CH620314A5 - Method and apparatus for chemical treatment in a luminescent discharge - Google Patents

Description

The present invention relates to a method and apparatus for chemical reaction in the glow discharge region of a plasma.

In plasma reactors of the type that the invention relates to, the gaseous reagent is excited by high frequency energy in a reaction chamber. These reactors are useful in very diverse applications, in particular for the chemical attack of various metals such as aluminum.

It is noted that, in plasma reactors of known type, the ionization current tends to concentrate towards the walls of the chamber, in the manner of the electrical effect of skin, or the tendency of high frequency currents to propagate near the surface of metallic conductors. In a plasma reactor, the ionized and excited species which forms the plasma is essentially formed in the current flow region.

In this same region, visible light from the glow discharge is created by the disappearance of various species in electronically excited states. The duration of certain electronically excited species is apparently so short that these species do not have time to diffuse outside the region of current concentration. It is noted that certain reactions, for example the chemical attack on aluminum, can only be used in the region of glow discharge, and, when this discharge takes place only near the walls of the reactor, only a small part of the room can be used.

In the method and the apparatus according to the invention, the ionization current is practically uniform throughout the region between the electrodes and the reactions can be carried out at any location in this region. The distribution of the current is uniform because the distance separating the electrodes is small and a distributed impedance is mounted in series with the plasma so that the impedance measured in the center of the plasma is practically the same as in the external part. In an advantageous embodiment, the electrodes are planar and the distributed impedance is formed by a sheet of dielectric material adjacent to one of the electrodes.

The invention therefore relates to a method and an apparatus for chemical reaction in a plasma formed by a gas.

It also relates to a method and an apparatus of the type described in which the ionization current and the luminescent discharge of the plasma are distributed throughout the region comprised between the electrodes.

This method and this device are particularly well suited to the chemical attack on aluminum.

Other characteristics and advantages of the invention will emerge better from the description which follows, given with reference to the appended drawing in which:

- Figure 1 is a diagram of an embodiment of the apparatus. This embodiment is particularly suitable for attacking aluminum;

- Figure 2 is a diagram of another embodiment of a platelet processing apparatus according to the invention; and

FIG. 3 is an enlarged partial section of a plate of a type which can be treated in the apparatus of FIG. 2.

The apparatus shown in FIG. 1 comprises two substantially flat and distant electrodes 11 and 12. The electrodes are formed from an electrically conductive material and they are held parallel to one another. In this embodiment, the electrodes form the upper and lower walls of a chamber 13 which also has a cylindrical side wall 14 of suitable insulating material, for example quartz. The electrode 11 is removably mounted at the upper part of the side wall 14 and allows access to the chamber.

A device allows the circulation of a gaseous reagent in the chamber 13. It comprises a gas inlet 16 having a flow control valve 17 and a vacuum pump 18 connected to an outlet 19. In the embodiment shown, the the inlet and outlet are mounted on a side wall 14 at opposite ends of the chamber, but it should be noted that they can be arranged at any suitable location in the chamber, for example on the electrode 11. The reactive gases suitable for the chemical attack of materials such as aluminum are chlorine and hydrocarbons containing chlorine, and we obtain particularly good results with carbon tetrachloride CCI4. When attacking silicon compounds and other materials, suitable gases include CF4, CUF, and other fluorine-containing compounds.

The flow rate of the gaseous reagent is adjusted so that the gas remains in the chamber for an optimal time so that the creation and disappearance of the active species which ensures the chemical attack are balanced, but this time must not be such that the

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620,314

reaction byproducts later inhibit the reaction.

In the case where the method is applied to CC14, for example it is found that a residence time of the order of '/ s gives particularly advantageous results. In the advantageous embodiment, CC14 enters a chamber of 300 cm3 s with a flow rate of the order of 40 to 70 cm-Vmin, at atmospheric pressure, and a pump 18 withdraws the gas from the chamber with a flow rate of about 7500 cm3 / s, so that the residence time is equal to 300/7500 = '/: <s, the pressure in the chamber being of the order of 0.1 torr. It can be seen that the pressures m of between 0.05 and 0.5 torr give satisfactory results.

A device is intended for supplying the electrodes 11 and 12 so that these create an electric field for ionizing the gas present in the chamber 13. The device used comprises a high-frequency generator 21 connected to the electrodes 11 and 12 by wires 22 and 23. In an advantageous embodiment, the generator operates at a frequency of the order of 13.56 MHz and at a power of the order of 50 W, although other frequencies and other powers can be used if: <> necessary.

