US20050103438A1

US20050103438A1 - Use of light emitting chemical reactions for control of semiconductor production processes

Info

Publication number: US20050103438A1
Application number: US10/508,628
Authority: US
Inventors: Mark Holbrook; David Reeve; Robert Grant
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 2002-03-15
Filing date: 2003-03-14
Publication date: 2005-05-19
Also published as: AU2003226491A1; AU2003226491A8; JP2005521242A; WO2003079411A3; WO2003079411A2; KR20040094794A; GB0206158D0; EP1485937A2

Abstract

A low-pressure processing vessel (3) used for the production of semiconductors is provided with a light-sensitive detector (1) at a distance from the reaction site, for example in the exhaust line (2). The detector (1) is used to detect light emitted by an intermediate which relaxes or recombines with the emission of light at a characteristic wavelength, the intermediate having a long lifetime such that the detector (1) can be positioned in a relatively remote location.

Description

This invention relates to the control of processes used in the production of semiconductors, particularly but not exclusively for endpointing semiconductor etch processes, or for endpointing deposition machine clean up processes.
Various proposals have been made for methods of predicting end point by optical techniques. These suffer from the difficulty that complex optical phenomena occur within the process chamber as the production or cleanup process proceeds.
An object of the present invention is to provide a method and apparatus which can be used in an improved form of process control.
The invention makes use of the fact that many chemical reactions—in particular those involving free radicals and ions produced within a plasma—proceed in a number of stages through a series of intermediates before a stable state or compound is produced. These intermediate stages can have lifetimes from a few nanoseconds to a few milliseconds. Transition from one stage to another can involve the emission of light. If the chemistry of the reaction is studied and understood, and a long lifetime intermediate is identified which relaxes or recombines with the emission of light at a characteristic wavelength, then a detector tuned to that wavelength can be placed at a convenient location outside and remote from the immediate site of reaction and its output used to monitor concentration of a species as a surrogate for material etch rate, and thus the progress of the clean-up of a vacuum processing system or alternatively the progress of a fabrication process that utilizes dry-etch.
Accordingly, the present invention provides a method of controlling a chemical process which takes place within a low-pressure enclosure, the process being such as to produce a species that emits photons of a known wavelength or wavelength distribution by a particular chemical recombination or relaxation process, the species having a lifetime characteristic which, at the pressure of said enclosure, enables it to be detected at a significant distance from the site of the primary reaction, the method comprising detecting said photons at said distance while rejecting other photons, and using the rate at which said photons are detected to control the process.
The term “significant distance” is used herein to mean a distance which is significant in relation to the size of the area within which the primary reaction occurs, and will typically be greater than 5 cm, and preferably is of the order of 0.5 m or more.
The method may be used particularly to control the processing of silicon with fluorine radicals, the chemical relaxation process being the combination of the silicon difluoride radical with the fluorine radical to yield electronically excited silicon trifluoride radical which subsequently returns to the ground state with the emission of a photon most probably between 380 and 650 nm.
The silicon process will most typically be dry etching of silicon/silicon dioxide, or the clean-up of silicon deposited on the walls of the enclosure during other processing. It is to be understood that “Silicon” includes Silicon Dioxide or other Silicon based deposits. The clean-up process may be one involving plasma enhanced chemical vapor etch, the plasma typically being produced within the enclosure.
The radicals may suitably be created upstream of the enclosure.
The photon detection may advantageously be carried out in an exhaust line from the enclosure, or in a vacuum pump to which the exhaust line is connected.
From another aspect, the present invention provides apparatus for use in conjunction with a low-pressure enclosure serving as a reaction chamber in which takes place a chemical process which is such as to produces species that emits photons of a known wavelength or wavelength distribution by a particular chemical recombination or relaxation process, the species having a lifetime characteristic which, at the pressure of said enclosure, enables it to be detected at a significant distance from the site of the primary reaction, the apparatus comprising a photon detector arranged at a significant distance from the primary reaction site, and means for monitoring the rate of photon detection.
