EP1523754A2

EP1523754A2 - Heating jacket for plasma etching reactor, and etching method using same

Info

Publication number: EP1523754A2
Application number: EP03763950A
Authority: EP
Inventors: Michel Puech
Original assignee: Alcatel CIT SA; Alcatel SA
Current assignee: CollabRx Inc
Priority date: 2002-07-11
Filing date: 2003-07-10
Publication date: 2005-04-20
Also published as: JP2005532693A; WO2004008477A2; WO2004008477A3; US20050224178A1; FR2842387A1; FR2842387B1

Abstract

The invention concerns a reactor wherein the reaction chamber (1) is delimited by a sealed wall (2) protected by a heating jacket (14). The heating jacket (14) is brought to a temperature higher than the polymer condensation temperature generated during the passivation steps of an alternating plasma etching process, so as to prevent polymer deposition on the sealed wall (2) of the reaction chamber (1) and on the heating jacket itself (14), thereby maintaining a constant etching speed.

Description

HEATING LINER FOR PLASMA ENGRAVING REACTOR, AND ENGRAVING METHOD FOR IMPLEMENTING SAME

TECHNICAL FIELD OF THE INVENTION The present invention relates to plasma etching reactors, and in particular reactors used for the implementation of micromachining or anisotropic etching processes of a silicon substrate by plasma according to the alternating process described in document US-A-5,501,893. During such an alternating process, stages of etching a substrate are alternated by a plasma of fluorinated etching gas such as SF ₆ , and steps of passivation of the surfaces using a plasma of pas.sivation CxFy gas such that C ₄ F ₈ for example.

The process steps are carried out under a low pressure atmosphere, allowing the establishment and maintenance of a plasma.

During the plasma etching step of fluorinated etching gas, the substrate is isotropically attacked by the fluorine atoms. The plasma passivation steps of CxFy passivation gas such as C ₄ F ₈ make it possible to deposit a polymer film on all the surfaces of the substrate exposed to the plasma. The vertical surfaces and the horizontal surfaces are thus covered. During the next plasma etching step of fluorinated gas etching, and under the combined action of vertical ion bombardment obtained by the negative polarization of the substrate, the polymer film is sprayed and removed on the horizontal surfaces, and the vertical etching of the substrate can continue, while the polymer remaining on the vertical surfaces momentarily opposes the action of the plasma on said vertical surfaces. By thus repeating the etching steps with a fluorinated etching gas plasma and the passivation steps with a CxFy passivation gas plasma, an anisotropic etching of the substrate is obtained.

The mechanism of etching of the substrate by the plasma of etching fluorinated gas is the following: one generates a plasma containing electrons, ions such as SF5 +, and fluorine atoms F. The fluorine atoms arriving at the surface of the substrate react chemically, for example in the case of a silicon substrate, depending on the reaction:

Si (s) + 4F (g) → SiF ₄ (g).

The reaction products such as SiF ₄ and the non-dissociated SF ₆ molecules as well as the SxFy radicals remain in gaseous form and are removed by pumping.

During the passivation step of surfaces by means of a plasma of passivation gas C ₄ F ₈ , for example the surfaces of a silicon substrate, a plasma is generated containing electrons, ions and radicals of CF type. , CF ₂ , ... etc. These radicals or monomers will bond to each other to form polymer chains [-CF-] n or [-CF ₂ -] n. These polymers condense on all surfaces exposed to plasma and cover them with a polymer film. These surfaces are of course the surfaces of the silicon substrate being etched, but also all the internal surfaces of the reaction chamber.

During the next plasma etching step of fluorinated etching gas, the surfaces subjected to ion bombardment, under the effect of negative polarization, are freed from the polymer film. This is particularly the case for horizontal surfaces of the silicon substrate, which can then be etched by atoms of atomic fluorine F. It is also the case for all surfaces ^other than the substrate which are subjected to bombardment. One problem with the alternate anisotropic etching methods according to US Pat. No. 5,501,893 is that the etching speed decreases progressively over time, in a substantially linear fashion, as illustrated in FIG. 1. Thus, starting at time 0 , with an etching speed of 10 microns per minute, the speed gradually decreases to reach 6 microns per minute after 12 hours of operation, in an example of operation of a given reactor and under plasma generation conditions kept constant.

