A METHOD OF DEPOSITING METAL CONTACT REGIONS ON A SILICON SUBSTRATE

Abstract: Metal, such as tungsten, contact regions for an integrated circuit are deposited on exposed portions of a silicon substrate having an apertured silicon dioxide layer thereon by the steps of: (1) disposing the substrate in a deposition chamber; (2) introducing a gaseous compound of a metal into the cham­ ber, which compound reacts with silicon so that metal from the compound substitutes for silicon in the surface of the subst­ rate to form a deposited metal layer in the apertures in the silicon dioxide layer; and (3) introducing hydrogen into the chamber in addition to the gaseous compound whereby the hydrogen reacts with the gaseous compound to form a further deposition of the metal, the metal being deposited during said further deposition on the surface of both the previously deposited metal layer and the silicon dioxide layer. During step (3) an etching gas (e.g nitrogen trifluoride) which etches the metal when activated, is introduced into the chamber and a plasma struck to activate the etching gas. By controlling the amount of the etching gas introduced into the chamber and the elec­ trical power coupled into the plasma, the silicon surface portions of the substrate are kept substantially free of the metal and a further deposit of the metal is produced on the previously deposited metal layer.

Description: This invention relates to a method of depositing metal contact regions on a silicon substrate, and is particularly useful in the production of integrated circuits. In a known technique (see, for example, US-A-4,349,408) for deposition of tungsten contact metal in a contact hole for subse­ quent planarising, a substitution reaction is used where W from WF₆ gas substitutes for Si in exposed silicon areas to produce a thin W surface. Hydrogen is then introduced into the CVD reactor and tung­ sten is deposited by reaction with the WF₆ source gas. More particularly, US-A-4,349,408 relates to a method of depo­ siting a refractory metal (tungsten) on a semiconductor substrate wherein a passivation material is also deposited around the peri­ phery of the metal. This is accomplished by depositing an oxygen doped polycrystal­ line silicon layer on one surface of the semiconductor substrate and then depositing a phosphorus doped polycrystalline silicon layer on the outermost surface of the first layer. Thereafter, windows are etched in the layers to expose surface areas of the substrate where the metal is to be deposited. Afterwards, the tungsten contact is deposited on the exposed surface areas of the substrate by CVD tech­ niques. In accordance with a CVD technique, tungsten (W) is deposited on the surface areas by placing the substrate in a CVD reactor and heating the substrate. Tungsten hexafluoride (WF₆) and an inert carrier gas such as argon (Ar) or nitrogen (N₂) are then fed into the reactor and the tungsten hexafluoride will react with the sili­ con in accordance with the following: 2WF₆ + 3Si → 2W ↓ + 3SiF₄ The deposition of tungsten will stop after a layer of from about 50 nm to about 200 nm thick has been deposited. Inasmuch as a layer of about 400 nm thick is desired, the deposition process must be altered. In order to deposit the additional 200 nm to 350 nm of tungsten, the temperature is lowered. At this point, hydrogen (H₂) is added to the tungsten hexafluoride and carrier gas. The tungsten hexafluoride will react with the hydrogen to deposit the desired additional tungsten in accordance with the following: WF₆ + 3H₂ → W ↓ + 6HF The present invention seeks to provide an improved method for forming metal contact regions on a silicon substrate. A method for depositing metal contact regions on a silicon substrate having an apertured silicon dioxide layer thereon, com­ prising the steps of: (1) disposing the substrate in a deposition chamber; (2) introducing a gaseous compound of a metal into the chamber, which compound reacts with silicon so that metal from the compound substitutes for silicon in the surface of the substrate to form a deposited metal layer in the apertures in the silicon dioxide layer; (3) introducing hydrogen into the chamber in addition to the gaseous compound whereby the hydrogen reacts with the gaseous compound to form a further deposition of the metal, the metal being deposited during said further deposition on the surface of both the previously deposited metal layer and the silicon dioxide layer; the method is characterised, according to the invention, by intro­ ducing into the chamber during step (3) an etching gas which when activated etches the metal and striking a plasma to activate the etching gas so that the silicon surface portions of the substrate are kept substantially free of the metal because of the etching action of the etching gas whereas a further deposit of the metal is produced on the previously deposited metal layer because the rate of deposition of the metal is higher on the metal than on silicon. How the invention can be carried out will now be described by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a schematic cross-section of part of an integrated circuit chip showing a planarising metal stud in a contact hole; Figure 2 represents a typical apparatus for carrying out a method embodying the present invention; and Figure 3 is a schematic representation of a cross-section of an integrated circuit chip including tungsten contact regions deposited by a method embodying the invention. Contact as well as inter-level vias on a Very Large Scale Inte­ grated Circuit (VLSI) chip present a serious concern from the metal coverage point of view. The severe topography results in poor step coverage and this results in problems, such as electromigration. These problems may be minimised by the use of a metal planarising stud in the via as shown in Figure 1 wherein a metal stud 10 is provided in a contact hole (via) in the insulation layer 12 on the semiconductor substrate 14. However, selective deposition of metal in the via alone and planarising this structure is a very complex process. Selective chemical vapour deposition (CVD) of tungsten techniques have been employed. In these techniques, use is made of a substitution reaction, where tungsten W from tungsten hexafluoride WF₆ source gas, substitutes for silicon Si in the exposed silicon areas. 2WF₆ + 3Si = 2W + 3SiF₄ This is a truly selective reaction but has the limitation that it is self-limiting. After the surface layer (perhaps a few 10 nm deep) is converted to W, no further substitution takes place. At this point, hydrogen is bled into the CVD reactor. Now tungsten is deposited by the net reaction WF₆ + 3H₂ = " + 6HF In this part of the process, W is deposited by a nucleation type of process. In such a process, unless extremely clean SiO₂ surfaces exist, it is impossible in the above reaction to avoid nucleation centres on the SiO₂ and to thus preferentially deposit W only on the W surfaces created by the initial part of the process. There are serious practical problems with this technique. It has been found that it is very difficult to maintain a clean SiO₂ sur­ face and that selective deposition is not possible in a reproducible and reliable manner. Further, it has been found that selectivity is lost whenever thick films of W (> 100 nm) are to be deposited. The present invention provides an improved technique to over­ come the above difficulties. An apparatus to carry out the process is shown in schematic form in Figure 2. The deposition process chamber 30 contains the specimens. Tungsten hexafluoride gas is introduced into chamber 30 through ports 32. A hydrogen-helium combination is introduced into chamber 30 through ports 34. Excita­ tion is provided by the rf generator 38 and an infrared sensor lamp assembly 36 is provided. A high vacuum valve 48, a throttle valve 46, a blower 44 and pump 42 are provided to exhaust chamber 30 through port 40. The initial portion of the process remains similar to the prior art steps described previously. The exposed Si sur­ faces on which the selective deposition is to be performed, are first converted to W surfaces by the substitution reaction: 2WF₆ + 3Si = 2W + 3SiF₄ After the surface has been substituted, the second part of the process is initiated. This involves three parameters. First, the Wf₆ is mixed with H₂ as before to give deposition of W by partially preferential nucleation on the already converted W surfaces. Then NF₃ is bled into the system and a plasma is struck in the reaction chamber. The effect of the latter two actions is to create a simul­ taneous etching condition for the tungsten. (here charge balance has been neglected). The amount of NF₃ and the plasma power coupled into the chamber are such as to ensure that the SiO₂ surface is kept clean at all times. Thus any nuclei that may be formed on the SiO₂ surface, are immediately cleaned out. Since the deposition rate on the exposed W surfaces is much higher than on the SiO₂ surfaces, there will be net deposition on these areas in spite of the etching action. The impor- tant advantage is that selectivity is maintained even for thick W films. Also, the deposition rate in the second part of the process is actually increased due to the plasma. The resultant contact structure is shown in the schematic illu­ stration of Figure 3. The metal contacts 10, formed of metal studs in the via holes in the silicon dioxide layer 12 interconnect alum­ inium layers 16 and the n+ regions 24. The recessed oxide isolation regions 20 and the polysilicon gate 22 with its gate oxide 18 form the rest of the integrated circuit. Thus, there has been described herein a differential deposition technique, which takes a reaction that is quasi selective and com­ bines it with an in-situ etching process to yield a truly selective deposition process that is capable of very thick selectively depo­ sited metal films. Although tungsten is the preferred contact metal, other contact metals may be similarly selectively deposited by a method embodying the invention.

