FR2677841A1 - Reactor for gas-phase plasma deposition of inorganic compounds on a polymer substrate - Google Patents

Abstract

The reactor includes a surface-wave plasma tube (13, 15) whose end (15) extending in the chamber (1) of the reactor has a profile (16) which is progressively flared and has an end opening surface covering the substrate (6) to be coated, the means for injecting the gas which is a precursor of the compound to be deposited including a plurality of injection orifices (18) situated substantially in a plane extending at a small distance from the end opening of the plasma tube, parallel to the latter and substantially regularly distributed in line with the substrate support (5) fitted with cooling means (30, 31) and connected to a radio-frequency generator (32) for modulating the ion bombardment of the substrate by virtue of the self-polarisation of the substrate under the effect of the radio-frequency field. Application in particular to the formation of protective and filtering layers on optical polymer substrates.

Description

Reactor for plasma deposition in gas phase of inorganic compounds on a polymer substrate
The present invention relates to a reactor for plasma deposition in the gas phase of inorganic compounds, in particular based on silicon, on a polymeric substrate, in particular optical, of the type comprising a sealed enclosure in which is disposed a substrate support, a plasma wave tube. surface, having one end extending in the enclosure, plumb with the substrate support, the tube being connectable to at least one source of plasma gas, typically and / or one or more nitrogen carrier gases , oxygen or carbon, and being associated with means for exciting these gases, and means for injecting at least one precursor gas of the inorganic compound disposed between the end of the tube and the substrate support.

 Such a reactor, of the post-discharge silicon precursor injection type, is described in document FR-A-2,614,317, in the name of the Applicant. The reactor described in this document is particularly suitable for the deposition at low temperature of silicon protective and / or filtering compounds on an optical polymer substrate, in particular made of polycarbonate, for the production of high performance optical elements, but does not allow to carry out rapid and uniform deposits on large substrate surfaces, nor to easily control the microstructure and the level of intrinsic stress of the layers.

The object of the present invention is to provide a reactor of the type defined above of flexible and easily controllable use, which is capable of carrying out substantially homogeneous deposits on larger substrates and with increased deposition rates, which can reach 0.2 ym per minute, as well as independent control of the microstructure of the layers and their constraints
x intrinsic.

 To do this, according to a characteristic of the invention, the end of the plasma tube extending into the enclosure has a profile which widens gradually and has a terminal opening surface at least equal to that of the zone of substrate to be coated.

 According to another characteristic of the invention, the injection means comprise a plurality of injection orifices directed towards the substrate support and situated substantially in a plane extending parallel to the terminal opening of the column, at a short distance from the latter, not exceeding 15 millimeters, typically less than 10 millimeters, the injection orifices being preferably substantially evenly distributed directly above the substrate support.

 According to another more particular characteristic of the invention, the substrate holder is connected to a radio frequency electric generator.

 With such an arrangement, a significantly enlarged diffusion plasma is obtained which remains radially homogeneous at a short distance from the substrate support where the deposit precursors are injected, the ion bombardment of the substrate being modulated easily and independently of the creation. precursors by self-polarization of the substrate under the effect of the radiofrequency field, which makes it possible to achieve a relatively uniform polarization of the surface of the substrate, even when it is of left shape.

Other characteristics and advantages of the present invention will emerge from the following description of an embodiment, given by way of illustration but in no way limiting, made in relation to the appended drawings, in which
FIG. 1 schematically represents in vertical section a reactor according to the invention,
- Figure 2 is a plan view of the injector of Figure I
- Figure 3 is a view similar to Figure I showing a variant of the injector; and,
- Figure 4 is a view similar to Figure 2 of the injector of Figure 3.

 In the embodiments shown, the reactor comprises a cylindrical enclosure 1 open at its upper end and in the bottom 2 of which is mounted an evacuation pipe 3 connected to a vacuum pump 4. In the enclosure is mounted centrally, spaced from the bottom 2, a metal substrate support 5 for a substrate 6 to be coated, in this case in the example shown, a "glass" correcting glasses of polycarbonate or polyallyldiglycolcarbonate (CR 39).

