FIELD OF THE INVENTION
The present invention is related to the fabrication of flat substrates such as semiconductor wafers, flat panel displays, solar cells for microelectronics applications or of flat substrates for other applications which require a cleaning or etching step of said substrates during manufacturing.
The present invention relates more particularly to an apparatus and a method for wet cleaning or etching of said flat substrate.
BACKGROUND OF THE INVENTION
In the processing of semiconductor wafers, the semiconductor surface preparation prior to various processing steps such as oxidation, deposition or growth processes, has become one of the most critical issues. With the rapid approach of subhalf micron design rules, very small particles with submicron dimensions and low levels of metal impurities (−1010 atoms/cm2) can have a detrimental effect on process yields. A commonly used technique to reduce foreign particulate matter contamination level on semiconductor surfaces, is the immersion of wafers in liquids such as chemical solutions. The semiconductor wafers are usually placed into a tank or vessel or wet bench, which holds a liquid for cleaning or etching. Such tank or vessel or wet bench is installed in a clean room environment. Often a wet bench contains several vessels or tanks.
Semiconductor wafers or substrates can be immersed into the liquid from the upper part of the tank. Another way to put the substrates in contact with the liquid is to use a door or an entrance at the side wall of the tank and to place a batch of wafers into the tank. After the door is closed and sealed, liquid is flowing into the tank, again to make contact between the cleaning or etching liquid and the substrates. In both approaches, one can switch from one liquid to the other liquid in the same tank. This method is commonly denominated as a single tank processor. Variations of these two techniques are also used for single wafer cleaning or etching purposes. In such a case, the wafer can be placed into a small vessel, which can be larger than the wafer. In some cases, the wafers are immersed into the liquid; in other cases, the wafers are loaded into an empty vessel and the liquid is flowing into the vessel.
A particular apparatus used after cleaning or etching of semiconductor wafers is disclosed in EP 0 385 536, the teaching of which is incorporated herein by reference.
EP-A-407-044 discloses a system having two rooms, each containing a liquid. The system can be used in a semiconductor wafer manufacturing line and allows a wafer to be transported from one room of the system to another room of the system without an intermixing of the liquids occurring. The level of the liquids in the two rooms is adapted as to adjust the respective pressures of the liquids and an elaborate shutter device is provided in the system for opening and closing an opening between the two rooms without causing intermixing through flow and turbulence of the liquids.
A first drawback of the above-mentioned tools and methods is the footprint necessary, i.e., the surface area within a clean room occupied by the tank or vessel. Above-mentioned tools making use of a tank or vessel are big and occupy a large area within a clean room. It is to be understood that clean room area is very expensive due to the costs of maintaining an ultra-clean atmosphere.
Another drawback is the throughput of wafers. When, for example, a single tank processor is used, switching from one liquid to another liquid takes time and the system can have a lower throughput compared to a classical wet bench approach.
Another drawback of the prior art cleaning or etching tools and methods is that a non-uniformity in the cleaning or etching process occurs remains. This is to be understood as follows: when a wafer is dipped in a tank, the bottom part of the wafer is always remaining some longer in the cleaning or etching liquid than the upper part of the wafer. Therefore, the contact-time of several parts of the wafer is not identical.
A further drawback of the prior art cleaning or etching tools and methods is that a drying and/or rinsing step is required after a cleaning or etching step. This drying and/or rinsing step takes extra time and therefore increases the processing time of wafers.
AIM OF THE INVENTION
It is an aim of the present invention to provide an apparatus and a method for wet cleaning or etching a flat substrate wherein the above-mentioned drawbacks of the prior art tools and methods have less impact. The apparatus and the method of the present invention allow a high throughput of flat substrates and a high cycle time, allow a more uniform contact time for several parts of one wafer, allow the development of manufacturing systems with a small footprint and allow flat substrate manufacturing with the wet cleaning or etching step and the required drying procedure being executed in one processing step.
SUMMARY OF THE INVENTION
The first object of the present invention is to provide an apparatus for wet cleaning or etching a flat substrate which comprises a tank with an opening for said substrate, said tank containing a cleaning or etching liquid and being installed in an environment consisting essentially of a gas or of a mixture of gases, such as a clean-room.
The said opening is present below the liquid-surface. The apparatus further comprises means to prevent the liquid from flowing through the opening out of the tank into the environment.
Advantageously, the substrate is transferred from within said environment into said apparatus through said opening. Said apparatus can be for instance used as part of a manufacturing line for the production of said flat substrate or as part of a manufacturing line for developing features in said substrate, such as integrated circuits in a silicon wafer.
In a first aspect of the apparatus according to the invention, said means are realized by the dimensions of the opening, said opening being such narrow that the liquid is prevented from flowing through it due to the surface tension and/or the capillary effect of said liquid.
A first embodiment of the apparatus to achieve this is characterized in that the opening is marginally larger than the thickness of said substrate and is, with respect to the thickness of the wall of the tanks, a lengthened passage.
A second embodiment of the apparatus to achieve this is characterized in that the opening is marginally larger than the thickness of said substrate and is towards the environment a converging passage.
In a second aspect of the present invention, said means comprise a portion in the tank, which portion is above the liquid and is filled with a gas or a gas mixture with a pressure being lower than the gas pressure within said environment. An advantageous embodiment of the apparatus to achieve this, is characterized in that said means comprise a pump connected with the room in the tank for sucking the gas and thereby reducing the gas-pressure in the tank and subsequently reducing the liquid-pressure near the opening.
In a third aspect of the apparatus of the invention, said means comprise a room adjacent to said tank, said room having an opening for said substrate and said room being filled with a gas or a gas mixture with a pressure being higher than the pressure within said environment. The wafer is transferred from said environment through said room into said tank.