SUMMARY OF THE INVENTION The object of the invention is to avoid such a negative drift in the etching performance of an etching equipment anisotropic silicon by an alternating anisotropic etching process according to US-A-5,501,893.

The invention results from an in-depth analysis of the phenomena appearing during the passivation and etching stages according to the alternating process, and leads to explaining this negative drift by the following process: during the passivation stages, all the parts of the reaction gradually cover with a polymer film. This film is not removed during the etching steps when the surfaces of the reaction chamber are connected to a low potential, for example to the electrical ground. Due to the low potential, the corresponding receiving surfaces of the reaction chamber are not subjected to ion bombardment, and thus retain a polymer film similar to that covering the surfaces of the substrate to be etched. Over time, this film thickens.

Although not subjected to ion bombardment, the polymer film deposited on the receiving surfaces connected to the mass potential is subjected to a light flux of ions and electrons of energy equal to E = Vp - 0 in which Vp represents the positive plasma potential.

In general, Vp is of the order of ten volts, typically 15 to 25 volts relative to the mass. This energy is insufficient to remove the polymer film by spraying, but it is sufficient to heat the walls and therefore the polymer film at temperatures of the order of 40 to 60 ° C.

Initially, when the reactor is cold and clean, that is to say free of polymer deposition of the type [-CF-] n or [-CF ₂ -] n, the etching speed of the silicon is optimal that is to say maximum. Then, as the substrate is etched, the polymer film condensed on the receiving surfaces of the walls not subjected to intentional ion bombardment will grow and thicken. At the same time, it will heat up under the effect of the flow of energy particles Vp. As it heats up, this film liberates, by partial vaporization, molecules of the CxFy type. These molecules are found in the gas phase, adding to the molecules of C ₄ F ₈ intentionally introduced by mass flow meters. These molecules are then deposited on the surfaces of the substrate, causing an uncontrolled increase in the passivation of the horizontal silicon surfaces during the passivation steps. During the following etching step, excess passivation increases the time required to destroy the polymer and begin the process of etching horizontal surfaces. This results in a decrease in the overall etching speed. The idea which is the basis of the invention is to prevent from the outset, during an alternating etching process, the formation of deposits of condensed polymer film on the receiving surfaces of the walls of the reaction chamber not subjected to ion bombardment, by bringing the receiving surfaces to a temperature sufficient to ensure the volatilization of any possible deposit of polymer.

Simultaneously, the invention aims to achieve this temperature rise without excessive expenditure of energy, and without risk of injury to intervention personnel circulating around the reactors.

It has already been proposed, in document US Pat ^. No. 5,788,799, to reduce the format of deposits on the walls of a deposition reactor by providing a ceramic heating jacket interposed between the plasma and certain parts of the wall of the reactor. The only methods described are deposition methods, and the ceramics chosen include oxides, nitrides or carbides of boron, aluminum, silicon, titanium, zirconium, chromium.

The solutions described would not be suitable for an etching reactor, for various reasons and in particular: contamination of the substrate to be etched, drop in etching yield.

For this, a plasma etching reactor according to the invention, comprising a reaction chamber surrounded by a sealed wall, containing substrate support means, and communicating with a plasma source, further comprises a heating jacket made of a metal or suitable alloy internally covering in leaktight manner all or part of the sealed reaction chamber wall, and an intermediate space thermal insulation provided between the heating jacket and the sealed reaction chamber wall.

In this way, the heating jacket has a temperature higher than that produced by plasma radiation alone, and the higher temperature of the heating jacket reduces the quantity of polymer molecules deposited on the jacket. Simultaneously, the heating jacket itself constitutes the receiving surface, and forms a screen preventing the deposition of the polymers on the sealed wall itself of the reaction chamber. And the heating jacket has a structure which avoids any contamination of the substrate to be etched and any drop in yield of the etching process.

