Photon Enhanced Reactive Ion Etching

This invention relates to plasma technology for use in either plasma etching the surface of a substrate or depositing material from the plasma on the substrate. More particularly, this invention relates to the use of vacuum ultraviolet radiation during etching or deposition.

Accordingly, it is a primary object of the present invention to provide an apparatus and process which permits real time control of plasma etching and deposition.

Another object of the present invention is to provide an apparatus and process which controls the disassociation of the plasma species adjacent the substrate to be etched or on which a layer is to be deposited.

A further object of the present invention is to provide an apparatus and method which permits control of plasma species, such as stable organic halides and molecular oxygen and fluorine.

A still further object of the present invention is to provide an apparatus and method which permits selection of particular plasma species for disassociation.

Another object of the present invention is to provide an apparatus and method which permits control of plasma etching and deposition in selected areas of the substrate.

In semiconductor manufacturing, etching is required to expose areas of the silicon substrate for diffusion or ion implantation of impurities so as to form integrated circuits in the silicon substrate. One method of etching is by a process of plasma etching or reactive ion etching. In this process, a chemically reactive gas such as CF.sub.4 is used. The surface of the substrate to be etched is covered with a mask, leaving selected areas of the surface exposed. The substrate with a surface to be etched is inserted into a chamber containing the reactive gas. To create the plasma, normally an RF voltage is applied across the gas to cause the gas to disassociate and form various species, such as a positive and negative ions, atoms, such as fluorine, and radicals. The plasma reacts with the surface and forms volatile products, thereby leaving an etched surface in the exposed areas.

By changing the gas and the conditions, as is well known in the art, plasma can be used for depositing a layer on the substrate rather than etching the surface of the substrate.

Plasma etching and deposition rate are dependent on the RF power and frequency (if the power source is RF) gas pressures and compositions, temperature, substrate bias, and gas flow dynamics. Varying any of the above parameters does not permit real time control of the plasma etching or deposition. Yet, such control is desirable. For example, as the plasma etching approaches a critical material interface, it is preferable to slow the etch rate.

Highly reactive species, such as halogen and oxygen, especially in their atomic state, whether neutral or ionized, are desirable in plasma etching. Atomic fluorine in a plasma created from a gas of CF.sub.4 with a small percentage of oxygen is believed to react with silicon and the oxides and nitrides of silicon according to the following reactions:

The etching or ashing of organic material, such as photoresist, also is preferably achieved with atomic oxygen. A general reaction would be as follows:

Because of the opacity of air (as well as various glass, quartz, and crystalline window materials) below wavelengths of about 2000 A and the consequent need to operate in a vacuum, this region of the electromagnetic spectrum is commonly called vacuum ultraviolet. The wavelength range corresponds to a photon energy range of 60 KeV to 6 eV. The ionization potentials of most organic compounds, such as CF.sub.4, lie in the energy range 8-13 eV so that organic molecules which absorb vacuum ultraviolet wavelengths will form ion species.

FIG. 1 is a schematic cross-sectional view of the preferred vacuum ultraviolet generating means in combination with the plasma reactor.

FIG. 2 is a schematic cross-sectional view of an alternative vacuum ultraviolet generating means in combination with the plasma reactor.

FIG. 3 is a schematic cross-sectional view of another alternate vacuum ultraviolet generating means in combination with the plasma reactor.

For a further understanding of the invention, and the objects and advantages thereof, reference will be had to the following description and the accompanying drawings, and to the appended claims in which the various novel features of the invention are more particularly set forth.

In order to achieve the foregoing objects of the present invention, the plasma reactor includes means for emitting selected wavelengths of vacuum ultraviolet and directing this radiation to the species of plasma, preferably adjacent the substrate, so as to cause a change in, at least, selected species and thereby control the plasma process, especially at the substrate. More specifically, the preferred plasma species for reactive ion etching are O.sub.2, F.sub.2 and stable organic halides, such as CF.sub.4, which require wavelengths in the vacuum ultraviolet for photoionization and disassociation. The plasma species for deposition also require the wavelengths to be in the vacuum ultraviolet regions of the electromagnetic spectrum.

As shown in FIG. 1, enclosed in a vacuum chamber 1 the means for generating vacuum ultraviolet radiation, is a Hopfield type repetitive spark discharge source 2 of vacuum ultraviolet having a continuum wavelength range of 600 to 1000 A. The source 2 includes power supply 3 and a spark gap 4, which breaks down when the voltage across the capacitor 5 is sufficiently great. The time required to charge the capacitor is determined by the time constant of the capacitor 4 and the resistor 6, the product of which determines the repetition rate. If desired, the spark gap 4 can be replaced by a thyratron circuit. Contained in the source 2 is purified helium which, during breakdown of spark gap 4, emits a continuum spectrum extending from 600 to 1000 A. A more preferred means for generating vacuum ultraviolet radiation is a duoplasmatron as described in FIG. 3. It can be substituted for the Hopfield type source 2. The vacuum ultraviolet continuum passes through an entrance slit 9 of a monochromator and is reflected and focused by a concave grating 10 through the exit slit 7 in the plasma reactor 8 and is absorbed by the plasma species adjacent the article, herein a silicon wafer carrying a SiO.sub.2 layer to be selectively etched. Typical intensity at the exit slit 7, of a mono-chrometer having 0.5 A band pass is of the order of 10.sup.8 photons/sec. The exit slit 7, entrance slit 9, and the grating 10 are mounted on a circle shown by the dashed lines 11 which is known as the Rowland circle. By rotating these optical components 7, 9 and 10 of the monochromator along the Rowland circle, different wavelengths from the continuum spectra can be selected so as to match the absorption wavelength of the plasma species, herein CF.sub.4, to cause disassociation and provide fluorine atoms for etching the SiO.sub.2. Because the vacuum ultraviolet source can be turned on or off at any time during the plasma etching (or deposition) process, the etching can be controlled on a real time basis.

