Method for the Local Removal of UV-transparent Insulation Layers on a Semiconductor Substrate

The present application is related to copending application Ser. No. 852,951, filed Mar. 17, 1992, of Christian Burmer, entitled "METHOD FOR CONTACTING CONDUCTIVE STRUCTURES IN VLSI CIRCUITS."

The invention is directed to a method for the local removal of insulating layers on a semiconductor substrate in a vacuum chamber with the assistance of laser light.

It is frequently necessary in semiconductor technology to undertake microwork at a semiconductor substrate during the manufacture of integrated circuits on the semiconductor substrate. Included are the local removal of passivation or insulating layers in circuits metallized in single-layer or multi-layer fashion, with the object of making conductive structures such as, for example, interconnects or pads accessible for various purposes. For example, analyses or electrical measuring and testing are such purposes. Furthermore, the locally exposed interconnects can be electrically conductively connected to one another by metal deposition in order to be able to simply implement potential design modifications and to check their compatibility. Semiconductor modules can also be repaired in the same way.

When the insulating layer to be removed is composed of silicon nitride or of an organic material such as, for example, polyimide, then it can be locally removed with the assistance of a pulsed UV laser on the basis of what is referred to as ablation in that the laser beam is directed or focused onto the desired location. Such an ablation, however, is not possible given an insulating layer having a high UV transmission such as, for example, silicon oxide and other glass-like layers since the layer lying therebelow would be damaged by the intense UV radiation. A number of laser-induced etching processes are known for the local removal of silicon oxide layers and are described in the article by G. Loper and M. Tabat in SPIE Vol. 459, Laser Assisted Deposition Etching and Doping (1984), pages 121-127, incorporated herein. Such laser-induced etching processes are based on the defined generation of highly reactive radicals from halogen-carbon compounds with the assistance of pulsed UV lasers in a vacuum chamber, so that a chemical reaction of the silicon oxide with these radicals occurs upon formation of volatile silicon and oxygen compounds. The gaseous chlorine and fluorine compounds utilized, however, have high toxicity and have an extreme corrosive effect on many materials, so that a high technological expenditure is required. Mechanical parts situated in the vacuum chamber such as, for example, xyz-tables for exact positioning of the semiconductor substrate to be processed become unusable within a short time due to the corrosion.

It is therefore an object of the present invention to specify a method with which a designational, local removal of UV-transparent insulating layers on a semiconductor substrate for the above indicated purposes is enabled in a simple way and, in particular, without employing toxic or corrosive chemical compounds.

This object is achieved by providing a method for local removal of a UV-transparent insulating layer on a semiconductor substrate in a vacuum chamber by use of laser light wherein a gas mixture is admitted into the vacuum chamber. The gas mixture comprises a carrier gas and a gaseous, layer-forming compound. The layer-forming compound is preferably a gaseous metal carbonyl. The carrier gas is preferably argon.

The method of the invention is based on the employment of gaseous, layer-forming compounds instead of the toxic halogen-carbon compounds. Surprisingly, UV-transparent insulating layers can be removed in this way. Given employment of gaseous, layer-forming metal compounds such as, for example, metal carbonyls, a particular advantage of the method is that metallic contacts can be deposited onto the exposed, conductive structures immediately subsequently thereto without interrupting the vacuum and with the same or with an only slightly modified method.

FIG. 1 shows an apparatus for the implementation of the method of the invention; and

FIG. 2 shows a schematic illustration of a semiconductor substrate treated in accordance with the method.