To be compatible with an alternating etching process, which can be used in particular in the microelectronics industry, for etching semiconductor substrates, the metal or suitable alloy is preferably chosen from metals or alloys which, on the one hand do not react with fluorinated etching and passivation gases to form volatile compounds, and on the other hand do not generate an emission of contaminating atoms under the effect of bombardment by the plasma. In particular, alkali metals, chromium, and heavy metals such as iron, copper, zinc should be avoided. Good results can be obtained with a heating jacket made of aluminum or titanium, aluminum being preferred for its low cost and its ease of implementation. In the case of an alternating etching process, the reactor according to the invention can also comprise:

- means for biasing the substrate support means, to control the bombardment of particles coming from the plasma,

- a source of gas ^"engraving, and etching rate control means for controlling one introduction of etching gas into the plasma source,

- a source of passivation gas, and means for controlling passivation flow to control the introduction of passivation gas into the plasma source, - a control device, suitable for alternately controlling the flow control means etching and passivation flow control means. According to an advantageous embodiment, the heating jacket is fixed to the sealed wall of the reaction chamber by a limited number of fixing points.

The intermediate space between the heating jacket and the sealed wall of the reaction chamber can advantageously communicate with the central space of the reaction chamber by an annular space of reduced thickness. The small thickness prevents the penetration of the plasma into the intermediate space.

The attachment points preferably have a thermally insulating structure which prevents the transfer of thermal energy by conduction from the heating jacket to the sealed wall of the reaction chamber.

The heating means of the heating jacket can be of several types. According to a first embodiment, the heating jacket is thermally coupled to heating means such as electrical resistors connectable to an external source of electrical energy.

The electrical resistors may for example comprise electrical resistors in a thin layer, and / or electrical resistors of the thermoaxial type.

Alternatively, the heating jacket is thermally stressed by radiation heating means such as infrared elements.

Preferably, the heating jacket is associated with thermal regulation means ensuring the regulation of its temperature within a range of suitable temperature values.

In practice, the heating jacket advantageously comprises heating means suitable for heating it to a temperature above 150 ° C. An additional problem with plasma etching reactors results from the presence of a conductive grid, limiting the reaction chamber downstream of the substrate support means. The purpose of this grid is to limit the propagation of the plasma, and to confine it in the reaction chamber. The problem is that ^'the gate tends to clog progressively, by accumulation of polymer particles. The invention solves this problem by ensuring that the conductive grid is in thermal contact with the heating jacket. It appears that the resulting rise in temperature on the grid prevents it from fouling and keeps it in correct working condition for a long time.

In addition, the presence of the heating jacket in the reaction chamber produces an advantageous effect on the means for holding a substrate on the substrate support: an advantageous embodiment of such substrate support means comprises attraction electrodes substrate electrostatic. In known reactors, these electrodes cover themselves fairly quickly with polymer, and their efficiency decreases rapidly over time.

The invention greatly reduces this problem, since the electrostatic attraction electrodes of the substrate remain with sufficient cleanliness for correct operation of the electrodes for a long period of time, apparently because the electrodes are no longer charged with polymer.

According to another aspect of the invention, there is provided a process for etching the substrate by plasma in a reactor as defined above, the process comprising alternating steps for etching the substrate with a plasma of fluorinated etching gas and of steps of passivation of the surfaces by a plasma of CxFy passivation gas, and at least during the passivation steps, the heating jacket is heated to a temperature higher than the condensation temperature of the polymers generated by the passivation plasma.

According to an advantageous embodiment, the heating jacket is heated continuously during all the stages of the process.

SUMMARY DESCRIPTION OF THE DRAWINGS Other objects, characteristics and advantages of the present invention will emerge from the following description of particular embodiments, made in relation to the attached figures, among which:

- Figure 1 illustrates the negative evolution of the etching speed in known reactors;

- Figure 2 is a schematic representation of a reactor according to an embodiment of the present invention; and - Figure 3 is a bottom view illustrating the lower surface of the upper wall of the heating jacket. DESCRIPTION OF THE PREFERRED EMBODIMENTS In the embodiment illustrated in FIG. 2, a plasma etching reactor comprises a reaction chamber 1 surrounded by a sealed wall 2 containing substrate support means 3 and communicating with a plasma source 4 .