 On the edge of the upper opening of the enclosure 1 is mounted, with the interposition of a seal 7, a metallic annular flange 8 supporting, by a metal gas supply pipe 9 passing through it in a sealed manner, a metal injector structure generally designated by the reference 10. On the flange 8 is mounted, with the interposition of a seal 11, a bell 12 comprising an upper tubular part 13 in which is mounted, by a weld 14, an extension 15 of a plasma tube whose lower end gradually flares to form an inverted funnel 16 whose lower terminal opening surface extends parallel to the substrate support 5 and has an at least equal surface, typically greater than that of the area of the substrate 6 to be coated.

 The bell 12 and the tube extension 15 are advantageously made of quartz.

 The upper end of the tube 15, above the bell 12, is associated with a rectangular transverse waveguide 17, coupled to a microwave generator 18 and to a coaxial tubular guide 19 in an application arrangement called Surfatron / Surfaguide maintaining a surface wave plasma inside the tube 15, which is supplied, at its upper end, with a plasma gas and / or one or more gases carrying oxygen, nitrogen or carbon from 20 'tanks, 20 through a conventional expansion, distribution and flow control system. Conventionally, the tubular guide 19 comprises an annular tuning piston 21 and the rectangular guide 17, a tunable short-circuit piston 22, the two pistons 21 or 22 being movable manually or, preferably, by remote control.

 The injector structure 10 must make it possible to distribute the flow of silicon carrier gas uniformly in a section perpendicular to the axis of the reactor so as to ensure uniform creation of the deposit precursors. The injection is therefore carried out in a delocalized manner in a horizontal plane cutting off the flow of plasma gas and oxygen and / or nitrogen, disturbing the flow of the latter as little as possible. Various solutions fulfilling this condition exist. Document US-A-4,692,343 describes a gas penetration device comprising radial tubular sections having openings on a part turned on the side opposite to that of the substrate. It is specified that these tubular sections may not all have the same length. The context of this application is however very different from the present case. All of the process gases are in particular injected by this device, while a radiofrequency plasma is created in a separate chamber opposite the substrate. This plasma does not intervene moreover in the deposition process which is purely thermal, but only serves to initiate the nucleation of the layers. The design details of the device, which should lead to the most uniform possible spatial distribution of the partial pressures of reactants in the chamber, are not specified. In particular, no mention is made of the importance of the diameter of the tubular sections as well as of the openings for controlling the gas flows. Likewise, the existence of buffer chambers, intended to achieve a quasi-equilibrium of pressure upstream of the tubular sections, is not specified.

 The above-mentioned document FR-A-2,614,317 describes a toric injector, the wall of which is pierced with a plurality of orifices 1 to 2 mm in diameter. The gas supply takes place at a single point on the circumference of the torus. With this device, a strong inhomogeneity in thickness and composition of the deposited layer is observed as soon as the transverse dimensions of the sample exceed 5 cm.

This is due to a significant variation in the gas flows injected between the different orifices, under the effect of the pressure drops. To overcome this, it is desirable that the carrier gas is in quasi pressure equilibrium in front of the orifices. This is possible if there is on the one hand a sufficient buffer volume just upstream of the orifices, and if these are calibrated to a sufficiently small diameter. In this case, all the orifices will inject equal gas flows into the enclosure.

 An improvement of the device described in US 4,692,343 therefore consists, as shown in FIGS. 1 and 2, of providing an injector structure with, in a metallic annular support 23, 26, one or more buffer chambers 24, 25 placing in communication, the means for supplying carrier gas 9 and a series of metallic tubular sections 27, extending radially inwards from the support 26 and each provided with several ejection orifices 28. These chambers may, although not exclusively, be of tonic shape, along the circumference of the deposition chamber.

Several toric chambers two, for example, can be superimposed, their interior volumes being in communication.

 However, one can do without these chambers always in the case where the tubular sections can themselves play this role by being dimensioned in a sufficiently bulky manner. In this case, however, it must be ensured that the tube structure does not disturb the flow of excited gas from the surface wave tube. The orifices 28 preferably open out facing the substrate holder.