As indicated previously, the spacing of the electrodes is small and a distributed impedance is placed in series with the plasma so that the distribution of the current and the ionization of the plasma are practically uniform throughout the region 2e. between the electrodes. As the distributed impedance is relatively large compared to the plasma impedance, the current passing through the center of the plasma is made almost equal to that passing through the outer part of the plasma. When the impedance of a path passing through the center of the plasma is low compared to that of a path going from the center to the outer wall of the chamber, the distribution of the current in the plasma is also practically uniform.

When the electrodes are circular and have a diameter of 203.2 mm (for example, when they are used for attacking aluminum) and when they receive a power of 50 W, it is possible to obtain Satisfactory results with an electrode spacing of 2.54 mm or less, a value of 10.2 mm giving particularly satisfactory results. In general, the distance between the electrodes must be markedly less than the diameter of the latter, and the ratio of the diameter of the electrodes to their spacing is advantageously at least 3/1.

In the embodiment of FIG. 1, a plate 26 of dielectric material is placed on the electrode 12 and constitutes a distributed impedance 45. Suitable dielectric materials are quartz, "Pyrex" glass and other glasses and satisfactory results are obtained with a "Pyrex" glass plate having a thickness of the order of 1.6 mm. The presence of the dielectric material ensures not only the distribution of the ionization current in the plasma, but also the passage of part of the ionization current in a wafer or another part placed in the plasma.

In the embodiment shown, a semiconductor wafer 27 rests on the plate of dielectric material. ss However, the relative positions of the dielectric material and of the wafer are not particularly delicate insofar as their arrangement is in series between the electrodes.

Thus, the dielectric material can be placed for example near the upper electrode and the wafer can be placed directly on the lower electrode.

By way of illustration, there is shown a wafer 27 which comprises a substrate 28 of a semiconductor material in which regions of suitable impurities are formed according to a well known method for the formation of semiconductor devices. A layer 29 of silica Si02 covers the substrate and is itself covered with an aluminum layer 31. A photographic resist layer 32 is formed on the aluminum and windows 33 are formed in the resist layer areas of aluminum that need to be removed are exposed.

In another possible application, a device is also used for the admission of an oxidizing gas such as oxygen or air, or a reducing gas such as hydrogen in the chamber 13 during the cycle of aluminum attack. This device comprises a gas inlet 38 having a flow control valve 39 mounted on the side wall 14. The introduction of the oxidizing gas allows the formation of aluminum compounds containing oxygen on the exposed surfaces of the aluminum, these compounds facilitating the fight against scouring of aluminum. The addition of a hydrogen-containing gas allows the formation of aluminum hydrides on the exposed faces of the aluminum and these aluminum compounds also facilitate scour control. It is found that air is particularly suitable for this purpose because it contains both oxygen and hydrogen.

A suitable heating device (not shown) is intended to maintain the electrodes at a predetermined temperature. This device can include any suitable heater. In the application to the etching of aluminum for example, the electrodes 11,12 are maintained at temperatures of the order of 75 to 125 ° C and 75 to 135 ° C respectively.

In the application to aluminum etching, this can form a layer 31 on a semiconductor wafer 27. It is assumed that the wafer 27 and the dielectric member 26 have been placed in the chamber 13 as shown and that the electrodes 11 and 12 are at a temperature between 75 and 125 ° C for one and 75-135 ° C for the other respectively. The valves 17 and 39 are closed and the pump 18 operates so that it reduces the pressure in the chamber to a low value much less than 0.1 torr. The valve 17 is then opened and transmits the gaseous reagent into the chamber with a suitable flow rate such as 49 to 70 cm3 / min, so that the pressure in the chamber reaches the working value of the order of 0.1 torr. The electrodes are then supplied so that the gas is ionized and forms a plasma which ensures chemical attack. As indicated above, the glow discharge is distributed uniformly between the electrodes and the wafer can be placed at any location in this region.

It is noted that the chemical attack process can be improved by cyclic use. In this application, another oxidizing gas can be introduced, the oxygen having a concentration which increases on the exposed aluminum surfaces, between the attack periods. The number and duration of cycles depend on the thickness of the aluminum to be removed and the content of aluminum in other materials. However, it is found that 3 to 5 attack periods of 3 to 3.5 min each are satisfactory in the case of most platelets.