The sealed wall 2 of the reaction chamber 1 comprises for example a peripheral portion 2a which is connected to a front inlet portion 2b which is itself open to communicate with an inlet tube 6 constituting the plasma source 4. The portion peripheral 2a and the front input portion 2b are metal portions, advantageously connected to the ground potential. The inlet tube 6 is made of dielectric material, and is surrounded by a coupling electrode 7 supplied with alternating electric current at radio frequency by a radiofrequency generator 8.

A source of etching gas 9a and etching flow control means 9b such as a solenoid valve make it possible to control the introduction of etching gas at the end of the inlet tube 6, into the plasma source 4. Likewise, a source of passivation gas 9c and means for controlling passivation flow rate 9d such as a solenoid valve make it possible to control the introduction of passivation gas at the end of the inlet tube 6, into the source of plasma 4. A control device 9e alternately controls the etching flow control means 9b and the passivation flow control means 9c.

The coupling electrode 7 excites the gases in the inlet tube 6 to produce a plasma which then moves towards the interior of the reaction chamber 1 in the direction of the substrate support means 3.

To control the bombardment of particles coming from the plasma, the substrate support means 3 are polarized by a radiofrequency generator 11 to which they are connected by a polarization line 10.

The reaction chamber 1 is connected by a pumping line 12 to pumping means 13 making it possible to establish and maintain in the reaction chamber 1 a low and controlled gas pressure, compatible with the production of a plasma.

Downstream of the substrate support means 3, the reaction chamber 1 is limited by a conductive grid 5 also connected to the potential of the mass, and the mesh of which is in relation to the ion density of the plasma.

The reactor of Figure 2 further comprises a heating jacket 14 internally covering all the portions of the sealed wall 2 which are at ground potential and which are in contact with the plasma. Thus, the heating jacket 14 comprises a peripheral wall 14a which covers the peripheral portion 2a, and comprises an upper wall 14b which covers the front inlet portion 2b. The heating jacket 14 is a wall made of a suitable metal, itself connected to the ground potential, and associated with heating means such as electrical resistors 17 or others. Thermal insulation means are interposed between the heating jacket 14 and the sealed wall 2 of the reaction chamber 1.

In the illustrated embodiment, the thermal insulation means consist of an intermediate space 15, of suitable thickness, for example of the order of approximately 0.5 to 1 mm, between the heating jacket 14 and the sealed wall 2 of the reaction chamber 1. Placed inside the reaction chamber 1, and communicating with the interior space of the reaction chamber by an annular space 14c of reduced thickness, for example of the same thickness as the space intermediate 15, the intermediate space 15 contains an atmosphere at very low pressure, and therefore having good thermal insulation properties. Simultaneously, the heating jacket 14 is fixed to the sealed wall 2 of the reaction chamber 1 by a limited number of fixing points, for example the three fixing points 16a, 16b and 16c illustrated in FIGS. 2 and 3.

The fixing points 16a, 16b and 16c have a thermally insulating structure, which further prevents the transfer of thermal energy by conduction from the heating jacket 14 to the sealed wall 2 of the reaction chamber 1. In the simple and effective embodiment illustrated in FIGS. 2 and 3, the heating jacket 14 is suspended from the sealed wall 2 of the reaction chamber 1 by fixing points

16a, 16b and 16c each consisting of a protuberance with a head, projecting below the watertight wall 2, and engaged in a respective slot 26a, 26b and 26c of the upper wall 14b of the heating jacket 14. The slots 26a, 26b and 26c are of the buttonhole type with a large portion of head passage and a narrow portion of head retention, as illustrated in FIG. 3. Preferably, the internal surface 14d of the heating jacket 14 is structured so as to present a slight radiation emission factor. In this way, we limit

1 heating of a substrate 23 placed on the substrate support means 3, and thus avoids disturbing the etching and passivation steps.

