METHOD FOR POLISHING SEMICONDUCTOR WAFERS

Abstract

A method for polishing a plurality of semiconductor wafers includes providing a polishing pad containing an abrasive substance bonded in the polishing pad; providing an alkaline polishing agent at a volumetric flowrate greater than or equal to 5 liters/min.; polishing the plurality of semiconductor wafers using the polishing pad; and circulating the polishing agent in a polishing agent circuit during the polishing.

Description

CROSS REFERENCE TO PRIOR APPLICATIONS

Priority is claimed to German Patent Application No. DE 10 2009 047 926.0, filed Oct. 1, 2009. The entire disclosure of which is incorporated by reference herein.

FIELD

The invention relates to a method for polishing semiconductor wafers.

BACKGROUND

Semiconductor wafers, in particular silicon wafers, are used for the fabrication of large scale integrated electronic components such as e.g. microprocessors or memory chips. In this case, stringent requirements are made in particular of the flatness of the front sides of the silicon wafers on which the electronic components are produced. This is necessary in order to minimize problems in the course of the exposure of the silicon wafers (lithography) and in the course of intermediate polishing processes (“Chemical Mechanical Polishing”, CMP) during the fabrication of the components.

Polishing the surfaces of semiconductor wafers pursues the goal of removing material from the surfaces of the semiconductor wafers in order to form a planar surface that is as uniform as possible. As a result, an undesirable surface topography and surface defects such as rough surfaces, crystal lattice damage or scratches can be removed and uniform surfaces can be made available for the subsequent further processing.

Therefore, after grinding, cleaning and etching steps in accordance with the prior art, the surface of the semiconductor wafers is smoothed by removal polishing.

In the case of double-side polishing (DSP), semiconductor wafers are introduced loosely into a so-called carrier and are polished on the front and rear sides simultaneously in a manner “floating freely” between an upper and a lower polishing plate covered with a polishing pad, with the aid of a polishing sol. The support parts supply a controllable pressure that presses the semiconductor wafer against the polishing support.

In the prior art, the polishing is carried out by relative movement between wafer and polishing pad under pressure and with a polishing agent (slurry) being supplied. The polishing agent is colloidally dispersed silica sol, for example. The polishing pad contains no abrasive. The interplay between mechanically abrasive action of the silica sol and chemical attack of the alkaline polishing agent then brings about the material removal that leads to the smoothing of the wafer surface.

One exemplary embodiment of DSP of silicon wafers is disclosed in US2003054650A. A suitable apparatus for such DSP polishing is presented in DE 100 07 390 A1.

In the case of CMP polishing, by contrast, only the front side is polished, for example by means of a soft polishing pad.

In conventional polishing agent preparation methods, the polishing agent constituents are procured as concentrated solutions and/or as solids. The solids, however, are generally dissolved with ultrapure water prior to use in separate preparation stations. These polishing agent constituents and ultrapure water are processed further in central preparation stations to form the finished polishing agent or to form partial mixtures. The solutions thus produced are usually stored temporarily in suitable containers. The ready to use mixture is conveyed to the consumers via a ring line. The polishing agent is then drawn off for the processing step via a branch line. The used polishing agent is subsequently discarded.

Conventionally, alongside the colloidally dispersed silica sol, an alkaline buffer solution is also used, e.g. K2CO3 or KOH solution.

SUMMARY OF THE INVENTION

In the use known from the prior art, the alkaline buffer solutions are suspected of causing metallic contamination on semiconductor wafers.

In an aspect of the invention this is avoided.

The invention relates to a method for polishing semiconductor wafers using a polishing pad containing an abrasive substance bonded in the polishing pad, and with an alkaline polishing agent being supplied, wherein a volumetric flow rate of the polishing agent is greater than or equal to 5 liters/min and the polishing agent is circulated in a polishing agent circuit during polishing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a flowchart of the method of the present invention.

DETAILED DESCRIPTION

FIG. 1 shows a method 100 for polishing a plurality of semiconductor wafers. A polishing pad is provided containing an abrasive substance bonded in the polishing pad (10). An alkaline polishing agent is provided at a volumetric flowrate greater than or equal to 5 liters/min. (20). The plurality of semiconductor wafers are polished using the polishing pad (30). The polishing agent is circulated in a polishing agent circuit during the polishing (40).

The alkaline polishing agent is preferably ultrapure water or deionized water containing compounds such as sodium carbonate (Na2CO3), potassium carbonate (K2CO3), sodium hydroxide (NaOH), potassium hydroxide (KOH), ammonium hydroxide (NH4OH), tetramethylammonium hydroxide (TMAH) or any desired mixtures of these compounds.

The use of TMAH is especially preferred.

The pH value of the polishing agent preferably lies in a range of 10 to 12.

The proportion of the stated compounds in the polishing agent solution is preferably 0.01 to 10% by weight, particularly preferably from 0.01 to 0.2% by weight.

