Microwave plasma processing process and apparatus

Abstract

A microwave plasma processing process and apparatus useful in the fabrication of integrated circuit (IC) or similar semiconductor devices, wherein the object or material to be processed, such as a semiconductor wafer, is processed with plasma generated using microwaves transmitted through a microwave transmission window disposed perpendicular to an electric field of the progressive microwaves in the waveguide.

Description

This application is a continuation of application Ser. No. 07/604,343, filed Oct. 25, 1990, which is a continuation of application Ser. No. 07/532,234 filed Jun. 4, 1990, which is a continuation of Ser. No. 07/416,002 filed Oct. 2, 1989, which is a continuation of Ser. No. 07/150,446, filed Feb. 1, 1988; which is a continuation of Ser. No. 07/016,513, filed Feb. 17, 1987; and which is a continuation of Ser. No. 06/802,332, filed Nov. 27, 1985, all now abandoned.

BACKGROUND OF THE INVENTION

introducted introduced into the plasma generating vessel 21 through a gas feeding tube 5, and ionized to form a plasma. Radicals generated in the plasma pass through the holes 4 to reach the object 26 in the processing vessel 3 and react with the object 26 forming volatile compounds which are exhausted by the vacuum pump. The distance between the plasma generating vessel 21 and the object 26 is approximately 0.8 cm. This length of 0.8 cm is equal to the distance where the plasma may intrude if the shielding means of plasma, or transmitting means for the radicals is taken away. The dimension of the plasma generating vessel 21, in the direction of the microwave electric field, is slightly reduced from that of the original waveguide 2, by 8 mm for example. The reduction in the dimension intensifies the microwave electric field inside the plasma generating vessel 21 thereby increasing the plasma generating efficiency.

Utilizing the apparatus of FIG. 2, as described above, several plasma processing experiments were performed, using a reactive gas mixture of O₂ +CF₄. The mixing ratio of CF₄ was 20% and its pressure inside the plasma generating vessel 21 was 0.5 Torr. The output power of the magnetron was 400 W. A number of silicon wafers of 4 inches in diameter having photoresist layers on them were processed, resulting in a higher ashing rate of 1.5 times that obtained with a conventional plasma processing apparatus. In this experimental processing, ashing of the photoresist layers was satisfactory. The photoresist material attached to a undercut portion or back side portion of the wafer under processing was completely removed. In addition, no damage to the wafer was found and the protection of the wafer was found satisfactory.

However, in this perpendicular incidence-type plasma etching apparatus, there are several problems. These are explained with reference to FIG. 3, which is a simplified illustration of the perpendicular incidence-type apparatus, and FIG. 4, which is an enlarged illustration of the microwave transmission window of FIG. 3.

As is illustrated in FIG. 3, the microwaves are guided through a waveguide 13, which is connected with a microwave power source (not shown). A reactor or etching chamber 14 is provided with a reactive gas inlet 11 and an evacuation outlet 15, connected to a conventional evacuation system (not shown) to form a vacuum in the chamber, and is connected with the waveguide 13 through a microwave transmission window 10 of silica or ceramic disposed perpendicular to the flow direction of the microwaves. An object 8 or material to be processed, such as a semiconductor wafer, is mounted on a stage (not shown; reference number 7 in FIG. 4) and is disposed in the reactor or vacuum chamber 14.

It is apparent from FIG. 4 that the microwaves perpendicularly incident on the microwave transmission window 10 are partially reflected at two portions. First, as is shown with an arrow R₁, the incident microwaves are partially reflected at an interface between the air or other atmosphere inside the waveguide and the ceramic or silica window. Second, the microwaves penetrating into the window are partially reflected at an interface between the window and the vacuum or plasma of the reactor. In addition to this, since the impedance in the reactor varies in accordance with the condition of the reactor, namely, from the vacuum to the plasma, it is substantially impossible to provide a system which results in satisfactory matching in both conditions of the vacuum and plasma.