Further according to the present invention there is provided apparatus for chemical processing, comprising a low-pressure chamber and an exhaust line extending from the chamber to a vacuum pump; the chamber defining a location in which, in use, a chemical process takes place which is such as to produce a species that emits photons of a known wavelength or wavelength distribution by a particular chemical recombination or relaxation process, the species having a lifetime characteristic which, at the pressure of said enclosure, enables it to be detected at a significant distance from the site of the primary reaction; the apparatus further comprising a photon detector arranged at a significant distance from said location, and means for monitoring the rate of photon detection.
The photon detector is preferably situated in an exhaust line of the enclosure, or in a vacuum pump to which the exhaust line is connected.
Preferably, the photon detector is provided with a light baffle to eliminate off-axis light and/or a light trap opposed to the entrance to the detector.
Embodiments of the invention will now be described, by way of example only, with reference to the drawings, in which:
FIG. 1 is a schematic cross-sectional view of a chemical process system used in one form of the invention;
FIG. 2 shows part of the apparatus of FIG. 1 in greater detail;
FIG. 3 is a graph showing one example of an output signal; and
FIG. 4 is a view similar to FIG. 1 of a second embodiment.
Before turning to the Figures, the physical basis underlying the embodiments will be described in more detail.
Many chemical reactions proceed through a number of intermediates. For example consider the following reaction of the material M with the free radical R:
M(s)+nR
MR_n(g) a)
MR_n+R
MR_n+1*(g) b)
MR_n+1*(g)
MR_n+1(g)+photon c)
MR_n+1(g)+R
MR_n+2(g) d)
In reaction a) the material is removed from the solid surface into the gas phase by combination with one or more individuals of the radical species. In reaction b) further stepwise combination yields an electronically energetic intermediate which decays to the ground state with the release of a photon. Further stepwise addition gives the final stable product which is pumped from the system.
In general, in the absence of quenching agents, the production of light at step c) will be directly proportional to the concentration of the electronically exited intermediate and can be used as an indicator that the reaction removing (etching) material M is proceeding.
In those particular circumstances where reaction b) is rate limiting the overall reaction cascade, and where the intermediate is sufficiently stable that b) is the predominant reaction, then the production of light at step c), which is necessarily a very fast process, will be a quantitative surrogate for the rate of removal of the material M. Even where reaction b) is not rate limiting the presence or absence of the production of light can be used as an endpoint indication for the removal of material M.
If the process pressure and the concentration of radical R are favorable, the emission of light associated with the etch process will be located remotely from the surface of the material M and may extend some metres into the surrounding space.
Given that the normal operating pressures in vacuum processing systems during the clean-up, preventative maintenance phase, or alternatively that during device fabrication by dry etch, is in the low mtorr range, the mean free path of the intermediates will be high compared with the geometries of typical process equipment. Given also that at such pressures the concentration of the reactants are necessarily low thereby enhancing the halflife of the intermediate MR_n, light emission due to step c) will occur as much as 1 metre away from the vacuum processing system's surfaces which are the subject of the cleaning process itself, or alternatively from the material being fabricated during a dry etch process. This distance is simply derived from the halflife of the species MR_nand its diffusion rate.
Since monitoring of the emitted light is carried out remotely from the reaction region it can be done with no interference or disruption to the reaction itself. For example, without prejudice to the generality of the technique, it may be possible to site the measurement means conveniently on the exhaust line from the processing vessel.
Without prejudice to the generality of the technique, one example of a useful reaction which is typical of the type of reaction described above, is the etching of silicon by the fluorine radical generated by plasma decomposition of compounds including, but not restricted to the perfluoronated hydrocarbons, sulphur hexafluoride and nitrogen trifluoride.
Si+2F
SiF₂ a)
SiF₂+F
SiF₃* b)
SiF₃*
SiF₃+photon c)
SiF₃*+F
SiF₄ d)
In a typical silicon etch process, where base pressures are in the range 1 to 100 mtorr, the silicon difluoride radical produced by reaction a) has a lifetime of several milliseconds, and can diffuse a considerable distance before conversion to the trifluoride radical takes place with the immediate relaxation of the excited state and comcomitant production of a photon.
The light emitted is a quasi-continuum ranging from 380 to 650 nm. Given the geometry of a typical process chamber, a substantial amount of this light will be emitted in the exhaust line.
A first embodiment of the invention will now be described with reference to FIGS. 1 to 3.
FIG. 1 shows a typical vacuum processing vessel 3 the diameter of which is of the order of 1 metre. The processing vessel 3 is maintained at a low pressure by a vacuum pump 11 via an exhaust line 2.
