MOISTURE-FREE SOG PROCESS

Abstract: A method is disclosed for applying spin-on glass (SOG) to a substrate over low-melting point, non-refractory materials such as aluminum. The spin-on glass is applied to the substrate in a moisture-free environment to minimize reverse hydrolysis during curing. This results in the formation of higher quality films, especially from inorganic SOGs.

Description: MOISTURE-FREE SOG PROCESS This invention relates generally to a process for applying spin-glass to a substrate, and more particularly to a process for the planarization of semiconductor wafers. The invention is especially applicable to inorganic spin-on glasses. Spin-on glasses (SOG) are proprietary liquid solutions containing siloxane or silicate based monomers diluted in various kinds of solvents or alcohols. They are commonly used for the planarization of semiconductor wafers, i. e. the filling and levelling of the trenches formed between interconnect paths deposited on the wafer. . On coating and curing of spin-on glasses, monomers are polymerized by condensation and release of water, solvent, and alcohol. The condensed material is a thin solid film having mechanical, chemical and electrical properties which depend on the starting solution, and the coating and curing process. There are more than one hundred different SOG solutions currently available. These are classified into two major families: 1) Inorganic silicates. 2) Quasi inorganic siloxanes ( ethyl- siloxanes, ethyl-, phenyl-, butyl-) . The various components of a SOG solution (silicon containing oligomer, solvents mixture, and residual water) are in equilibrium in the liquid phase. Immediately after coating, volatile products (solvents and water) evaporate, and polymerization occurs due to the formation by condensation of SUBSTITUTE SHEET silanol, Si-OH, bonds. These produce more wate. according to the following reaction: Polymerization continues until the distance between neighbouring silanol groups, Si-OH, becomes too large or when too much by-product, such as water, blocks the condensation reaction. Heating is then required to permit further densification. ' Both families of SOG solutions can incorporate boron or phosphorus organometallic catalyst to improve the properties of the films, such as: higher density, reduced hydrogen content, higher coefficient of thermal expansion, better flexibility and higher resistance to cracking. In the SOG solution, the boron or phosphorus organometallic molecules are generally not well bonded to the silicon-containing compounds. Strong bonding generally occurs in the solid state when the film is exposed to relatively high temperatures. These organometallic molecules can nevertheless polymerize in the solution to form SUBSTITUTE SHEET poorly bonded polymers that dissociate and form stable polymers during coating and condensation of the film. As an example, a Japanese SOG solution is alloyed with a phosphorus organometallic molecule, • p w°χ( OH )y(° c 2 H 5)z' which is in dynamic equilibrium with the solutions water and ethanol, C2H 5 OH: If the ethanol C2H5OH, or water concentration suddenly drops, as during coating and solvent evaporation, the equilibrium is broken and, at high temperatures, the phosphorus organometallic molecule will polymerize and connect to the forming Siθ x Hy film by producing more water and ethanol, C 2 H 5 OH: SUBSTITUTESHEET In theory, the phosphorus atom connects to the Si0 2 network with three P-O-Si bonds. These bonds are formed by the condensation of -P-OH or -P-OC H 5 , and -Si-OH. Water and ethanol are formed as by-products and water must be eliminated quickly to prevent reverse hydrolysis. SUBSTITUTE SHEET This reverse hydrolysis is extremely undesirable because it contributes to the incorporation of hydrogen and inhibits the action of the phosphorus organometallic catalyst. Residual hydrogen forms silanol groups, SiOH. It is very difficult to remove and causes serious yield and reliability problems. If this reverse hydrolysis continues, it is possible to totally disconnect the phosphorus by forming many different acids: hypo- phosphorus acid, H3PO2; eta-phosphorus acid, HPO2; pyro-phosphorus acid, H 4 P2O5; ortho-phosphorus acid, H 3 PO 3 ; hypo-phosphoric acid, H 4 P 2 O 6 ; meta-phosphoric acid, HPO 3 ; pyro-phosphoric acid, H 4 P 2 O 7 ; and ortho- phosphoric acid, H 3 PO 4 . As an example, the formation of ortho-phosphoric, H 3 PO 4 , due to reverse hydrolysis by residual or ambient moisture, is: SUBSTITUTESHEET H • O • Si • O Si < 0 O H • O • Si • O si' O o H • O • si • Ό Si ' The acids