In the illustrated apparatus, the dielectric constant (ε₁) inside the waveguide 13 is 1, since air occupies the waveguide 13. Further, the dielectric constant (ε₃) inside the reactor 14, before the plasma is produced therein, is 1. This is because the reactor 14 is maintained at a vacuum condition. The dielectric constant (ε₂) of the microwave transmission window 10 depends on the type of the insulating material used. For example, the dielectric constant (ε₂) of a silica window 10 is of the order of 3, and that of a ceramic window 10 is of the order of 9. Accordingly, in the two interfaces of the window 10 discussed above, there is a relationship of the dielectric constants ε₃ <ε₂ <ε₁.

Under the above relationship of dielectric constants, in order to attain appropriate matching or minimum reflection (R₁ +R₂) of the microwave, it is required to control the difference in reflection (R₁ -R₂) so that it equals ##EQU1## wherein λ is the wavelength of the microwaves, namely, odd times of λ/2, so that a phase shift or difference of half a wavelength or λ/2 results between the phase of R₁ and that of R₂. R₁ and R₂ were previously described. It is, therefore, conventionally carried out to adjust a thickness of the window 10 to λ/4.

The appropriate matching, however, is destroyed when the plasma is then produced in the reactor 14, whereby the relationship of the dielectric constants is changed to ε₃ >ε₂ >ε₁. In this state, the maximum reflection (R₁ +R₂) of the microwaves is caused, as a result of the phase shift of one wavelength λ in total. In other words, if matching is previously obtained while the reactor is under vacuum, such matching cannot be maintained during and after the plasma is produced in the reactor, because, as described above, the reflection of the microwave is increased with the production of the plasma. Further, the inappropriate matching results in unsatisfactory production of the plasma, and the sudden and large reflection of the microwaves damages the apparatus or system. In practice, it has been observed from our experiments using oxygen (O₂) as a reactive gas and a vacuum of 1 Torr that the reflection of the microwaves was 70% (without matching) and 30% (with matching).

Another problem is decay of the microwaves in the reactor. The microwaves incident on the reactor, when the plasma is contained in the reactor, rapidly decay upon introduction into the reactor. In addition, the density of the plasma in the reactor decreases with the decay of the microwaves. Accordingly, in order to attain uniform processing of the object or material in the plasma, it is necessary to dispose the object in the neighborhood of and parallel to the microwave transmission window. This disposition is illustrated in FIG. 3, referring to reference number 8 (the disposition of the object illustrated by reference number 28 must be avoided).

In addition, there is a problem concerning the distance between the window and the stage on which the object is supported (see the reference symbol "l" "l" in FIG. 4). When the object has an electrical conductivity or when the stage is of a metallic material, the strength of the microwave electric field is lowest at an interface between the object and the stage. This means that, in the above instance, it becomes difficult to attain effective production of the plasma independent of the distance (l) (l) of the stage from the window. Therefore, hereinbefore, a long distance (l) (l) has been set so that the the stage can be positioned where the microwave electric field is maximum in strength. It has been observed that when the distance (l) (l) is less than λ/4, no effective production of the plasma is attained, while, when the distance (l) (l) is greater than λ/4, a remarkable decay of the plasma density at the neighborhood of the stage is caused at a pressure of 1 Torr or more. Our experiments using as a reactive gas oxygen (O₂), which produces radicals having a short life, indicated that no resist material was ashed at a pressure of 4 Torr and the distance (l) (l) of 2 cm and that, at a pressure of 1 Torr and the distance (l) (l) of 4 cm, the resist material was ashed, but its ashing rate was slow. In addition, it should be noted that the prior art generally recognizes a contradictory phenomenon that, in the ashing process of resist material using oxygen, the ashing can be attained only if the object is close to the window, while an increase of the efficiency of the plasma processing can be attained only if the distance (l) (l) between the window and the stage is long.

Further, there are problems in connection with the disposition of the object. For example, it is difficult to dispose a cooling means for the metallic stage in the reactor and to reduce the size of the apparatus.

