Method for Contacting Conductive Structures in VLSI Circuits

The present application is related to copending application, Ser. No. 852,952, filed Mar. 17, 1992, of Christian Burmer, entitled "METHOD FOR THE LOCAL REMOVAL OF uv-TRANSPARENT INSULATION LAYERS ON A SEMICONDUCTOR SUBSTRATE".

The invention is directed to a method for contacting conductive structures in VLSI circuits in a vacuum chamber with the assistance of laser light.

It is frequently necessary in semiconductor technology to undertake micro-work at a semiconductor substrate during the manufacture of integrated circuits on a semiconductor substrate. These include, among other things, the local removal of passivation or insulating layers in circuits metallized in one-layer or multi-layer fashion with the goal of making conductive structures such as, for example, interconnects or pads accessible for various purposes. Such purposes, for example, are analyses of electrical measuring and testing purposes. Furthermore, the locally exposed interconnects can be electrically conductively connected to one another by metal deposition in order, for instance, to implement design modifications in a simple way and to check their compatibility. Semiconductor modules can also be repaired in the same way. It is important in all of these purposes to produce a good electrical contact to the exposed, conductive structure.

When the insulating layer to be eroded is composed of silicon nitride or of an organic material such as, for example, polyimide, then it can be locally removed on the basis of what is referred to as ablation with the assistance of a pulsed UV laser, wherein 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 121-124, incorporated herein. Such laser-induced etching processes are based on the defined production of highly reactive radicals of 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 or fluorine compounds employed, however, have high toxicity and have an extremely 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 the exact positioning of the semiconductor substrate to be processed, become unusable due to corrosion within a short time. The exposed conductive structures often have surfaces that do not meet the demands made for a good electrical contact. In particular, an oxidation in air can generally not be prevented before, for example, metal is deposited thereon or before measuring tips are put in place.

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 these purposes is enabled in a simple way and, in particular, without the employment of toxic or corrosive chemical compounds. Furthermore, the exposed structures should be immediately subsequently provided with contacts in a simple way in order, for example, to enable the placement of measuring tips.

This object is achieved in accordance with a method of the invention for contacting conductive structures in VLSI circuits on a semiconductor substrate in a vacuum chamber with use of laser light wherein at least one gaseous metal compound is provided in the vacuum chamber. At least one UV-transparent insulating layer covering the conductive structure is locally removed and a metallic layer is locally deposited on the conductive structure immediately after locally removing the insulating layer. The method is controlled with the assistance of the following parameters: laser power, pressure of the gaseous metal compound, and irradiated substrate area.

The method of the invention is based on the employment of gaseous metal compounds instead of the toxic halogen-carbon compounds; gaseous metal carbonyls are particularly suitable. Surprisingly, UV-transparent insulating layers can be removed in this way and metallic contacts can be immediately subsequently deposited on the exposed, conductive structures without interrupting the vacuum and with the same or with only a slightly modified method.

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

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

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 metal 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 due to 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) generally composed of lenses and diaphragms in order to achieve a suitable, parallel light 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 to satisfy 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 can be either locally eroded or covered with a tungsten layer dependent on the W(CO).sub.6 pressure set in the chamber, on the energy of the laser pulses and on the substrate surface itself.

Example: silicon oxide erosion with subsequent tungsten deposition.

According to FIG. 2, an aluminum interconnect is situated as conductive structure 22 on the semiconductor substrate 20, and a silicon dioxide layer approximately 1 .mu.m thick is situated thereover as UV-transparent insulating layer 21. The SiO.sub.2 layer is eroded in approximately 2 minutes with the assistance of the laser beam 23 given the following setting of the process parameters. Subsequently, a tungsten layer 24 deposits on the exposed Al.

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.

The aluminum structure is not attacked by this process since the pulse energy of the laser is too weak. It can be advantageous for the growth of a tungsten layer to lower the laser pulse energy to approximately 0.7 .mu.J and to elevate the W(CO).sub.6 pressure to approximately 1 through 2 mbar. The deposition rate then amounts to approximately 200 through 250 nm per minute.

Due to scattered radiation of the laser, a slow tungsten deposition generally occurs in the environment of the uncovered Al structure, i.e. 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 of the method of the invention is based on the following mechanism. First, a metal-containing surface layer is deposited on the surface composed of silicon by photolytic decomposition of the metal compound in the vapor phase, i.e. a tungsten-containing layer in the exemplary embodiment. 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 number 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 procedure begins anew until the entire silicon oxide has been eroded. A photolytic deposition of tungsten from tungsten carbonyl then occurs on the conductive structure that has been exposed, as known from the literature (see, for example, R. Solanki, Solid State Technology, June 1985, pages 220-227, incorporated herein).

Perceptibly, 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 known photolytic erosion processes. The method, for example, can therefore also be employed for combinations of UV-transparent insulating layers. Furthermore, other gaseous metal compounds can be utilized from which a photolytic deposition of metal is possible. Such metal compounds are known, for example, from the afore-mentioned 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.