that are formed corrode aluminum interconnects. Consequently, for planarization over aluminum interconnects, when maximum temperature is limited to about 450°C or so, serious problems are encountered with film quality and reliability. These effects are not observed in the planarization of dielectrics over refractory materials, such as polysilicon, suicides and refractory metals, that allow very high temperature curing (over 800°C) due to more complete thermally induced condensation of pairs of silanol bonds, Si-OH, over long distances. This type of planarization over refractory materials is relatively easy. The difficulty of using silicate SOGs for the planarization of low-melting point materials, such as aluminum has caused a worldwide trend not to use purely inorganic silicate (including phosphosilicate) SOGs and to use instead members of the quasi-inorganic siloxane family. However, there are significant disadvantages in using quasi- inorganic SOGs relating to their electrical properties, and their use is becoming uncertain and questionable for advanced applications. A further problem is caused by the absorption of water during curing. SOGs, particularly phosphorus SUBSTITUTESHEET alloyed SOGs, are extremely hygroscopic materials, and rapidly absorb ambient moisture during curing. This moisture pick-up promotes the irreversible reactions described above, and the resulting SOG films have poor properties and reliability. An object of the invention is to alleviate the afore-mentioned problems by permitting the obtention of higher quality SOG films, mainly inorganic SOG films, over aluminum or other materials that cannot tolerate high temperature processing. According to the present invention there is provided in a method of applying spin-on glass (SOG) to a substrate over low-melting point, non-refractory materials such as aluminum, the improvement wherein the spin-on glass is applied to the substrate in a moisture-free environment to minimize reverse hydrolysis during curing. The non-refractory materials may also be, for example, Titanium, Titanium nitride, Titanium - Tungsten, Tungsten, or a combination thereof. By eliminating the moisture from the curing process. a dense, low hydrogen content, high coefficient of thermal expansion, flexible, crack-resistant and corrosion-free SOG film can be produced. After deposition of the first layer of dielectric, the spin-on glass coating/cure process is preferably carried out with a dedicated SOG processor, typically in the following manner, which is described with reference to the planarization of a series of semiconductor wafers: a) The wafer is transported from sending cassette SUBSTITUTESHEET to a coating chamber. b) A few ml of a SOG solution are dispensed at the centre of the wafer to be planarized. c) The wafer is spun at a given RPM to spread uniformly the solution and to permit the evaporation of volatile compounds and film solidification. d) For SOG curing, the wafer is sequentially transported to in-line hot plates which are temperature controlled at temperature roughly between 80°C and 250°C. e) The wafer is slightly cooled at an idle station. f) The wafer is stored and cooled in a receiving cassette. g) When all the wafers are received in the ambient exposed receiving cassette, they are all together transferred to the sending cassette -for a second coat (steps a to f are repeated) . When all the wafers are received in receiving cassette, they are all together transferred to the sending cassette for a third coat (steps a to f are again repeated) . h) When sufficient coats have been applied, the wafers are transferred to the station for the next process step. During steps a) to g) . an operation lasting about two hours, the wafers are not exposed to ambient atmosphere and, unlike the prior art, are maintained in a moisture-free environment. As a result, the SOG films, particularly phosphorus alloyed SOG films, cannot therefore absorb ambient moisture and extremely high quality films are produced. The invention also relates to an apparatus for applying spin-on glass (SOG) to a substrate over low-melting point, non-refractory materials such as aluminum, which provides a moisture-free environment for the application of said spin-on glass to SUBSTITUTESHEET minimize reverse hydrolysis during curing. The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which:- Figures la and lb show repectively plan and side views of a SOG planarization apparatus for carrying out the process according to the present invention; and Figure 2 is a plot of the stress in a SOG film plotted