Except for the modification of the exit slit 7 in the wall of the plasma reactor 8, the reactor can be any one of a number of commercial reactors or the reactor described in Application Ser. No. 865,811, filed Dec. 30, 1977. However, the carrier 12 of the reactor 8 should be movable, herein rotatable as shown by the arrow 13, so as to position different portions of the article to be etched and different articles under the vacuum ultraviolet beam. In this way, selected portions of the article can be etched at a different rate. Also, the deposition rate on selected portions of the article can be varied by exposing selected portions of the article. Further details regarding the spark gap source of vacuum ultraviolet and monochromator can be found in Techniques of Vacuum Ultraviolet Spectroscopy, by James A. R. Samson, published by John Wiley & Sons (1967).

In operation herein, the article to be etched (or have a layer deposited thereon) is placed in the plasma reactor 8 in the normal CF.sub.4 +O.sub.2 environment.

During conventional etching, the vacuum ultraviolet beam is turned on to maximize the etching rate initially by generating additional fluorine atoms for etching by disassociation of the CF.sub.4 and oxygen atoms by the disassociation of O.sub.2 to combine with carbon atoms to form CO.sub.2 gas. The beam is turned off as the etching approaches its end point to slow the etch rate. Of course, the beam can be turned on or off at anytime so as to alter the etch profile or for any other reason which requires control of the etching on a real time basis. The same is true during a deposition process.

In the etch process, the substrate preferably is one wherein all the components are capable of forming volatile substances under the conditions of the etching process. Moreover, the etchant should be a material whose etching characteristics are activated or greatly enhanced by the presence of radiation under the common operating pressures and temperatures. This normally will necessitate an absorption spectra of the material which encompasses the wavelength(s) of the vacuum ultraviolet beam so that disassociation will occur for etching (the same being true for deposition). The spectra of desired etchants can be found in Some Aspects of Vacuum Ultraviolet Radiation Physics by B. Vodar and J. Romand, Pergamon Press (1974) and Chemical Spectroscopy and Photochemistry in the Vacuum-Ultraviolet by Sandorfy, Ausloos and Robin, D. Reidel Publishing Co. (1974).

The preferred etchant employed in the present process is CF.sub.4. However, F.sub.2 may also be used as an etchant and a source of fluorine atoms. In ashing photoresist, O.sub.2 is the preferred etchant.

Of course, the process is carried out under vacuum conditions, normally at pressures of 5.times.10.sup.-3 torr to about 5.times.10.sup.-1 torr. The minimum flow rate of the etchant is generally determined by the desired etch rate. The maximum flow rate is primarily determined by the capacity of the pumping system. Room temperature (e.g. about 300.degree. K.) is preferred for the process of the present invention, but higher temperatures can be employed when desired.

Alternatively, instead of employing the spark gap source of FIG. 1 as the means for generating the vacuum ultraviolet, a synchrotron (not shown) can be used to provide the beam 20 shown in FIG. 2. For example, a 180 MeV synchrotron as described in the Journal of Applied Physics 36, 380 (1965) emits an intense radiation continuum throughout the vacuum ultraviolet spectrum. Further details about this synchrotron and others can be found in that reference and in Techniques of Vacuum Ultraviolet Spectroscopy and the references cited therein and in Chemical Spectroscopy and Photochemistry in the Vacuum Ultraviolet. As shown in FIG. 2, the beam 20 from the synchrotron first is reflected off a pre-mirror 21 before striking a grating 22 of the monochromator and being reflected through an exit slit 23 onto the article 24 to be etched in the plasma reactor 25.

A further alternative embodiment is shown in FIG. 3 in which the etchant gas molecules are exposed to vacuum ultraviolet radiation so as to disassociate the molecules into reactive species prior to introducing the gas into the plasma reactor. In this embodiment, the vacuum ultraviolet source is a duoplasmatron 30 (partially shown), which emits line radiation depending on the gas introduced in the inlet 31 and present at the filament 32. Accordingly, the gas is selected for its emission line(s) so as to match the absorption spectra or disassociation energy of the gas molecules in the chamber 33, which are to be used as the plasma etchant. While not shown in this embodiment, it is desirable to use a monochromator as shown in FIG. 1, because the duoplasmatron also emits a continuum of wavelengths and it may be necessary to tune the emission to the disassociation energy of the gas molecules. After exposure to the vacuum ultraviolet radiations, the etchant gas now containing reactive species, such as fluorine atoms, is fed into the plasma reactor 34. The presence of the reactive species effects the etching rate so that controlling the formation of the reactive species in the chamber 33, the etching of the article 35 in the reactor 34 can be controlled. Further details on the duoplasmatron can be found in Techniques of Vacuum Ultraviolet Spectroscopy.

From the foregoing, it is apparent that the present invention provides the advantage of being able to select those plasma species desired for disassociation by tuning the monochromator to the disassociation energy of those species.

While the invention has been illustrated and described with reference to preferred embodiments thereof, it is to be understood that the invention is not limited to the precise construction herein disclosed and the right is reserved to all changes and modifications coming within the scope of the invention as defined in the appended claims.