An example of a microwave plasma processing apparatus according to the present invention is illustrated in FIG. 5. A waveguide is indicated with reference number 30, through which the microwave produced in a conventional microwave generator 39 is transmitted in the direction of the arrow. A microwave transmission window 31 of an insulating material such as silica or ceramic defines a part of the waveguide 30 and separates a reactor or vacuum chamber 32 from the microwave transmission region of the waveguide 30. The reactor 32, as is shown in FIG. 5, is provided with a reactive gas inlet 35, an evacuation outlet 36 connected with a conventional evacuation system (not shown), and a stage or susceptor 34 on which an object or material to processed to be processed 33, for example, a semiconductor wafer, is laid. The object 33 is disposed parallel to the window 31.

As is apparent from FIG. 5, the microwave transmission window 31 is disposed perpendicular to the direction (arrow 40) of the electric field of the progressive microwaves in the waveguide. In other words, the window 31 is parallel to the direction of the progressive microwaves. Namely, the direction of the window 31 is shifted 90° from that of the window 10 in the conventional perpendicular incidence-type plasma etching apparatus shown in FIG. 3, for example. As a result, the mode of the microwaves traveling from the waveguide 30 to the reactor 32 is not adversely affected, and the microwaves are effectively absorbed into the reactor 32. Therefore, in the illustrated plasma processing apparatus, it has been found that matching can be easily accomplished.

In order to verify the above effects, we produced a microwave plasma processing apparatus of the illustrated structure in which the microwave transmission window 31 was formed from silica and had a thickness of 12 mm. The distance (d) between the window 31 and the stage 34 was 3 mm. The distance (D) between the upper wall of the waveguide 30 and the stage 34 (a total of the height of the waveguide 30 in the direction of the electric field of the microwaves, the thickness of the window 31, and the distance (d) described above; the distance (D) hereinafter is also referred to as "chamber height") was 50 mm. The resulting apparatus was very small in comparison with the prior art perpendicular incidence-type apparatus. Microwaves having a frequency of 2.45 GHz, supplied from a microwave generator (not shown), were transmitted through the waveguide 30. Three hundred cc of oxygen gas was were introduced through the inlet 35 into the reactor 32. Applying a vacuum of 0.3 Torr and an output power of 1.5 kW, plasma etching was carried out to remove the resist material on the object 33 (silicon wafer). The results gave an etching rate about five times higher than that of the prior art plasma etching process.

In connection with the above results, it has been found from our-further our further experiments that, in the oxygen plasma etching process at a vacuum of 1 Torr, the reflection of the microwaves is 30% (70% in the prior art process) when no matching is made and 5% (30% in the prior art process) when matching is made. Such a reduction of the reflection of the microwave enables a higher etching rate.

In the illustrated apparatus of the present invention, it is easy to dispose a cooling means in the apparatus, since the lower portion of the stage 34 does not have to be maintained in a vacuum condition. In fact, according to the present invention, it is possible to carry out the plasma processing at a temperature of 100°C or less. It should be noted that, during the plasma processing, the object 33 is generally heated to a higher temperature exceeding 200°C if the apparatus has no cooling means.

In the practice of the present invention, it is preferred that the dimensions of the object or material to be processed be smaller than those of the microwave transmission window. This is because, when the microwaves transmitted through the window are incident on the material they must cover all of the material. This enables uniform plasma processing under limited plasma generating conditions.

Moreover, the distance or chamber height (D) discussed above is preferred to be less than λ/2, wherein λ is the wavelength of the microwaves. It has been found that such a chamber height results in reduction of the deflection of the reflected wave in the presence of the plasma, thereby extending the possible matching range. Further, when the chamber height (D) is less than λ/2, a tuning operation can be easily carried out.

Further, while not illustrated in any of the accompanying drawings, it is preferred that the microwave transmission window be supported with a holder which is fitted to the waveguide and be replaceable. If the window is replaceable, it is easy to change the size and material of the window depending upon the conditions of the plasma processing. Further, an apparatus of this structure can be commonly used for both plasma processing and after-glow discharge processing, for example. This means that the apparatus of the present invention can be widely used in various different processes.