against relative humidity. SOG processing equipment for coating and in-line curing of the SOG film is relatively new. Prior art SOG processors do not allow wafer manipulations, SOG coating, in-line curing, cooling and storage under continuously controlled ambient stations. These now essential in-line ambient controlled stations are provided in the SOG processor shown in Figure 1. Referring now to Figure 1, the SOG processor comprises a main unit 1 defining a pularity of processing stations. The unit includes sending cassettes 4, transport mechanisms 2, in-line cure plates 6 (up to 9) , and receiving cassettes 7. The unit 1 further has a coating area 5 and cooling area 3. More particualrly , the SOG processor comprises the following elements: 1) One or more sending cassettes 4 to store the wafers to be processed. 2) Transport mechanism 2 to transfer, one by one, the wafers from the sending cassette to the coating SUBSTITUTESHEET area . 3) Coating area 5 where the SOG coating and wafer spinning is done. 4) Transport mechanism 2 to transfer the coated wafer from the coating area to the first in-line temperature controlled hot plate. 5) A first in-line hot plate area 8. 6) Transport mechanism 2 to transfer the wafer from the one in-line hot plate to the next. 7) A last in-line hot plate area 9. 8) A transport mechanism to transfer the wafer from the last in-line hot plate to a wafer cooling area. 9) A wafer cooling area 7. 10) A transport mechanism to transfer the wafer from the wafer cooling area to a receiving cassette area. 11) One or more receiving cassettes 7 to store the wafers that received the first SOG coat. The entire processor is provided in an inert environment. The inert gas ambient protects the wafer at locations 1 to 11. The inert gas is typically nitrogen, but can be argon or any other noble gas or any other non-reactive moisture-free gas. This gas prevents reverse hydrolysis and permits the production of films with considerably improved properties. In order to determine the effects of moisture on film properties,- various SOG films were prepared using the apparatus shown in Figure 1. The results are shown in Figure 2. Film stress was monitored for films processed under uncontrolled atmospheric ambient and correlated with relative humidity to show its effect on film properties. Moisture pick-up and reverse hydrolysis pushes SOG SUBSTITUTE SHEET film stress toward compression (less tensile) because of an internal volumetric expansion due to the formation of silanol, Si-OH, pairs from more compact Si-O-Si bonds. The water pick-up effect is shown in Figure 2, which shows that the equilibrium SOG mechanical stress is mainly controlled by relative humidity. The compressive stress effect due to the formation of silanol pairs is effectively observed; the higher the relative humidity, the lower the tensile stress. Since the ambient conditions such as dew point, relative humidity, and duration of the ambient exposure are not constant from wafer to. * wafer and day to day, the resulting films properties fluctuate and manufacturing is difficult. While the invention is mainly applicable to inorganic SOGs, it can be applied with success to quasi-inorganic SOGs. Either type can be alloyed or not with phosphorus, boron, arsenic or lead. The benefits of the invention are more noticeable with alloyed SOGs, but, since a non-alloyed SOG is also extremely hygroscopic, the technique also apply to unalloyed SOGs. The number of coats can vary. Generally speaking, the higher the number of coats, the better, the end results. The in-line high temperature hot plates can be replaced by an in-line oven, an in-line plasma cure device, an in-line microwave device, or an in-line ozone device, or an in-line UV-ozone device, to permit cure in a moisture-free gas. SUBSTITUTE SHEET This inert gas it typically nitrogen, but can be argon or another noble gas, any other moisture free gas, or a mixture thereof. It can be heated or at room temperature. The inert gas ambient can also be replaced by vacuum which is also a. moisture-free environment. The gas can be ionized (plasma) or not. The equipment described is an in-line one-wafer-at- a- time processor but the invention also extends to single wafer or batch systems where the SOG can be applied by spin coating, immersion, spraying or other techniques that can be used to apply the SOG film to a substrate in which the environment is controlled. While the invention has been described in connection with semiconductor planarization techniques, it can be applied to other operations, such as: a) To other steps in the manufacturing of IC's • Diffusion source • Dielectric layer • Diffusion barrier • Encapsulation • Adhesion layer • Buffer layer • Antireflective layer • Corrosion protection layer To other semiconductor devices : • Emission diodes • Liquid crystal display • Electro chromic display • Photodetectors • Solar batteries SUBSTITUTE SHEET • Sensors To other fields: • Optical fibers • Corrosion protection 5 • Adhesion promoters • Friction reduction • Optical/thermal reflectance adjustments SUBSTITUTESHEET