A typical vacuum processing technique which is the basic function of the vessel would be the deposition of polysilicon by introduction into the vessel a compound such as, but not restricted to, an organo-silicon compound, and then dissociating that compound at the heated substrate surface 5. A side effect of this procedure is to deposit silicon on the walls of the process vessel 3. It is to be understood that “Silicon” includes Silicon Dioxide or other silicon based deposits. Such silicon is a disadvantage to the basic function of the vessel as it contributes to particulates and consequential failure of devices. A typical clean-up technique would be the introduction of nitrogen trifluoride into an up-stream plasma region 4 where it is dissociated to yield the free fluorine radical. The fluorine radical reacts with the silicon which has been deposited on the walls of the vessel, and follows the reaction sequence already outlined above for silicon. The pressure in the reaction vessel during the clean up process will be of the order of 1 to 100 mtorr.
In order to maximize the efficiency of the clean up process it is desirable to monitor the rate of etch of the silicon contaminate and then stop the clean up process once it has dropped below a pre-determined level.
In FIG. 1 a detector 1 is shown connected to the exhaust line 2 of the process vessel 3. FIG. 2 shows a detail of one suitable form of detector which in this case consists of a light-baffle 7 in front of a wavelength discriminating filter 8 and a photomultiplier tube 9. Opposite the detector 1 is placed a light-trap 6. The light-baffle 7 in this example consists of a number of opaque parallel plates in front of the filter 8, each perforated with a multiplicity of apertures, the pattern and spacial arrangement of which has the function of rejecting light which may be reflected from the reaction region along the exhaust line 2. The light-trap 6 comprises an open-ended cylinder whose walls are provided with projecting baffles, all the internal surfaces being matt black for maximum light absorption.
In a specific embodiment, the light-baffle 7 would consist of a series of opaque plates with apertures in the plates arranged one after the other so that only on-axis light can reach the photon detector.
The apertures in the plates are arranged such that their distribution pattern in the direction in the plane of the individual plates is aperiodic so as to avoid the situation where off-axis light could pass through one aperture in the first plate at a particular angle so that a multiple rule would allow it to pass through not the associated line-of-sight aperture in the next plate but one off-axis and, by virtue of the same multiple rule then pass through other line-of-sight apertures in subsequent plates. The size of the apertures are arranged to increase from one plate to another so that the smallest sized apertures are in the plate adjacent to the photon detector and the largest sized apertures are in the plate furthest away with the increase in size arranged in such a way that an observer located at the photon detector would only be able to see the edges of the apertures in the plate adjacent to him and not be able to view any of the edges in the apertures in plates not adjacent to him.
The combination of the light-baffle 7 and the opposed light-trap 6 has the effect that only light emitted within the tightly confined detection region is detected.
FIG. 3 shows a typical output from the photomultiplier in graphical form which may be used to determine the rate of removal of silicon contaminant. By reference to historical cleaning cycle data, digital signal process techniques such as, but not restricted to, curve shape recognition and perturbation analysis can be used to yield an endpoint decision which can then used to automate the clean-up cycle.
With reference to FIG. 4, a second embodiment is used for controlling the etching of silicon.
FIG. 4 shows a typical vacuum processing vessel 3 the diameter which is of the order of 1 metre. A typical processing technique would be the etching of silicon by introduction into the vessel a fluorine source such as, but not restricted to, sulphur hexafluoride as a gas. The pressure in the reaction vessel will be in the range 1 to 100 mtorr and an electric field is applied to two electrodes 13 in such a manner that a plasma is formed between them. The silicon substrate 14 to be etched is placed on the ground connected electrode and the fluorine radical produced by the dissociation of the sulphur hexafluoride reagent reacts with the silicon according to the reaction sequence already outlined above.
In this example, consider that a layer of silicon dioxide exists within the silicon wafer 14 and it is desired to etch down to this buried layer, and stop at it in a timeous manner. Once the etch of silicon has reached the silicon dioxide layer, the etch rate decreases and this event may be detected by the reduction in concentration of silicon reaction intermediates.
In FIG. 4, as in FIG. 1, a detector 1 is connected to the exhaust line 2 of the process chamber 3. The detector 1 may suitably be the same as is shown in FIG. 2, and the output from the detector will be of the same form as is shown in FIG. 3.
Modifications may be made to the foregoing embodiments within the scope of the invention as defined in the claims. The detector could be placed within the processing vessel itself at a suitable distance form the reaction site, or could be incorporated in the vacuum pump; however, positioning the detector in communication with the exhaust line is likely to be the most convenient arrangement. The invention may be used with processes other than silicon/fluorine wherever a suitable light-emitting intermediate stage is present.