FIG. 6 illustrates an embodiment of the apparatus according to the present invention. As is illustrated, the height (L in FIG. 5) of the waveguide 30 in the direction of the electric field of the microwaves is decreased in the direction of travel of the microwaves. For example, the height (L₂) is smaller than the height (L₁). FIG. 7 is a perspective view of the apparatus shown in FIG. 6.

Similarly, in the apparatus of FIG. 8, which is a modification of the apparatus of FIG. 6, the height of the waveguide is gradually decreased so that the height (L₂) of the waveguide 30 is smaller than the height (L₁) of the same. FIG. 9 is a perspective view of the apparatus shown in FIG. 8.

In any case, when the height of the waveguide 30 is gradually decreased in the manner shown, for example, in FIGS. 6 to 9, it effectively compensates for the loss of the strength of the electric field at an end portion of the waveguide 30. As a result of the decrease of the waveguide height, a constant distribution of the strength of the electric field in the waveguide 30 is provided, and reduction of the reflection of the microwaves therefore results. For example, we could reduce the reflection power of the processing apparatus using an incident power of 1500 W from 400 W (prior art) to 150 W (present invention).

In the practice of the present invention, the microwave transmission window generally comprises a disc-shaped element of an insulating material such as silica or ceramic. However, as an alternative, it may comprises comprise two or more rectangular, for example, stripe-shaped, elements which are parallel to each other, the distance between the two adjacent elements being λg/4, wherein λg is the wavelength of the microwaves in the waveguide. See FIG. 10. FIG. 11 is a perspective view of the apparatus of FIG. 10, in which 37a and 37b are notches. The separation of the window into two or more elements in the illustrated manner is effective to prevent the breakage of the window without narrowing the plasma area, when a window of a weak material such as alumina is subjected to atmospheric pressure. This is because the openings 37a and 37b act as an additional waveguide.

Finally, FIG. 12 shows that a stage 34 reciprocatable in the direction of the electric field of the progressive microwaves in the waveguide 30. The reciprocatable stage 34 is effective to control the distance (d) between the window 31 and the stage 34 depending upon various factors, such as conditions of the object 33 or the objects of the processing, thereby preventing damage of the object 33. It has been found that, in oxygen plasma processing for removing resist material from an aluminum substrate, at 0.3 Torr, the aluminum substrate was damaged at a distance (d) of 5 mm, but was not damaged at a distance (d) of 20 mm.

Claims

1. A microwave plasma processing apparatus for processing a material, comprising:

a reaction gas source for supplying a reaction gas;

a microwave generator generating microwaves having a wavelength λ;

a waveguide, connected with said microwave generator, having a rectangular prism shape with a longitudinal axis and side-walls extending in parallel to the longitudinal axis, said waveguide receiving the microwaves propagating in a first direction parallel to the side-walls and for transferring the microwaves along the longitudinal axis of said waveguide;

a dielectric window, formed of at least two rectangular elements parallel to each other, superposed over an opening in a first side-wall of said waveguide, for transmitting the microwaves therethrough to an exterior surface thereof, the distance between the two rectangular elements being λg/4, where λg is a wavelength of the microwaves in the waveguide;

a plasma processing chamber connected to said reaction gas source and formed adjacent to the first side-wall of said waveguide to entirely enclose the exterior surface of said dielectric window, said plasma processing chamber offset on the exterior of said waveguide from the first side-wall of said waveguide in a second direction perpendicular to the longitudinal axis of said waveguide, said plasma processing chamber receiving the microwaves transmitted through said dielectric window and the reaction gas supplied from said reaction gas source to generate plasma therein; and

a stage for holding the material to be processed thereon, said stage disposed in said plasma processing chamber with a top surface of said stage substantially in parallel with said dielectric window.