The polishing agent solution can furthermore contain one or more further additives, for example surface-active additives such as wetting agents and surfactants, stabilizers acting as protective colloids, preservatives, biocides, alcohols and complexing agents.

In this case, the polishing agent is free of solids.

The polishing agent can also be a slurry containing abrasives.

The proportion of the abrasive substance in the polishing agent slurry is preferably 0.25 to 20% by weight, particularly preferably 0.25 to 1% by weight.

The size distribution of the abrasive substance particles is preferably markedly monomodal.

The average particle size is 5 to 300 nm, particularly preferably 5 to 50 nm.

The abrasive substance is preferably composed of one or more of the oxides of the elements aluminum, cerium or silicon.

A polishing agent slurry containing colloidally dispersed silica is particularly preferred.

The silicon wafers can either be pressed with the aid of a polishing head with the side surface to be polished against the polishing pad lying on a polishing plate and be polished in this way or alternatively be subjected to free floating double-side polishing—in a manner guided by carriers.

In the case of polishing with a polishing head, the latter preferably also includes a retainer ring that laterally encloses the substrate and prevents it from slipping from the polishing head during polishing.

In modern polishing heads, that side surface of the silicon wafer which is remote from the polishing pad bears on an elastic membrane that transmits the polishing pressure exerted. The membrane is part of a possibly subdivided chamber system that forms a gas or liquid cushion.

However, polishing heads are also in use in which an elastic support (“backing pad”) is used instead of a membrane.

The substrate is polished with the polishing agent being supplied between the substrate and the polishing pad and with rotation of the polishing head and of the polishing plate.

In this case, the polishing head can additionally also be moved translationally over the polishing pad, whereby more comprehensive utilization of the polishing pad area is achieved.

The method according to the invention can be carried out equally on single-plate and multi-plate polishing machines; in the form of both a single-side and a double-side polishing process.

Preference is given to the use of multi-plate polishing machines having preferably two, especially preferably three, polishing plates and polishing heads.

In the method according to the invention, a polishing pad is used which contains an abrasive substance bonded in the polishing pad (FAP or FA pad).

Suitable abrasive substances comprise, for example, particles of oxides of the elements cerium, aluminum, silicon, zirconium and particles of hard substances such as silicon carbide, boron nitride and diamond.

Particularly suitable polishing pads have a surface topography shaped by replicated microstructures. These microstructures (posts) have, for example, the form of pillars having a cylindrical or polygonal cross section or the form of pyramids or truncated pyramids.

More detailed descriptions of such polishing pads are contained in WO 92/13680 A1 and US 2005/227590 A1, for example.

The grain sizes of the FAP polishing pads used are preferably greater than or equal to 0.1 μm and less than or equal to 1.0 μm.

In principle, the polishing agent is preferably led past the polishing machine and circulated by means of ring lines (polishing agent circuit).

The media circulated in the polishing agent circuit are preferably cooled. The cooling is preferably effected by heat exchanger elements incorporated into the polishing agent circuit.

In the vicinity of the polishing machine, the polishing agent is preferably drawn off by means of a distributor unit and then introduced between polishing pad and semiconductor wafer.

The flow rates of the polishing media used are preferably set by means of measuring and regulating circuits as follows: the flow of the polishing media is set by means of flow controllers with actuating or needle valves or by means of metering pumps. The valves are set by means of a change in the line diameter. The flow rates are measured by means of impeller-type flow meters, for example. It goes without saying that automatic control by means of suitable software is possible and preferred.

The media regulated in this way, which can comprise abrasive polishing agent components, solutions and water, can be led into a mixing unit (e.g. to a pipe with static mixing elements), in which the polishing agent is mixed.

The polishing agent mixed in this way can then be led either firstly into a holding station or else directly into a holding container serving as a recycling holding container and as a container for possibly topping up the polishing agent.

From the holding container, the polishing agent is preferably conveyed, via a ring line, to one or more consumers and, by means of a branch line, to the polishing machine.

Polishing agent which has not been used flows back to the holding container and is conveyed to the consumer(s) again via the ring line.

The polishing agent consumed during polishing is collected by means of a suitable collection system and returned to the holding container via a line system. Said line system preferably provides a discharge, that is to say that, preferably, part of the consumed polishing agent is removed and not returned to the holding container. A corresponding amount of new, unconsumed polishing agent is then fed to the holding container.

Consequently, a specific portion of the polishing agent already used is preferably always replaced by new polishing agent.

Preferably, the alkaline component of the polishing agent is used for topping up the same.

Preferably, the pH value of the polishing agent in the holding container is always monitored by means of online analysis. If necessary, by discharging consumed polishing agent and topping up the polishing agent in the holding container, the pH value is correspondingly corrected, preferably by feeding in an alkaline solution.

The filling level of the holding container is preferably always at a specific minimum level. This is ensured by corresponding preparation with fresh polishing agent.