 Another embodiment, represented in FIGS. 3 and 4, comprises several toric metal tubular rings, preferably at least three (40, 41, 42,), centered on the axis of symmetry of the enclosure and pierced with ejection orifices 46 facing the substrate holder. The internal volumes of these rings communicate with each other by a small number of radial tubes of substantially the same diameter 43, 44, 45 and the external ring 42 is connected to the means for supplying silicon carrier gas 26.9.

 As in the previous case, the O-rings can themselves act as a buffer chamber, unless separate chambers are provided, as before (24, 25).

 According to one aspect of the invention, the distance between the plane of the injection orifices 28, 46 and the opening surface of the roof 16 is less than or equal to 1 centimeter, the distance from the substrate 6 not exceeding 3 centimeters.

 This arrangement makes it possible to obtain a high concentration of excited species at the surface of the film in the process of growth, making it possible to improve the quality of the microstructure of the latter by compensating for the absence of thermal energy to assist the rearrangement of condensed atoms on the surface.

 These excited species are metastable atomic or molecular energetic states whose lifespan is much greater than that of ions, but such that their number very quickly becomes negligible if the substrate holder is far from the plasma excitation substantially beyond the prescribed distance. In the latter case, the layers deposited are of poor quality, with a diffuse appearance, low adhesion and resistance to abrasion.

 The ion density of the microwave plasma decreases very rapidly (almost exponentially) as soon as the inverted funnel 16 ends. At the level of the substrate holder, this density is low if not negligible. When no RF voltage is applied to the substrate carrier, the floating potential which results therefrom is clearly insufficient to induce ion bombardment of notable effect on the microstructure of the deposited layer.

 As can be seen in the drawings, the substrate support 5 has a box shape defining an internal cavity 29 supplied with cooling fluid, for example water, by a distribution pipe 30, the return of the cooling fluid s 'effecting through a pipe 31. The metallic structure of the substrate support 5 is connected to a polarization radio-frequency generator 32 via a circuit 33 for automatic impedance tuning. The RF generator 32 and the microwave generator 18 are advantageously electrically connected to a synchronization and control unit 34 (not shown). The substrate support 5 comprises, at its upper part, a thermocouple probe 35 for rough monitoring of the change in its temperature. According to a particular characteristic of the invention, to achieve a relatively uniform polarization of the surface of the substrate 6, when the latter is of left shape, provision is made to attach to the substrate support 5 a removable metallic imprint 36 matching the shape of the substrate 6 to be coated and ensuring electrical and thermal contacts between the substrate 6 and the substrate support 5.

 According to one aspect of the invention, to meet the severe specifications imposed by the specifications of end users, the films constituting the coatings must be deposited with precise control of their microstructure and their intrinsic constraints. This is the case in particular for the hard structural layer of silicon nitride, which will invariably be deposited in the presence of radiofrequency polarization.

 Under the conditions according to the invention, the effect of the radiofrequency field is to initiate a second plasma between the support 5 of the substrate 6 and the sheet of tubes (27, 40-42) of the injector structure, the latter being comprising substantially, from an electrical point of view, like a solid electrode. This radiofrequency plasma therefore provides part of the charged species responsible for the self-polarization effect of the substrate support, but this effect is also influenced by the microwave plasma which injects a significant amount of charged particles between the RF electrodes. The contribution of RF plasma to the generation of deposit precursors is negligible compared to that of microwave plasma.

 In addition, the deposition rate is mainly controlled by the incoming flow of silicon precursor gas, and it is possible to vary slightly the location of the limit of the luminescent zone in the surface wave plasma tube, by acting on the power. incident microwave and tuning conditions, without significantly changing the growth rate of the layer.

 Due to the very rapid variation of the charge density in the microwave plasma outside the surface wave structure 15, 16, it is thus possible to modulate significantly the flow of charges injected between the RF electrodes by the plasma. microwave.

By playing on the other hand on the power supplied by the RF generator, it is possible to adjust the current and the energy of the ions bombarding the surface of the substrate relatively independently of the generation of deposit precursors.

 The practical exploitation of these possibilities implies that the stability and reproducibility of the two plasmas are well controlled. It is in this sense advantageous to have an optical diagnosis of the microwave plasma by emission spectroscopy, relating to lines recognized as significant, in order to overcome the problems of adjusting the external parameters, incident microwave power and position. tuning pistons 21 and 22.