2. A microwave plasma processing apparatus in which material is to be processed using microwaves received by said microwave plasma processing apparatus, comprising:

a reactor for plasma processing therein, having a stage for holding the material to be processed;

a waveguide for transferring the microwaves in a propagating direction within the waveguide; and

a dielectric window, having first and second surfaces substantially parallel with the propagating direction of the microwaves adjacent said dielectric window, said first surface formed coplanar with an inner surface of said waveguide and said second surface forming part of an inner surface of said reactor.3. A microwave plasma processing apparatus according to claim 2, wherein said dielectric window comprises insulating material, selected from at least one of silica and ceramic and alumina.4. A microwave plasma processing apparatus according to claim 2,

further comprising a microwave transmitter, disposed adjacent said reactor, to propagate microwaves in the direction of propagating microwaves, said microwave transmitter having a first dielectric constant,

wherein said reactor has an interior with a second dielectric constant, and

wherein said dielectric window comprises a material having a third dielectric constant with a value between the first and second dielectric

constants.5. A microwave plasma processing apparatus according to claim 4, wherein said dielectric window comprises material selected from a group of silica and ceramic.6. A microwave plasma processing apparatus according to claim 2, wherein said dielectric window comprises a disc-shaped element.7. A microwave plasma processing apparatus according to claim 2, wherein said dielectric window comprises plural windows, each of which has a size small enough to prevent arising breakage in said plasma processing.8. A microwave plasma processing apparatus according to claim 7, wherein each

of said plural windows is made of alumina.9. A microwave plasma processing apparatus according to claim 2, wherein said reactor further includes a reactive gas inlet and an exhaust gas outlet.10. A microwave plasma processing apparatus according to claim 2, wherein the surface of the material to be processed has dimensions smaller than said dielectric window.11. A microwave plasma processing apparatus according to claim 2, wherein the microwaves have a wavelength λ,

wherein said microwave plasma processing apparatus further comprises a microwave transmitter, disposed adjacent said reactor, to propagate microwaves in the direction of propagating microwaves, and

wherein said stage has a top surface supporting the material to be processed and separated from an opposing wall of said microwave transmitter opposite said dielectric window by a distance of less than λ/2 during processing of the material.12. A microwave plasma processing apparatus according to claim 2, further comprising a microwave transmitter, disposed adjacent said reactor, propagating microwaves in the direction of propagating microwaves and having a height, measured perpendicular to said dielectric window, gradually decreasing from a first value above said dielectric window to a second value at an end wall of said microwave transmitter downstream from said dielectric window.13. A microwave plasma processing apparatus according to claim 2, further comprising:

a microwave transmitter, disposed adjacent said reactor, to propagate microwaves in the direction of propagating microwaves; and

a holder, supporting said dielectric window, replaceably fitted to said microwave transmitter.14. A method for processing a material by plasma, comprising:

placing the material in a chamber having a dielectric window disposed in substantially the same plane as an inner surface of a waveguide; and

introducing microwaves via the waveguide along a first direction, substantially parallel to the inner surface of the waveguide and along the dielectric window, so that the plasma is generated in the chamber.15. A microwave plasma processing method according to claim 14, wherein the dielectric window comprises insulating material, selected from at least one of silica and ceramic and

alumina.16. A microwave plasma processing method according to claim 14,

further comprising transmitting the microwaves within a microwave transmitter in a direction of microwave propagation,

wherein the microwave transmitter and the reactor have interiors with first and second dielectric constants, respectively, and

wherein the dielectric window comprises a material having a third dielectric constant with a value between the first and second dielectric constants.17. A microwave plasma processing method according to claim 16, wherein the dielectric window comprises material selected from a group of silica and ceramic.18. A microwave plasma processing method according to claim 14, wherein the dielectric window comprises disc-shaped element.19. A microwave plasma processing method according to claim 14, wherein the dielectric window comprises plural windows, each of which has a size small enough to prevent arising breakage in said plasma processing.20. A microwave plasma processing method according to claim 19, wherein each of the plural

windows is made of alumina.21. A microwave plasma processing method according to claim 14, wherein the reactor includes a reactive gas inlet and an exhaust gas outlet.22. A microwave plasma processing method according to claim 14,

further comprising the step of transmitting the microwaves within a microwave transmitter in a direction of microwave propagation, and