During the cyclic process, the ionization current can be maintained for a period of the order of 3 to 3.5 min, and the current is stopped and the valve 17 is closed so that the gaseous reagent can no longer enter the bedroom. The pump 18 continues to operate until the pressure in the chamber returns to the base level below 0.08 torr, and the pump is then stopped. The valve 39 is then opened and allows air or another suitable oxidizing gas to enter. The air enters with a suitable flow and for a suitable time so that the oxygen-containing gas accumulates on the exposed surfaces of the aluminum. When the chamber has a volume of the order of 300 cm3, suitable results are obtained by admitting air at the rate of 300 cm3 / min for a time of the order of 45 s. After the air intake, the valve 39 is closed and the pump 18 is controlled again in order to

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let it reduce the pressure value to the basic level. The suitable, after training in a conventional manner. A valve layer 17 is then opened again and the electrodes are 67 of insulating material such as SiO 2 covers the substrate and one supplied again for the next cycle. layer 68 of aluminum is formed on silica. Aluminum

We now consider an example of application of the process through the windows 69 of the silica layer and forms the cedes of the invention. The techniques described and a contact with the silicon substrate are used, and a cover 71 of reserve aluminum layer whose thickness is of the order of 10 000 photographic Å is formed on the aluminum and delimits a design is removed from a semiconductor wafer during three connections to be formed by attacking the exposed parts of cycles each comprising an attack period of 3.5 min and the aluminum.

an oxidation period of 45 s. The volume of the chamber is _,,

a iL 3 »1 - t-f 1 a * 1 * a - • Gaseous reactants suitable for 1 attack of 1 aluminum of 300 cm-, and the gaseous reagent is CCI4 whose time of seiour m,„ ... , „', ,,

„J .1 - ■,. „. in 1 application of 1 device of figure 2 are chlorine and is of the order of V25 s. During the oxidation periods, 1 air,, lt, ,,? _

, .., -,,, o,. ,. hydrocarbons containing chlorine such as CCI .. In the case of entering the room at the rate of 300 cnr / min. Pressure , , , , ,

j. ,,, "Jnrvo". 1, plates of the type represented in FIG. 3, in which working in the chamber is 0.08 torr, and the electrodes r n. r .. b j1 ....

. . c- • * _ • <. a parts of 1 aluminum are directly in contact with silicon,

upper and inner are maintained at temperatures of *,. , .... ,

-. . „. O„ ... t, - t ti c - we can see that we can get better results either by

75 and 125 C respectively. The generator 21 operates at an i5. . „,,,

vs. , ** ti • has 7 .1 arrangement of a quartz plate under the plates, on a frequency of 13.56 MHz, with a power of 50 W, and the. ,,. n ... 1. ,,. 1

-,,, .., n - ... a m n 1 electrode 54, either by using a gaseous reagent which

electrodes 11 and 12 are separated by a distance of 10.2 mm. . . ,,,,,

T,,,, is helium or some other heat conducting inert gas.

The device represented in FIG. 2 comprises a chamber T. ,. ... °,

vs, ' , . co ,,, The results are particularly good when using a

51 having walls 52 of a suitable material, for example „,, n ■

1. • * T - „mixture containing 110 parts of CCI. and 40 parts of helium,

in aluminium. A removable cover, not shown, allows the vojyme to arrange platelets in the chamber and their removal. In general, the parallel and flat electrodes 53 and 54 are mounted. We now consider the operation and 1 use in the chamber and connected to a high energy source 56 of the apparatus of FIG. 2 in the application to the aluminum attack

frequency placed outside the chamber. A plate 50 of "um, allowing the formation of connection patterns on the insulating glass is placed between the electrode 53 and the superplate wall of the type shown in FIG. 3. When the reins 52, and additional plates glass 55 are platelets were placed on the electrode 54, the chamber is arranged along the edges of the plate 50 and the electrode 53. closed and the pump 58 operates so that it reduces the pressure

The electrode 54 is advantageously provided with passages allowing it to be in the chamber at an initial level of 1 order of 0.10 to 0.50

as long as the circulation of water or other suitable torr fluid- Before chemical attack, an oxygen plasma is formed

cooling the electrode to a cool temperature in the chamber so that it removes any water which may be weak. present. This plasma is for example maintained for a time of 30 s. Then the gaseous reagents are admitted and the

An inlet 57 is disposed near one end of the attack plasma is formed. This plasma is maintained during a chamber and allows the introduction of a gas therein, and a time which is sufficient for the attack of aluminum, for example 5 to 7

pump 58 of evacuation is connected to the exit 59 placed near min by micron of aluminum, with a plasma which contains the other end of the room. With this arrangement, the gas mixture of CC14 introduced with a flow rate of 110 cm3 / min and enters the chamber on the side of the first end, of helium introduced with a flow rate of 40 cm'Vmin. In the case circulates between the electrodes and leaves by the other end. It is possible to use a reactor having the dimensions indicated above, other gas circulations if necessary, the gas capable of electrodes being for example supplied with a power of for example being introduced by the lower electrode, direct- 40 1000 at 1100 W, and the lower electrode 54 is held at a ment in the region between the electrodes. temperature below 30 ° C. The pressure in the chamber,