 The reproducibility of the microwave plasma being thus ensured, it remains to establish that of the substrate polarization conditions. The RF generator 32 and its automatic impedance tuning deliver constant power WRF to the RF plasma. In particular, if the electrical characteristics of the polarization circuit drift over time, the significant quantities of the ion bombardment, namely the flow of ions arriving on the substrate and their energy, will correlatively vary in a generally undesirable manner.

 This is particularly the case when the insulating deposit material is deposited on the initially bare surfaces of the substrate support. To overcome this drawback, according to one aspect of the invention, the free surface of the substrate support, around the substrate 6 and on its side faces, is advantageously covered with a layer 37 of thick dielectric resistant to bombardment. ionic, for example ceramic. In this way the additional thickness of dielectric generated during a deposition is low in relative value and no longer influences the polarization parameters. On the other hand, the power WRF is generally read on a wattmeter integrated into the generator 32 and imprecise. It appears more advantageous, all other things being equal, to be based on the value VDC of the DC voltage appearing between the metal part 35 of the substrate support and the mass.

 Similarly, to more effectively control the RF plasma, additional conductive rods can be added to the tube bundle 27, but preferably an independent reference electrode will be provided in the form of a metal grid 38 interposed between the substrate support and the plane of the injection orifices 28, 46, at a short distance (5 to 15 mm) from the latter and connected to ground by being supported by the annular structure 26.

This arrangement has the advantage of confining the RF plasma at a distance from the injector, preventing the latter from being covered by deposits rich in partially conductive silicon compounds, requiring frequent cleaning.

 For the deposition of protective and filtering layers on an optical substrate 6, the precursor gas injected through the orifices 28 is typically silane stored in a tank 39 to which are added appropriate equipment for expansion, distribution and flow control of a pyrophonic gas.

 The plasma gas is advantageously argon stored in the reservoir 20. The addition of argon to the carrier gas of oxygen, nitrogen or carbon makes it possible to adjust the luminescent zone of the plasma with sufficient flexibility by playing on incident power and increases the pumping speed.

 We will now give, by way of example, an embodiment of the device for obtaining deposits based on silicon on a polycarbonate substrate 6. The plasma gas is argon with which oxygen, nitrogen, and / or ammonia are mixed, as the case may be, for the initial phase of the process, and the silicon precursor is silane. Once the substrate has been positioned on the substrate support, a conditioning vacuum of less than 10-2 Pa is established in enclosure 1, typically of the order of 5 × 10 Pa.

 The microwave generator 18 operates at the standard frequency of 2.45 GHz and at a power of the order of a few hundred Watts; the radio frequency generator 32 typically operates at a frequency of 13.56 MHz with a power of a few tens of Watts.

First of all an activation of the surface of the substrate is carried out in two stages with the following parameters 1. flow rate: Ar: 85 sccm
pressure: 10 Pa
power MW: 200 W 2. flow rate: Ar: 76.5 sccm
NH3: 8.5 sccm
pressure: 10 Pa
MW power: 200 W
Next, an a-Si: H adhesion layer is deposited with the following parameters
flow rate: Ar: 17.0 sccm
He: 5.0 sccm (injected with SiH4 in post
dump)
pressure: 14 Pa
MW power: 200 W
then the deposition of a hard and transparent structural layer of silica with the following parameters
flow rate: Ar: 125 sccm
O2: 15 sccm
SiH4: 6> 5 sccm
total pressure: 10 Pa
MW power: 400 W
It is also possible to deposit layers of low-carburetted hydrogenated silicon nitride with the following parameters
flow rate: Ar: 125 cm3 / min
3
N: 15 cm / min
SiH4: 4 to 5 sccm
pressure: 8 Pa
MW power: 200 W
These nitrum layers are imperatively deposited in the presence of radiofrequency polarization, with VDC, as the case may be, of the order of a few tens of Volts.