wherein the microwaves have a wavelength λ, and the stage has a top surface supporting the material to be processed and separated from an opposing wall of the microwave transmitter opposite the dielectric window by a distance of less than λ/2 during said generating of the plasma.23. A microwave plasma processing method according to claim 14,

further comprising the step of transmitting the microwaves within a microwave transmitter in a direction of microwave propagation, and

wherein the microwave transmitter has a height, measured perpendicular to the dielectric window, gradually decreasing from a first value above the dielectric window to a second value at an end wall of the microwave

transmitter downstream from the dielectric window.24. A method for fabricating an integrated circuit semiconductor device, comprising:

transmitting microwaves within a microwave transmitter having an inner surface in a plane substantially parallel to a direction of microwave propagation;

disposing on a stage inside a reactor a semiconductor wafer having a surface substantially parallel to the direction of microwave propagation, with at least one of photoresist films to be removed from and protective layers to be etched off the surface of the semiconductor wafer;

reducing pressure in the reactor sufficiently to permit generation of plasma; and

generating the plasma by transmitting the microwaves through a dielectric window, having a first surface in substantially the plane of the inner surface of the microwave transmitter and thereby, parallel to the direction of microwave propagation adjacent the dielectric window, without altering the direction of microwave propagation in the microwave transmitter.25. A microwave plasma processing method according to claim 24, further comprising cooling the material to be processed on the stage using a cooler, disposed below the stage, during at least said generating of the plasma.26. A microwave plasma processing apparatus for processing material therein, comprising:

a reactor for plasma processing therein, having a stage for holding the material to be processed;

a waveguide for transferring microwaves; and

a dielectric window, disposed between said reactor and said waveguide in substantially the same plane as an inner surface of said waveguide and thereby substantially parallel with a direction of propagation of the

microwaves adjacent the dielectric window.27. An apparatus for processing a material by plasma comprising:

a chamber having a dielectric window and a stage for placing the material;

a generator for generating microwaves; and

a waveguide for introducing the microwaves from the generator to the chamber, wherein the dielectric window is disposed in substantially the same plane as an inner surface of said waveguide and thereby substantially parallel to a direction along which the microwaves propagate adjacent the dielectric window so that the plasma is generated in said chamber.28. A method for processing a material by plasma, comprising:

placing the material in a chamber having a dielectric window; and

introducing a microwave along a waveguide in a first direction adjacent the dielectric window substantially in parallel with an outer surface of the dielectric window disposed in substantially the same plane as an inner surface of the waveguide, so that the plasma is generated in the

chamber.29. A microwave plasma processing method for processing a material with microwave generated plasma, comprising:

disposing the material to be processed on a stage inside a reactor, and positioning the stage facing a dielectric window formed as a part of the reactor; and

introducing microwaves along a waveguide having an inner surface in a plane, the dielectric window being disposed with an outer surface substantially in the plane of the inner surface of the waveguide, with the microwaves thereby propagating adjacent the dielectric window in a direction parallel to the dielectric window, so as to generate plasma between the dielectric window and the stage.30. A microwave plasma processing apparatus to process material therein, comprising:

a microwave transmitter to propagate microwaves in a direction of microwave propagation inside containment walls, each having an inner surface;

a reactor having a stage to hold the material to be processed with a surface of the material substantially parallel to the direction of microwave propagation; and

a dielectric window, disposed with a first surface in substantially a same plane as the inner surface of one of the containment walls of the microwave transmitter and thereby substantially parallel to the direction of microwave propagation adjacent the dielectric window, and a second surface substantially parallel to the first surface, to provide a pressure seal between said microwave transmitter and said reactor enabling

generation of a plasma inside said reactor.31. A microwave plasma processing apparatus in which material is to be processed therein, comprising:

a reactor for plasma processing therein, having a stage for holding the material to be processed;

a waveguide for transferring microwaves; and

a dielectric window, disposed so as to be part of a surface of said reactor and be substantially on the same plane as an inner surface of said waveguide, and thereby substantially parallel with a direction of propagating microwaves.