In the method according to the invention, a defined amount or concentration of polishing agent is preferably collected in a holding container and then reused, a defined amount of consumed polishing agent being replaced by fresh polishing agent. This takes place in such a manner that, firstly, fresh polishing agent is produced and, after it has been used once, it is guided into a holding container. If the filling level in the holding container reaches its highest level, the processing process is changed over completely to the polishing agent which has already been used once and is located in the collection container. Depending on the particular application, the filling level in the holding container decreases as a result of media losses/rinsing losses, clarification rate, etc. When a specific filling level in the holding container is reached or after a predefined duration of use or after a specific number of wafers, the polishing agent which has already been used one or more times is topped up by the addition of fresh polishing agent from a supply container.

Suitable for carrying out the method is a polishing installation for semiconductor wafers which has means which are able to replace polishing agent within defined limits of amount and concentration. A polishing installation of this type preferably has at least one holding container for polishing agent which has already been used and a container for fresh polishing agent, and also means for discharging consumed polishing agent and adding fresh polishing agent.

One preferred polishing installation for semiconductor wafers is one which has a regulator, the polishing agent being replenished on the basis of the chemical consumption. This is carried out by means of an apparatus which determines the chemical consumption during the polishing method in the outgoing flow of polishing agent and then supplies correspondingly fresh polishing agent or specific polishing agent components. The chemical consumption is determined by means of an electrode, for example.

A further preferred polishing installation for semiconductor wafers has means which discharge and replace a defined amount or concentration of polishing agent. These means are, for example, commercially available measurement systems and pumps.

A further preferred polishing installation for semiconductor wafers has means which collect a defined amount or concentration of polishing agent in a holding container for reuse and means which replace consumed polishing agent by a defined amount of fresh polishing agent.

The advantages of the present invention reside in the fact that the composition of the polishing agents remains stable even in the case of regenerated polishing agents.

The polishing abrasion rate remains virtually constant and there is no need to readjust the polishing time, in which case the polishing times also remain constant and a better quality, for example with respect to the desired thickness, is achieved.

Use of relatively high volumetric flow rates of greater than or equal to 5 liters/min is provided according to the invention. Preferably, the volumetric flow rate of the polishing agent is 5-10 liters/min, particularly preferably 5-9 liters/min, and especially preferably 6-8 liters/min. The comparatively high volumetric flow rate leads to better dissipation of heat from the semiconductor wafers and from the immediate surroundings thereof, that is to say polishing agent and polishing pad.

The invention enables higher flexibility with regard to the parameters and variations in process optimization, development and expansion of the method. Using process-relevant measurement data, the invention allows the composition of the polishing agent to be varied.

Preferably, in a polishing sequence which requires three media, for example, all three media are reused, topped up and/or replenished in the manner described.

Owing to the process-technological precautions, in particular by virtue of the fact that the polishing agent with the critical alkaline components is continuously circulated, topped up and replenished, the ingress of metal contamination in the bulk of the semiconductor wafer can be reduced. Critical metals are copper and nickel, in particular.

The polishing agent recycling also makes it economic to use particularly pure alkaline media such as TMAH, which further reduces the metal contamination.

Finally, the diffusion rates of the metal ions can be reduced by the polishing agent circuit being cooled.

Claims

1. A method for polishing a plurality of semiconductor wafers comprising:

providing a polishing pad containing an abrasive substance bonded in the polishing pad;

providing an alkaline polishing agent at a volumetric flowrate greater than or equal to 5 liters/min.;

polishing the plurality of semiconductor wafers using the polishing pad; and

circulating the polishing agent in a polishing agent circuit during the polishing.

2. The method as recited in claim 1, wherein the alkaline polishing agent is ultrapure water containing at least one of sodium carbonate, potassium carbonate, sodium hydroxide, potassium hydroxide, ammonium hydroxide and tetramethylammonium hydroxide.

3. The method as recited in claim 1, wherein the alkaline polishing agent includes a slurry containing at least one abrasive selected from the group consisting of oxides of aluminum, cerium and silicon.

4. The method as recited in claim 2, wherein the polishing agent has a pH value in a range of 10 to 12.

5. The method as recited in claim 1, wherein the polishing agent includes colloidally dispersed silica and TMAH.

6. The method as recited in claim 1, wherein the polishing pad includes at least one abrasive selected from the group consisting of particles of silicon carbide, boron nitride and diamond and of oxides of cerium, aluminum, silicon and zirconium, and wherein the particles have an average particle size greater than or equal to 0.1 μm and less than or equal to 1.0 μm.

7. The method as recited in claim 1, wherein the circulating includes cooling the polishing agent during the circulating.

8. The method as recited in claim 1, further comprising replenishing a defined amount or concentration of the polishing agent in a manner regulated according to a chemical consumption.

9. The method as recited in claim 1, further comprising discharging a defined amount or concentration of used polishing agent and replacing the discharged used polishing agent with unused polishing agent.

10. The method as recited in claim 1, further comprising collecting a first defined amount or concentration of used polishing agent in a holding container, reusing the first defined amount or concentration of the used polishing agent and replacing a second defined amount or concentration of the used polishing agent with a new polishing agent.