The electrical resistors 17 or other means for heating the heating jacket 14 are supplied by a line 21 controlled by thermal regulation means comprising a control device 19 which receives by a line 20 temperature information from the heating jacket 14 taken by a temperature sensor 18. The control device is designed so as to regulate the temperature of the heating jacket 14 and to maintain it within a range of suitable temperature values making it possible to avoid the deposition of polymer molecules [-CF- ] n or [-CF ₂ -] n on the heating jacket 14.

The temperature of the heating jacket 14 can be chosen as a function of the type of gas CxFy used, and therefore as a function of the type of polymer deposited during the passivation steps.

In practice, the heating means 17 are adapted to heat the heating jacket 14 to a temperature above 150 ° C., sufficient to avoid condensation of the polymers generated during the passivation steps.

Preferably, the conductive grid 5 is in thermal contact with the heating jacket 14 in a peripheral contact zone 22. In this way, the heating of the conductive grid 5 prevents its progressive fouling and considerably prolongs its duration of use. The heating of the conductive grid 5 by specific heating means constitutes in itself an independent invention capable of being applied to reactors without the heating jacket 14.

A schematic illustration has been shown in FIG. 2 of specific means for holding a substrate 23 on the substrate support means 3: these particular means are electrostatic electrodes 3a for attracting the substrate, which attract the substrate 23 by electrostatic attraction. In this case, it is necessary to maintain satisfactory cleanliness of the electrostatic electrodes 3a, failing which the substrate 23 is not correctly held on the substrate support means 3.

The adaptation of the heating jacket 14 and the means making it possible to heat it satisfactorily considerably reduces the fouling of the electrostatic electrodes 3a, and also makes it possible to increase the correct operating time of the electrodes for satisfactory maintenance of the substrate 23.

During operation, the pumping means 12 and 13 maintain an appropriate low gas pressure inside the reaction chamber 1. Appropriate etching or passivation gases are introduced by the gas generation means 9. The supply of the coupling electrode 7 by the RF generator 8 generates a plasma 24 in the inlet tube 6, and the plasma 24 propagates in the reaction chamber 1 in the direction of the substrate 23 thanks to the polarization of the substrate 23 by the radio frequency generator 11. Simultaneously, the electrical resistors 17 supplied by the line 21 and the control device 19 maintain the heating jacket 14 at a appropriate temperature avoiding any deposition of passivation polymer, and simultaneously protecting the sealed wall 2 of the reaction chamber 1. In this way, at the end of the passivation step, the monomer molecules are rapidly removed by the pumping means 12 and 13, and the introduction of the etching gases into the inlet tube 6 rapidly causes an etching effect on the substrate 23, without the appearance of progressive reduction in engraving speed. Thus, a process for etching substrate 23 with a plasma

24 in a reactor according to the structure described above comprises alternating stages of etching the substrate 23 by a gas plasma 24. fluorinated etching and passivation steps of surfaces by a plasma 24 of CxFy passivation gas. During this process, the heating jacket 14 is heated to a temperature higher than the condensation temperature of the passivation polymer generated by the plasma, at least during the passivation steps.

For simplicity, the heating jacket 14 can be heated continuously during all the stages of the process.

Thanks to the thermal insulation means 15 interposed between the heating jacket 14 and the sealed wall 2 of the reaction chamber 1, the electric power necessary to maintain the heating jacket 14 at the desired temperature is limited, and unnecessary heating is avoided. sealed wall 2 of the reaction chamber 1. As a result, the external temperature of the sealed wall 2 remains compatible with security requirements, that is to say that this temperature is bearable and the intervention personnel during of use can touch the wall without risk of burns.

The present invention is not limited to the embodiments which have been explicitly described, but it includes the various variants and generalizations which are within the reach of those skilled in the art.

Claims

1 - Plasma etching reactor, comprising a reaction chamber (1) surrounded by a sealed wall (2), containing substrate support means (3), and communicating with a plasma source (4), characterized in that '' it further comprises a heating jacket (14) made of a suitable metal or alloy internally covering in an unsealed manner all or part of the sealed wall (2) of the reaction chamber (1), and an intermediate space of thermal insulation (15) provided between the heating jacket (14) and the sealed wall (2) of the reaction chamber

(1).

