plasma deposition of hydrogenated amorphous silicon, (b) reactive sputtering of tin oxide films and (c) anisotropic etching of silica through a photoresist mask.

based on chlorine or fluorine compounds. The plasma deposition of Figure 4a is based on the destruction of a carrier gas (or a mixture of gases) to form a solid. Semiconductive amorphous hydrogenated silicon ~2, hard carbon ~3, alloys or insulators can be prepared by combination of gases such as Sill4, CH4, GeH4, NH3, N20 etc. Adding boron or phosphorus carriers (B2H6, PH3) in minute quantities can also provide semiconductor doping ~4.

3. Vacuum technology for low pressure plasma processes

Figure 5 shows a typical plasma reactor, a vacuum system used for a low pressure plasma process. We are considering the most common case where the plasma is generated between two parallel plates, one of which is electrically grounded, the other being connected to an electrical power supply. The substrate to be processed is attached to one of the electrodes. Heaters are typically needed to supply the required process temperature. The process gases sustaining the plasma enter through the gas inlet via a flow controller. At the pump side, the effluent gases go through a throttling device needed to attain the required

I CONTROL \ ' ,-_

ITEMPERATUREI/ I MATCH [ ~ ~ l Ill

[GENERATOR I ~

Figure 5. Typical set-up for a plasma process system.

1958


process pressure. In the case of toxic or corrosive gases a system for treatment of the exhaust gases will be needed ~5.

Low pressure plasma processes taking place in such a system are imposing a number of important boundary conditions on the vacuum technology.

Process gases. Process gases are typically needed at a high flow. In large industrial-type machines the flow can be in the range of liters per minute. The gas flow to be pumped away may even be higher due to dissociation processes and chemical reactions (e.g. S iH4~Si +2H2, i.e. doubling the volume) within the plasma and may contain large amounts of hydrogen. Many process gases are poisonous (PH3, As2H3), corrosive (C12, NH3) , flammable or spontaneously flammable (Sill4). The effluent stream of gases coming out of the plasma reactor can sometimes be even more dangerous due to chemical reactions generating highly poisonous compounds 15 such as COC12 or extremely corrosive ones such as HCI or HF.

Consequences: industrial-type processes using high gas flows will typically go for roots pump systems. However, provisions have to be taken to prevent operation in the end pressure regime to inhibit oil backstreaming. An interesting alternative for lower flows are specially designed turbomolecular pumps with corrosion resistant blades and lower speed 16 run with a throttling valve.

If oxygen is used as a process gas, there is the risk of explosion or polymerization of the normal mineral pump oils--special fluorinated oils such as Fomblin should be used LS. However, a general use of these special oils for all plasma processes using reactive gases is not mandatory.

Any type of cryogenic pumping, however, should be banned except for non-reactive feedstock and effluent gases, since any accidental warm up could cause a catastrophe. Great care finally has to be taken in multipurpose use of pumps: remaining gases from previous processes are dissolved in oil and may react with the recent one and lead to an explosion (e.g. Sill4 and air).

For the vacuum vessel materials which are resistant to etchant gases have to be chosen, stainless steel may not be sufficient in etching processes, fairly pure or anodized alu- minum is a good choice. Other components like pressure gauges have to be chosen accordingly.

Particulate formation. Due to homogenous gas phase reactions, particulates may form in the plasma. Typically these particulates are very hard materials (a-Si, SiC, SiN) with strong abra- sive properties.

Consequences: pumps have to be protected e.g. by mounting them outside the line of sight from the plasma. However, problems due to particulates can be so severe that the best recipe is to work outside the parameter range of particulate formation.

Uniformity of gas composition. Since the process gas composition is typically modified due to the plasma process, care has to be taken to prevent a process gas depletion along the path of the gas in the machine. This could cause the process to be non-uniform. Such an effect is typically a major problem in large area (m-size) plasma treatment.

Consequence: the gas feed has to be arranged in such a way that each part of the substrate is supplied with fresh process gas, as shown in Figure 6, by means of a porous electrode

GAS II INLET

LA

GA INLET

e . . . . . . . . . ~

, i , , IZ

' R F

- E , ! im i

! ', ,, [,,1 I S

PROCESS PUMP

Figure 6. Conventional capacitive plasma reactor (left) and pressurized plasma box reactor (right). In the first case the degassing impurity flow (gray arrow) mixes with the process gases while in the second case most of this flow is pumped out,

distributing the gas uniformly. This may need sophisticated engineering, however, it has given excellent results.

Impurities. All deposition-type plasma processes are extremely sensitive to residual gas impurities in the plasma reactor, since the growth of a plasma deposited layer is fed from the gas phase. Typically these residual gases contain mostly undesirable elements like oxygen or carbon and depending on the d ~ o s i - tion rate in a PECVD process can cause considerable thin ]ilm contamination m7 (this will be discussed in more detail in Chap- ter 4). In addition one has to keep in mind that all plasma processes are taking place at such high pressures (mbar range) that pumping must be throttled during process, hence higher residual gas pressures are allowed to build up (since their saturation vapour pressures are typcially much higher) in the vacuum vessel.