Claims

1. A method of controlling a chemical process which takes place within a low-pressure enclosure, the method comprising conducting a chemical process which produces a species that emits photons of a known wavelength or wavelength distribution by a particular chemical recombination or relaxation process, the species having a lifetime characteristic which, at the pressure of said enclosure, enables it to be detected at a significant distance from the site of the primary reaction, the method further comprising detecting said photons at said distance while rejecting other photons, and using the rate at which said photons are detected to control the process.

2. A method according to claim 1, in which said significant distance is greater than 5 cm.

3. A method according to claim 2, in which said significant distance is of the order of 0.5 m or more.

4. A method according to claim 1, in which the chemical process comprises the processing of silicon with fluorine radicals, the chemical relaxation process being the combination of the silicon difluoride radical with the fluorine radical to yield electronically excited silicon trifluoride radical which subsequently returns to the ground state with the emission of a photon most probably between 380 and 650 nm.

5. The method of claim 4, in which the silicon process is dry etching of silicon/silicon dioxide.

6. The method of claim 4, in which the silicon process is the clean-up of silicon/Silicon Dioxide or other silicon based material deposited on the walls of the enclosure during other processing.

7. The method of claim 6, in which the clean-up process makes use of plasma enhanced chemical vapor etch, the plasma being produced within the enclosure.

8. The method of claim 7, in which the plasma is produced from radicals created upstream of the enclosure.

9. A method according to claim 1, in which the photon detection is carried out in an exhaust line from the enclosure, or in a vacuum pump to which the exhaust line is connected.

10. Apparatus for use in conjunction with a low-pressure enclosure serving as a reaction chamber in which takes place a chemical process which is such as to produce a species which emits photons of a known wavelength or wavelength distribution by a particular chemical recombination or relaxation process, the species having a lifetime characteristic which, at the pressure of said enclosure, enables it to be detected at a significant distance from the site of the primary reaction, the apparatus comprising a photon detector arranged at a significant distance from the primary reaction site, and means for monitoring the rate of photon detection.

11. Apparatus for chemical processing, comprising a low-pressure chamber and an exhaust line extending from the chamber to a vacuum pump; the chamber defining a location in which, in use, a chemical process takes place which is such as to produce a species that emits photons of a known wavelength or wavelength distribution by a particular chemical recombination or relaxation process, the species having a lifetime characteristic which, at the pressure of said enclosure, enables it to be detected at a significant distance from the site of the primary reaction; the apparatus further comprising a photon detector arranged at a significant distance from said location, and means for monitoring the rate of photon detection.

12. Apparatus according to claim 11, in which the photon detector is situated in the exhaust line or in the vacuum pump to which the exhaust line is it connected.

13. Apparatus according to claim 11, in which the photon detector is provided with a light baffle to eliminate off-axis light and/or a light trap opposed to the entrance to the detector.

14. Apparatus according to claim 13, in which the light baffle includes a number of plates having apertures which are arranged aperiodically.

15. Apparatus according to claim 14, in which the size of the apertures are arranged to increase from one plate to another such that the apertures of smallest size are in the plate adjacent the photon detector and the apertures of largest size are in the plate furthest from the photon detector.

16. Apparatus according to claim 15, in which the sizes and arrangement of the apertures are such that an observer located at the photon detector can only see edges of the apertures in the plate adjacent to him whilst being unable to view any edges in the apertures in the plates which are not adjacent to him.

17. Apparatus according to claim 11, in which the photon detector is spaced at least 5 cm from said location.

18. Apparatus according to claim 17, in which the photon detector is spaced 0.5 m or more from said location.