2 - Reactor according to claim 1, characterized in that the metal or suitable alloy is chosen from metals or alloys which on the one hand do not react with the fluorinated gases of etching and passivation to form volatile compounds, and on the other hand do not generate an emission of contaminating atoms under the effect of bombardment by the plasma.

3 - Reactor according to claim 2, characterized in that the suitable metal is aluminum or titanium. 4 - Reactor according to any one of claims 1 to

3, characterized in that it further comprises:

- polarization means (10, 11) of the substrate support means (3), for controlling the bombardment of particles coming from the plasma, - a source of etching gas (9a), and means of etching flow control ( 9b) to control the introduction of etching gases into the plasma source (4),

a source of passivation gas (9c), and means for controlling passivation flow (9d) to control the introduction of passivation gas into the plasma source (4),

- A control device (9e), adapted to alternate control the etching flow control means (9b) and the passivation flow control means (9d).

5 - Reactor according to any one of claims 1 to 4, characterized in that the heating jacket (14) is fixed to the sealed wall (2) of the reaction chamber (1) by a limited number of fixing points ( 16a, 16b). 6 - Reactor according to claim 5, characterized in that the intermediate space between the heating jacket (14) and the sealed wall (2) of the reaction chamber (1) communicates with the central space of the reaction chamber ( 1) by an annular space (14c) of reduced thickness.

7 - Reactor according to one of claims 5 or 6, characterized in that the fixing points (16a, 16b) have a thermally insulating structure which prevents the transfer of thermal energy by conduction from the heating jacket (14) towards the sealed wall (2) of the reaction chamber (1).

8 - Reactor according to any one of claims 5 to

7, characterized in that the heating jacket (14) is suspended from the sealed wall (2) of the reaction chamber (1) by three head protrusions, protruding beneath the sealed wall (2), and engaged in buttonhole type lights with a large head passage portion and a narrow head retention portion.

9 - Reactor according to any one of claims 1 to

8, characterized in that the heating jacket (14) is thermally coupled to heating means such as electrical resistors (17) connectable to an external source of electrical energy.

10 - Reactor according to claim 9, characterized in that the electrical resistors (17) comprise electrical resistors in thin layer and / or electrical resistors of thermocoaxial type.

11 - Reactor according to any one of claims 1 to 8, characterized in that the heating jacket (14) is thermally stressed by means of heating by radiation such as infrared elements. 12 - Reactor according to any one of claims 1 to 11, characterized in that the heating jacket (14) is associated with thermal regulation means (18-21) ensuring the regulation of its temperature in a range of temperature values appropriate. 13 - Reactor according to any one of claims 1 to 12, characterized in that the heating jacket (14) comprises heating means (17) adapted to heat it to a temperature above 150 ° C.

14 - Reactor according to any one of claims 1 to 13, characterized in that the internal surface (14d) of the heating jacket (14) is structured so as to have a low emission coefficient n of radiation.

15 - Reactor according to any one of claims 1 to 14, characterized in that, downstream of the substrate support means (3), the reaction chamber (1) is limited by a conductive grid (5) in thermal contact with the heating jacket (14).

16 - Reactor according to any one of claims 1 to 15, characterized in that the substrate support means (3) comprise electrostatic electrodes (3a) of substrate attraction.

17 - Method for etching the substrate (23) by a plasma (24) in a reactor according to any one of claims 1 to 16, characterized in that it comprises an alternation of steps for etching the substrate (23) by a plasma (24) of fluorinated etching gas and stages of passivation of the surfaces by a plasma (24) of CxFy passivation gas, and in that, at least during the passivation stages, the heating jacket (14) is heated ) at a temperature higher than the condensation temperature of the polymers generated by the plasma (24). 18 - Process according to claim 17, characterized in that the heating jacket (14) is heated continuously during all the stages of the process.