A device is intended to distribute the ionization current during the attack, for example of the order of 0.29 torr. So after the active species of plasma in the region between the end of the attack, a second oxygen plasma is formed to

electrodes. This device comprises glass plates 61 and 62 which it removes the chlorine remains. This plasma is for example adjacent to the upper electrode 53. As indicated, the pia- 45 maintained for a period of the order of 2 min. Then,

that 61 is a plate full of glass the upper face of which is the apparatus is purged by a dry inert gas such as argon, and the adjacent to the lower face of the electrode, and the plate 62 is chamber is open and allows the removal of the platelets

a perforated glass plate having an adjoining upper face. Before processing another batch of platelets, the surface centered on the underside of the plate 61. The perforations or of the lower electrode is advantageously cleaned with acer

orifices 63 pass vertically through the plate 62 and concentrate 50 tone.

The plasma in the regions which are below the The method and the apparatus according to the invention have an orifices. The size of the orifices is not essential and there are a number of important advantages and characteristics,

obtains satisfactory results with orifices whose diameter The small spacing of the electrodes and the relatively meter impedance is of the order of half the diameter of the wafers. high mounted in series with the plasma tend to distribute the

In an advantageous embodiment suitable for etching 55 ionization current throughout the plasma which is then very chemical 76 mm platelets, the plates are rectangular-uniform. During the aluminum attack, the use of a pro-

res and have a length of approximately 61 cm and a width of cyclic envi- ronment allows a significant attack without scouring-

30.5 cm, the thickness of the plate 61 being approximately 3.2 mm, so that the withdrawal of the aluminum is accelerated and is very that of the plate 62 of approximately 9.5 mm , the holes having their own. The luminescent discharge of the plasma fills practical-

diameter of the order of 38 mm. The plates 64 rest at the w) ment the entire region delimited between the electrodes and the reaction of the upper face of the electrode 64 in alignment with the orifices, can be carried out at any location in this region,

and the distance separating the underside of the electrode 53 from the Although the application of the method and the apparatus has been described

upper face of the electrode 54 is of the order of 25.4 mm. reil of the invention with reference to the attack of carried aluminum

Figure 3 shows an example of a wafer structure with semiconductor wafers, it should be noted that they

64, comprising a substrate 66 of semiconductor material such come to any chemical reaction implemented in the region as silicon, whose regions contain a luminescent discharge impurity from a plasma.

C 1 sheet drawings

Claims (7)

620,314 2 1. A method of chemical treatment of a material in a plasma formed by ionization of a gaseous reagent in a reaction chamber comprising two parallel electrodes under electrical supply, between which the material to be treated is disposed, characterized in that it comprises maintaining a sufficiently small spacing between the electrodes so that the impedance of the ionizing current directly between the central parts of the electrodes is substantially less than the impedance of a path going from the central parts to the outside along the m of the electrodes and along the wall of the chamber between the edges of the electrodes, creating an impedance greater than the plasma impedance between the electrodes in order to distribute the ionizing current and the ionization of the gas throughout the entire region between the electrodes and the arrangement of the material in direct contact with the ionized gas in the ionization region. 2. Method according to claim 1, characterized in that the distributed impedance is formed by the provision of a dielectric material between the electrodes. 3. Method according to claim 1 or 2, characterized in that the electrodes are connected to the power supply and disconnected from it cyclically. 4. Method according to claim 3, characterized in that it further comprises the introduction of a gas into the region between the electrodes, when the latter are not supplied. 5. Apparatus for implementing the method according to claim 1, characterized in that it comprises a device delimiting a reaction chamber, a device for introducing a gaseous reagent into the chamber, two parallel electrodes , a device for supplying the electrodes so that the gas ionizes and forms a plasma containing at least one chemically active species in the chamber, the spacing of the electrodes being small enough that the impedance to the ionizing current directly between the parts center of the electrodes is substantially less than the impedance of a path from the center to the outside along the electrodes and along the wall of the chamber between the edges of the electrodes, a dielectric material having an impedance greater than that of the plasma placed between the electrodes to ensure the distribution of the ionization current 40 and the ionization of the electrodes, and a support device for the material to be treated dir ectely in the ionization region, the main surface of the material being parallel to the electrodes. 6. Apparatus according to claim 5, characterized in that the dielectric material is quartz or glass. 7. Apparatus according to one of claims 5 or 6, characterized in that the dielectric material is a flat body having orifices perpendicular to the electrodes, the material to be treated being aligned with one of the orifices. so