Claims: 1. A method for depositing metal contact regions on a silicon sub­ strate having an apertured silicon dioxde layer thereon, comprising the steps of: (1) disposing the substrate in a deposition chamber; (2) introducing a gaseous compound of a metal into the cham­ ber, which compound reacts with silicon so that metal from the compound substitutes for silicon in the surface of the subst­ rate to form a deposited metal layer in the apertures in the silicon dioxide layer; and (3) introducing hydrogen into the chamber in addition to the gaseous compound whereby the hydrogen reacts with the gaseous compound to form a further deposition of the metal, the metal being deposited during said further deposition on the surface of both the previously deposited metal layer and the silicon dioxide layer; the method being characterised by introducing into the chamber during step (3) an etching gas which when activated etches the metal and striking a plasma to activate the etching gas so that the silicon surface portions of the substrate are kept substantially free of the metal because of the etching action of the etching gas whereas a further deposit of the metal is produced on the previously deposited metal layer because the rate of deposition of the metal is higher on the metal than on silicon. 2. A method as claimed in claim 1, in which the gaseous compound is tungsten hexafluoride and the deposited metal is tungsten. 3. A method as claimed in claim 1, or claim 2, in which the etching gas is nitrogen trifluoride.

Inventors: IYER, SUBRAMANIAN SRIKANTESWARA, JOSHI, RAJIV VASANT
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: H01L21
IPC codes: C23F4, C23C16, H01L21