According to FIG. 1, a semiconductor substrate 2 having at least one UV-transparent insulating layer on its surface, or having an integrated circuit to be processed, is secured on a carrier 3 in a vacuum chamber 1, this carrier 3 enabling an exact positioning of the semiconductor substrate 2. Via a valve 4, the chamber 1 can be evacuated down to a pressure of approximately 10.sup.-6 mbar; an inert gas 6, for example argon, can be admitted via a further valve 5. The gaseous, layer-forming compound employed in the exemplary embodiments is tungsten hexacarbonyl W(CO).sub.6, a crystalline powder 7 at room temperature that is heated to approximately 50.degree. C. in an evaporator vessel 8. The gas thereby arising by sublimation is admitted into the vacuum chamber 1 via a valve 10 with the assistance of an inert carrier gas 9, usually argon. Light pulses of a UV-laser 12 can be introduced into the chamber 1 through a silica glass window 11; an ArF excimer laser having a wavelength of 193 nm is preferably utilized. Every laser pulse passes through a conventional optical arrangement (not shown) usually composed of lenses and diaphragms in order to obtain a suitable, parallel laser beam 15 and thereby images a rectangular diaphragm 13 onto the surface of the semiconductor substrate 2 via a suitable microscope 14. The image of the diaphragm 13 defines the size of the irradiated substrate surface 2 and can be set according to the demands; typical values thereby lie in a range from 2 .mu.m.times.2 .mu.m through 15 .mu.m.times.15 .mu.m. The energy of the laser pulses is regulated with the assistance of an attenuator 16. The irradiated specimen surface is either locally eroded or covered with a tungsten layer dependent on the W(CO).sub.6 pressure established in the chamber 1, on the energy of the laser pulses and on the substrate surface itself.

FIG. 2 shows a 1 .mu.m thick silicon dioxide layer as a UV-transparent insulating layer 21 on the semiconductor substrate 20 having a conductive structure 22. When the following process parameters are set, it is eroded in approximately 2 minutes with the assistance of the laser beam 23:

W(CO).sub.6 pressure: 0.4 mbar

Laser pulse energy: 1.2 .mu.J

Pulse frequency: 50 Hz

Laser wavelength: 193 nm

Irradiated Area: 12 .mu.m.times.12 .mu.m

A conductive structure lying under the silicon dioxide layer is not attacked by this process since the pulse energy of the laser is too weak.

Tungsten can be immediately subsequently deposited with the recited process on a conductive structure exposed by the erosion. As soon as the overlying silicon oxide layer is eroded, the exposed, conductive structure of, for example, aluminum is not attacked by the process; on the contrary, tungsten is deposited on its surface with a laser-induced process. The parameters can also be modified; for example, a higher W(CO).sub.6 pressure or a lower laser power can be advantageous.

Due to scattered rays of the laser, a slow tungsten deposition generally also occurs in the environment of the exposed Al structure, on the oxide surface. This can be advantageous since disturbing charging phenomena in some analysis methods are thereby reduced.

The erosion of the silicon oxide layer in conformity with the method of the invention is based on the following mechanism. First, a surface layer is deposited on the surface composed of silicon oxide on the basis of a photolytic decomposition of the gaseous, layer-forming compound in the vapor phase, i.e. a tungsten-containing layer in the exemplary embodiments. This leads to an increased absorption of the laser radiation and, thus, to a heating of the surface. When adequate energy can be absorbed after a plurality of laser pulses, typically 2 through 5, material, namely tungsten and silicon oxide, is eroded by the next laser pulse. A surface composed of silicon oxide is again present and the process begins anew until the entire silicon oxide has been eroded. A photolytic deposition of tungsten from tungsten carbonyl then occurs on the exposed, conductive structure, as known from the literature (see, for example, R. Solanki, in Solid State Technology, June, 1985, pages 220 through 227, incorporated herein).

Perceptively, only the UV-transparency of the insulating layer is of significance for the method of the invention, but not the chemical composition thereof, as in the case of known photolytic erosion processes. The method, for example, can therefore also be employed for combinations of UV-transparent insulating layers.

Further, other gaseous compounds can be utilized from which a photolytic deposition of radiation-absorbing layers is possible; compounds of this type are known, for example, from the aforementioned article by R. Solanki.

Although various minor changes and modifications might be proposed by those skilled in the art, it will be understood that I wish to include within the claims of the patent warranted hereon all such changes and modifications as reasonably come within my contribution to the art.