In a fourth aspect of the apparatus according to the invention, said tank can be adjacent to a process chamber wherein, for instance, a gas phase etching process of semiconductor wafers can occur. In this embodiment, wafers are transferred from within said environment through said tank into said process chamber. In this way, the process chamber can be isolated from the environment thereby avoiding the introduction of moisture, oxygen or other rest gases in the chamber. Also, the introduction of a wafer into the process chamber through said tank can occur with a relatively simple automation setup. By preference, one tank is located adjacent to the process chamber at the inlet opening of said process chamber and another tank is located adjacent to the process chamber at the outlet opening of said process chamber. Said process chamber can be an etcher or a RTP-system or a EPI-system or another system being part of the manufacturing line of said substrate.
In a fifth aspect of the apparatus of the invention, said means comprise means outside the tank for directing a gas flow towards the opening. In an advantageous embodiment of the invention, the gas flow comprises N2. In another embodiment the gas flow comprises a vapor which is not condensing on said substrate at said opening, said vapor being chosen from a group of substances which are miscible with said liquid and which will form with said liquid a mixture having a surface tension lower than that of said liquid alone. Preferably the vapor is an IPA vapor.
In a further aspect of the apparatus of the invention, said apparatus comprises an inlet opening and an outlet opening. Said inlet opening can be a slice in a first sidewall of said tank and said outlet opening can be in another wall of said tank, said substrate being passed essentially horizontally through said apparatus. In another embodiment said inlet opening is a slice in the bottom of said tank and said outlet opening is in the top of said tank, said substrates being passed essentially vertically through said apparatus.
Yet another embodiment of the apparatus according to the invention is characterized in that the tank further comprises an inlet and an outlet for said liquid, said outlet being near said opening and said means comprises a pump connected to the outlet for sucking the liquid and thereby reducing the liquid-pressure near the opening.
The invention furthermore pertains as a second object to a method of wet cleaning or etching a flat substrate in particular a semiconductor wafer, comprising the steps of putting a substrate via an opening in and out of a tank filled with a cleaning or etching liquid. The method in accordance with the invention is characterized in that the substrate is put into and/or taken out of the cleaning or etching liquid at a level underneath the liquid-surface via an opening in the tank present below the liquid-level in the tank and thereby preventing the liquid from flowing through the opening out of the tank.
More particularly, the method of wet cleaning or etching of a flat substrate in a tank filled with at least one cleaning or etching liquid comprises the steps of:
loading from within a gaseous environment said substrate through an inlet opening in said tank, said inlet opening being at a level underneath the surface of said liquid; and
unloading said substrate out of the tank into said gaseous environment while preventing said liquid from flowing out of said tank.
An embodiment of the method according to the invention is characterized in that, gas present above the liquid in the tank is sucked out of the tank to reduce the pressure inside the tank.
A further embodiment of the method according to the invention is characterized in that the liquid is sucked out of the tank near said opening to reduce the liquid-pressure near the opening.
Another embodiment of the method according to the invention is characterized in that a gas flow is directed towards said opening from outside the tank to dry the substrate and to prevent the liquid from flowing out of the tank.
The method of wet cleaning or etching a flat substrate comprises the step of transferring from within a gaseous environment said substrate through a tank filled with a cleaning or etching liquid, said substrate being introduced into said tank through an inlet opening and taken out of said tank through an outlet opening underneath the liquid-surface and are transferred at a uniform speed through said liquid thereby exposing essentially each part of said substrate to said liquid for the same time period.
Said apparatus and said method offer as an advantage the possibility of placing tanks no longer side by side but on top of each other. This approach makes it possible to reduce drastically the footprint of a cleaning or etching system and keeping a high throughput. Also a cleaning tank can be placed directly besides of a next apparatus for further processing. The footprint of the apparatus of the invention furthermore is much smaller than the footprint of a classical wet bench. At the same time the apparatus and method disclosed in the present application allow for keeping a high throughput. Said apparatus and said method offer as another advantage the possibility of achieving a uniform cleaning or etching of wafers being transferred through the cleaning or etching liquid in said tank. As wafers can be transferred at a uniform speed throughout said tank, each part of said wafers is exposed for the same time period to said cleaning or etching liquid. Another advantage of said apparatus and said method is that by the use of dilute chemical mixtures as a cleaning liquid in said tank, a rinsing step after said cleaning or etching step in said tank can be avoided. In this way, a fast processing time of wafers and a high throughput can be achieved. It is disclosed in the prior art that if a cleaning solution is dilute enough, being diluted preferably with DI-water, wafers may be dried immediately without ultra pure water rinsing after cleaning or etching. As a result, no detrimental residues are left behind on the wafer surface. This works well for instance with dilute HCl being diluted with DI-water in the range of pH 2-4. Another possibility is to have a very dilute SCl cleaning step with a volume mixing ratio of NH4OH/H2O2/H2O in a range of about 1/1/1000. Another possible solution that can be used to achieve this purpose is to use ozonated water. A detailed description of the use of such dilute chemicals for wafer cleaning/rinsing purposes is given in the co-pending patent application EP 96309145.9 “Drying of Wafers Directly From Chemical Baths with No Ultra-Pure Water Rinse” being co-owned by IMEC and Texas Instruments.
It is to be understood that the apparatus and method disclosed in this document can be used for cleaning or etching of any kind of flat substrates. Thus, flat panel display substrates or solar cell substrates or glass substrates or metal plates or printed circuit boards can be cleaned or etched using a specific embodiment of the apparatus according to this invention.