Claims: Claims : 1. A a method of applying spin-on glass (SOG) to a substrate over low-melting point, non-refractory materials such as aluminum, characterized in that the spin-on glass is applied to the substrate in a moisture-free environment to minimize reverse hydrolysis during curing. 2. A method as claimed in claim 1, characterized in that said moisture-free environment comprises an inert gas. 3. A method as claimed in claim 2, characterized in that said inert gas is selected from the group consisting of: nitrogen, argon or other noble gas. 4. A method as claimed in claim 1, characterized in that said moisture-free environment comprises a vacuum. 5. A method as claimed in claim 1, characterized in that said spin-on glass is an inorganic spin-on glass. 6. A method as claimed in claim 5, characterized in that said spin-on glass is a phosphorus-alloyed spin-on glass. 7. A method as claimed in claim 1, characterized in that the following steps are carried in said moisture-free environment: 1) placing the substrate(s) to be processed in a sending cassette; 2) transferring, one by one, the substrates from the sending cassette to a coating and spinning zone; SUBSTITUTESHEET 3) dispensing a few ml of SOG solution at the centre of the substrate, and spinning said substrate; 4) transferring the coated substrate from the coating and spinning zone to a first in-line temperature controlled hot plate; 5) transferring the substrate from the said first in-line hot plate to the next and so on to the last in-line hot plate; 6) transferring the substrate from the last in¬ line hot plate to a substrate cooling zone; 7) transferring the substrate from the substrate cooling zone to a receiving cassette zone; and * 8) storing the substrate(s) that have received a first SOG coat in a receiving cassette; and 9) repeating steps 1 to 8 as many times as required. 8. A method as claimed in claim 1, characterized in that said substrate is a semiconductor wafer that is planarized with said spin-on glass. 9. An apparatus for applying spin-on glass (SOG) to a substrate over low-melting point, non- refractory materials such as aluminum, characterized in that said apparatus includes a chamber providing a moisture-free environment for the application of said spin-on glass to minimize reverse hydrolysis during curing. 10. An apparatus as claimed in claim 9, characterized in that said chamber is a vacuum chamber. 11. An apparatus as claimed in claim 9, characterized in that it comprises: a) a sending cassette to store substrates to be SUBSTITUTESHEET processed; b) a transport mechanism to transfer, one by one, the substrates from the sending cassette to the coating area; c) a coating zone where the SOG coating and substrate spinning is carried out; d) a transport mechanism to transfer the coated substrate from the coating zone to a first in-line temperature controlled hot plate; e) a transport mechanism to transfer the substrate from one in-line hot plate to the next; f) a transport mechanism area to transfer the substrate from the last in-line hot plate to. a substrate cooling zone; g) a substrate cooling zone; h) a transport mechanism area to transfer the substrate from the substrate cooling zone to a receiving cassette zone; i) a receiving cassette to store the substrates that have received the first SOG coat. j) a transport device to transfer the substrate from the receiving cassette area to the sending cassette zone; and h) a transport device to transfer the substrate from the receiving cassette area to a zone for carrying out a further process step. 12. A method as claimed in claim 1, characterized in that said materials are selected from the group consisting of Titanium, Titanium nitride. Titanium - Tungsten, Tungsten, or a combination thereof. 13. A method as claimed in claim 1, characterized in that said spin-on glass is a quasi-inorganic spin-on glass. SUBSTITUTESHEET

Inventors: OUELLET LUC, OUELLET, LUC
Applicants: MITEL CORP, MITEL CORPORATION
Application year: 1991
Main classification: H01L21
IPC codes: B05C11, C04B41, H01L21