Consequences: despite the fairly high gas pressure during plasma processes the residual gas situation particularly in deposition processes demands at least high vacuum standard for the vacuum vessel. This will lead in most cases to the requirement of a high vacuum pumping stage for cleaning the system. This stage will be required in addition to the process gas pumping stage which should automatically be interlocked against accidental pumpdown into the oil backstreaming pressue range. An interesting alternative for this problem could in the lower flow range be a plasma-type turbomolecular pump which could serve both purposes.

4. The purity challenge in PECVD

As stated above, the high purity (up to 99,999%) feedstock gases for depositing materials in PECVD will within the system be inevitably mixed with gaseous impurities desorbed from the walls of the vacuum system. This is a veritable problem for presumably high purity semiconductor thin films like e.g. a- Si: H.

How bad the situation really is, is shown in the following estimate of the process gas contamination in a typical HV PECVD system.

Let us assume a fairly clean HV-system with a base pressure of Po = 2 x 10 -4 Pa, using a Co = 0.3 m 3 s - ' unthrottled turbomolecular pump; the process pressure is P = 10 Pa at a flow of process gas F = 100 seem, a typical value for development size

1959


deposition machines. We assume that vacuum leaks in the chamber can be neglected and that the base pressure of the system is due to the degassing flow from a normal vacuum vessel (mostly H20, CO, oil e t c . ) - - a fairly realistic situation. Then the contamination factor r of the process gas phase is in the order of the ratio of degassing pressure P* and process gas pressure P:

r = p , p - i = P o C o F - i

Inserting the values chosen above we end up with a contamination ratio of r = 4 × 10 -4. It is a worldwide experience ~7 that for example a -S i :H deposited in standard vacuum equipment has an oxygen content in the range of 0.1 at%. This value, somewhat systematically larger than the estimated gas pressure ratio, suggests a gettering-like effect at the growing film active surface for oxygen-containing impurities. For semiconductive thin films this impurity level is very unsatisfactory, it may change the position of the Fermi level and, for oxygen relative content above 2 × 10 -5, a-Si long-term stability is affected.

Two strategies can be pursued at this point. (a) reduce degassing: this is accomplished by baking and

thus degassing the system. If this has to be done with the whole vacuum vessel, we end up with an uhv system. The benefit was demonstrated by various teams ~s but going to uhv is not acceptable costwise in industrial processes.

(b) increase the flow of process gas: thereby residual gas impurities would be diluted and thus reduced. Typically, however, this solution also cannot be accepted costwise due to the expensive process gases and due to strongly enlarged pumping equipment.

We propose an industrially acceptable alternative ~9, a PECVD system of the type shown in Figure 6. In this design the plasma is confined to the volume of a so-called plasma box which is placed in a conventional vacuum vessel. This box can easily be degassed separately from the outer vacuum vessel and remain clean if it is kept at an elevated temperature (150- 250°C) which is fortunately needed for many semiconductor deposition processes anyhow. The box is slightly leaky but keeps up a pressure gradient of one or two orders of magnitude between the high process gas pressure inside the box and the outer vessel connected permanently to a high rate pump. A completely vacuum-tight plasma box is unpractical since e.g. in a multichamber system substrates have to be transferred in and out. The fraction of the dirt degassing flow from a conventional HV outside vessel which will enter the plasma box is the ratio of the box leakage conductance to the large pump conductance. This ratio amounts easily to 20-200. One could call this a "poor man's" uhv system.

5. A new approach to industrial plasma processing: the plasma box reactor

The above described plasma box system was developed in our company in cooperation with Arthur Pfeiffer Vakuumtechnik in Asslar, FRG, as a tool for PECVD of large area amorphous silicon (a-Si :H) based solar cells on a high volume industrial scale. Although the impurity situation was one of the most important aspects, the plasma box system was developed be- cause we came to the conclusion that a machine which is fully adapted to large area PECVD processes was not available on the market. Such a machine was required to respond well to all boundary conditions which are described in Section 3.

We could clearly show that in such a system the effect of reactive gases is confined to the plasma box itself, a -S i :H deposition and (accidental) generation of particulates is clearly restricted to the plasma box; this makes any kind of cleaning process much easier. Sophisticated engineering parts of the outer vacuum vessel like the transfer system of gate valves were not affected at all by any kind of coating or particulates.

Large area uniformity of a -S i :H PECVD was shown to be excellent and moreover could be achieved within an unusually wide power and pressure parameter range. The thickness uniformity of a 400 nm thick a -S i :H coating was shown to be better than 5% over an area of 48 x 58 cm 2. This result is due to the gas feed via a porous electrode and to the well-defined regular shape of the plasma within the box.