 In all cases, the gas flow rates and the pressure are adjusted beforehand and the deposition is initiated by the ignition of the microwave plasma by operating in the following manner: the tuning pistons 21, 22 are first positioned, then the generator 18 is started and the incident power is adjusted to the appropriate value. The microwave plasma does not initiate spontaneously, it is started using an auxiliary radio-frequency electrode placed outside the plasma tube 13. This procedure is preferable to that consisting in first establishing the microwave plasma in the absence of the injection of the silicon precursor gas. In fact, taking into account the stabilization time of the mass flow regulators, the characteristic deposition speed and the thicknesses deposited, the flow rate setpoint is often not reached during the step considered.

 With such an arrangement, growth rates of silicon oxide greater than 0.2 gm / minute are obtained, the rise in temperature of the substrate remaining sufficiently low to allow treatment for more than 25 minutes at a stretch, or more using the cooling of the substrate support. Similar results are obtained in the case of silicon nitride.

The device according to the invention applies to the production of deposits according to documents FR-A-2,614,317, FR-A-2,631,346 and of French patent application No. 90.05529, all in the name of
Applicant, the content of which is assumed to be incorporated here for reference.

 The arrangement described makes it possible to obtain sufficient radial homogeneity of the density of excited species at the outlet of the plasma tube, to control the distribution of the concentrations and transit times of the excited species, to obtain an optimized spatial distribution of the flow of gaseous precursors injected, and to control in a substantially independent manner the flow and energy of the ions bombarding the surface of the substrate for controlling the microstructure and the intrinsic stress of the films deposited.

 Although the present invention has been described in relation to a particular embodiment, it is not limited thereto but is on the contrary subject to modifications and variants which will appear to those skilled in the art.

Claims (14)

 1. Reactor for plasma deposition in gaseous phase of inorganic compounds on a polymer substrate, comprising: a sealed enclosure (1) in which is disposed a substrate support (5); a surface wave plasma tube (13, 15) having one end (15, 16) extending in the enclosure directly above the substrate support, the tube being connectable to at least one source of plasma gas (20 , 20 ') and being associated with means (18, 17) for exciting the plasma gas and means (10) for injecting at least one precursor gas of the inorganic compound disposed between the end of the tube and the support substrate, characterized in that the end of the tube has a profile (16) which widens gradually and has a terminal opening surface at least equal to that of the substrate zone (6) to be coated.  2. Reactor according to claim 1, characterized in that the injection means (10) comprise a plurality of injection orifices (28; 46) directed towards the substrate support (5) and located substantially in a plane s 'extending parallel to the terminal opening of the plasma tube, at a distance therefrom not exceeding 15 millimeters.  3. Reactor according to claim 2, characterized in that the injection orifices (28) are substantially regularly distributed perpendicular to the substrate support (5), at a distance from the latter not exceeding 30 mm.  4. Reactor according to claim 2 or claim 3, characterized in that the injection orifices (28) are formed by a metallic structure (27) and fed by at least one annular chamber (25) disposed externally relative to the 'terminal opening of the column (15).  5. Reactor according to one of claims 1 to 4, characterized in that the substrate support (5) is metallic and comprises cooling means (30, 31).  6. Reactor according to claim 5, characterized in that the substrate support (5) has an interchangeable metal imprint (36) adapted to the profile of the substrate (6) to be coated.  7. Reactor according to claim 5 or claim 6, characterized in that the substrate support (5) is electrically connected to a radio frequency generator (32).  8. Reactor according to claim 4 and claim 7, characterized in that it comprises a metal grid (38) interposed between the metal injection structure (27) and the substrate support (5).  9. Reactor according to claim 7 or claim 8, characterized in that the substrate support (5) has transverse dimensions greater than those of the substrate (6), the surfaces of the substrate support not covered by the substrate being covered with 'a coating of ceramic material (37).  10. Reactor according to one of the preceding claims, characterized in that the surface wave plasma tube comprises, above the flared end (16), a bell-shaped part (12) mounted in leaktight manner on the enclosure (1).  11. Reactor according to claim 10, characterized in that the enclosure (1) has an upper access opening, the bell-shaped part (12) being supported by an annular flange (8) mounted in leaktight manner on the 'access opening.  12. Reactor according to claim 11, characterized in that the injection means (10) are supported by the annular flange (8).  13. Reactor according to one of the preceding claims, characterized in that the means for excitation of the plasma gas include a microwave generator (18).  14. Reactor according to one of the preceding claims, characterized in that the plasma gas contains argon.