Improvement in material purity could be clearly shown in a -S i :H layers deposited with and without plasma box 2°. Oxy- gen content (as determined by SIMS) in a -S i :H layers from a plasma box reactor is reduced by a factor of 50 as compared to the equivalent coating deposited in the same but open plasma box. These data were taken using a machine without load lock. In our machine with a load lock the effect is less pronounced since the outer chamber around the plasma box is cleaner being protected by the load lock. After 3 weeks of continuous HV pumping--i .e , no venting of the process chamber - -and about 30 a-Si deposition runs, a quantitative SIMS-analysis on an a-Si layer was performed. The layer was halfway deposited with the plasma box in full operation and halfway with an open plasma box. Even after such a long pumping time for the outer chamber, the oxygen concentration [O] improved by a factor of three with the plasma box in operation. The lowest value achieved was [O] = 5 x 10 ~8 cm -3. This value is close to the [O] = 2 x 101Scm -3 achieved by Tsuda e t a121 from Sanyo using a "super chamber" uhv-system.

6. Summary

We have given an overview on low pressure plasmas for solid material processing. The most important problems of the vacuum technology used for realization of industrial low pressure

Table 1.

Technical problems in Solutions in the plasma box concept low pressure plasma processes

Gas phase purity in PECVD

Corrosive gases

Parasitic deposition and formation of particulates in PECVD Uniform plasma processing in large area applications Heating

Potential reduction of impurities by factor 20-200 in comparison to a conventional HV reactor Corrosion effects restricted to plasma box only Confined to plasma box only, therefore easy cleaning

Easily achieved due to porous electrode and regularly shaped confined plasma Isothermal uniform process environment easily achieved due to uniform multireflection heat radiation from outside the box

Vacuum technology well-adapted to low pressure plasma processes: important features of the plasma box reactor

1960


plasma processes are pointed out and technical suggestions are given how to solve them. We finally offer a vacuum technological concept, the plasma box, which is well adapted to plasma processes, in particular to those for industrial applications.

This concept is particularly useful for achieving higher purity materials in plasma CVD processes. The essential features of the plasma box are listed in Table I.

References

I A v o n Engel, Ionized gases, Clarendon Press, Oxford (1988); B Chapman, Glow discharge processes, Wiley, New York (1980); R N Franklin, Plasma Phenomenon in gas discharges, Clarendon Press, Oxford (1976). 2 H F Winters, In Swarm Studies and Inelastic Electron-Molecule Collisions (Edited by L C Pitchford, B V McKoy, A Chutjan and S Trajmar), p. 347, Springer, New York. 3 M Henchman, In Ion-molecule Reactions (Edited by J L Franklin), p. 189, Butterworths, London (1972). 4F A Houle, Appl Phys A41, 315 (1986); H H Gilgen et al, In Laser Processing and Diagnostics (Edited by D Bauerle), p. 225, Springer, Berlin (1984).

P Sigmund, Phys Rev, 184, 383 (1969) and 187, 768 (1969). 6S A Self, Phys Fluids, 6, 1762 (1963).

7 M S Butler and G S Kino, Phys Fluids, 6, 1346 (1963). s D L Flamm and V M DonneUy, Plasma Chem Plasma Process 1, 317 (1981). 9 A K Kolfschofen, R A Haling, A Hating and A E de Vries, J Appl Phys, 55, 3813 (1984). I°W W Holzen, J Vac Sci Technol, 12, 99 (1975). )1 A Reinberg, Proc Syrup Etching, Electrochem Soc, 91, (1976). 12j p M Schmitt, JNon Cryst Solids 59 & riO, 649 (1983); J Perrin, In Proc 18th Int Conf Phenomena lonized Gases, Swansea, Wales (Edited by W T Williams), p. 54, Adam Hilger Publishers, Bristol, U.K. (1987). 13 A Bubenzer, B Dischler, G Brand and P Koidl, J Appl Phys, 51, 4590 (1983). )4 W E Spear and P G Le Comber, Solid State Comm, 17, 1193 (1975). 15j F O'Hanlon and D B Fraser, J Vac Sci Technol A6, 1235 (1988). 16 j Henning, J Vac Sci Technol A6, 1196 (1988). 17 D E Carlson, In Tetrahedrally bounded amorphous semiconductors, (Edited by D Adler and H Fritzsche), p. 165, Plenum, New York (1985); F Jansen and D Kuhman, J Vac Sci Technol A6, 13 (1988). I sc C Tsai et al, J Non Cryst Solids 59 & (sO, 731 (1983); S Tsuda et al, Japan J Appl Phys 26, 33 (1987). 19 US Patent 4798739, Jan 1989. 2o j p M Schmitt, J MSot, P Roubeau and P Parrens, In Proc 8th EEC Photovohaic Solar Energy Conf, Florence, May 1988, p. 964, Kluwer Academic Publishers, Dordrecht, The Netherlands (1988). 2~ S Tsuda, T Takahama, H Tarui, K Watanabe, N Nakamura, H Shibuya, S Nakano, M Ohnishi, Y Kishi and Y Kuwano, In Proc 18th IEEE Photovoltaic Specialists Conf, p. 1295, Las Vegas (1985).

1961

Documents

Plasma processes under vacuum conditions