JPH09193137A - Production of silicon carbide wafer and wafer of 4h silicon carbide - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a silicon carbide wafer from a silicon carbide boule without generating mechanical stress or damage like a case using a diamond saw. SOLUTION: A wire electrode is positioned in adjacent relation to a silicon carbide boule doped so as to be sufficient to have conductivity. The flow of water is held between the boule and a wire. A current sufficient to generate a discharge arc between the boule and the wire is cyclically applied to the wire electrode. Further, the wire is moved so as to traverse the boule while the current is cyclically applied to slice a wafer of 4H silicon carbide from the boule.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a semiconductor wafer from a semiconductor bulk crystal, and more particularly, to a 4H silicon carbide wafer from a silicon carbide bulk crystal or "boule" using electrical discharge machining. On how to do.

[0002]

BACKGROUND OF THE INVENTION Semiconductor devices are fabricated on a wafer of semiconductor material, a thin, generally circular slice of semiconductor material having a diameter of about 25.4 to about 102 mm (about 1 to 4 inches), in a common environment. Often formed.

As part of the overall process of manufacturing a wafer, first, bulk crystals (called ingots or boule) using one of several well-known techniques, such as growth from melting or sublimation. There is).
The bulk crystal is then typically polished along one side to form a surface called the "primary flat," and then along another side to form a "secondary surface." Plane (secondary flat) "
Form a surface called. The relative position of such planes is often used to determine the nature and orientation of the crystal.

After growing the ingot and then determining the orientation and characteristics according to the plane, the ingot is sliced into wafers. The most common slicing technique is to slice a wafer from a boule using a diamond saw or diamond saw. The sliced wafers are then "lapped" with an appropriate abrasive to provide a uniform surface within 1-2 microns of perfect flatness. The wafer is then subjected to chemical etching or polishing.

In recent years, the increasing utility and use of silicon carbide as a suitable material (and indeed an excellent material) for many semiconductor applications has been demonstrated. Silicon carbide is stable at high temperatures, has a wide bandgap width, high saturated electron drift velocities, and good radiation hardness.

Silicon carbide is also one of the physically hardest materials currently in existence. In fact, silicon carbide, either alone or in the form of composites, is widely used as an abrasive. Thus, the relatively simple operation of slicing bulk crystals into wafers with a diamond saw in softer materials becomes very difficult in silicon carbide.

Although diamond saws can be used to cut bulk silicon carbide crystals into wafers, diamond saws have several drawbacks. As the most basic problem, a diamond saw causes stress on the crystal when cutting the crystal. This stress heats the material being sliced, causing stress cracks on the wafer,
It causes sintering strain and surface damage. In general,
All such stress cracks and the like have a detrimental effect on devices formed using the wafer and may even render the wafer unusable.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of forming a silicon carbide wafer from a silicon carbide boule that eliminates the mechanical stress and damage that techniques such as diamond saw tend to produce. That is.

[0009]

The present invention accomplishes the above objectives by providing a method of making portions of silicon carbide from bulk crystals of silicon carbide. The method cuts a boule of 4H silicon carbide that has been sufficiently doped to be conductive by electrical discharge machining with an electrode tool and a dielectric fluid.

The above and other objects, advantages and features of the invention, and the manner in which these objects, advantages and features are achieved, will be readily understood from the following detailed description of the invention.

[0011]

DETAILED DESCRIPTION OF THE INVENTION The present invention is a method for producing a portion of silicon carbide from a bulk crystal of silicon carbide, the method comprising: fully doping electrically conductive 4H.
A step of electrical discharge machining ("EDM") is performed on the silicon carbide boule using an electrode tool and an electric fluid.

As is known to those who know silicon carbide, 4
The H polytype has a particularly low resistivity, which makes it a good candidate for processing the above types. The suitability of the n-type 4H material is much higher than that of, for example, 6H silicon carbide, and thus in the present invention using 4h silicon carbide, a relatively high EDM cutting rate (25 mils per minute: 1000
A mil = about 25.4 mm (1 inch) can be achieved. In contrast, cutting high resistivity materials such as lightly doped p-type silicon carbide, or semi-insulating materials, is very difficult using electrical discharge machining.
In particular, 4H silicon carbide having an n-type carrier concentration higher than about 10 ¹⁵ cm ^-3 is most suitable for the present invention.

In general, electrical discharge machining is a relatively well known technique, but its use in the environment of the present invention is novel. Therefore, the properties and operation of the EDM machine are not described in detail herein, but in EDM, the dielectric medium (water or other) between the work piece (here, silicon carbide boule) and the electrode is used. It is only briefly mentioned that a discharge is established before and after (for example, the dielectric fluid). Such discharges tend to remove pieces of the work piece. When using wires as electrodes, such wires are usually oriented vertically and can make relatively fine cuts in the work piece. A recent example of an EDM machine is
Hosaka US Pat. No. 5,171,95
5 and 5,243,165. Such examples are given by way of example and not limitation, the use of EDM is relatively widely established for cutting metals.

In typical EDM techniques, and in the present invention, the discharge is usually established before and after a dielectric fluid, which for the present invention preferably comprises deionized water. As mentioned above, when using wires as EDM electrodes, the method preferably includes slicing a 4H silicon carbide wafer from a bulk single crystal. However, different electrodes can be used to machine 4H silicon carbide work pieces into some other form or shape, and such examples are also within the scope of the invention. For example, silicon carbide boules may be cored and EDM is expected to work properly in such cases.

Expressed as a number of steps, the method of the present invention comprises the steps of:
Positioning the wire electrode adjacent the boule of silicon carbide. A flow of deionized water is maintained between the boule and the wire, while at the same time a current is periodically applied to the wire electrode. The applied current is large enough to create a discharge arc between the boule and the wire. The wire is moved across the boule while periodically applying a current to slice a 4H silicon carbide wafer from the boule.

The method preferably further comprises the step of removing material from between the wire and the boule dislodged from the boule by electrical discharge. If such material is not sufficiently removed, the wire and such material will tend to short rather than cause a proper cutting discharge.

EDM technology can be described as a "spark" process. In other words, EDM uses a periodic discharge rather than a continuous discharge to cut 4H silicon carbide material. Generally speaking, the occurrence of periodic sparks is determined by "on" and "off" times. The longer the on-time, the more power is applied to the wire, which generally results in a faster disconnection. However, as a corresponding factor, the finish produced by such faster cutting is less than that produced by cutting the same material more slowly.
Poor quality, which can lead to accuracy problems. Also, if the on-time is too long, the wire tends to break.

To make this modest, a particular off-time is chosen. The off-time is determined as the length of time that the supply of electricity to the wire is stopped during the spark.
The off time gives the opportunity to remove the cut particles from the bulk single crystal and prevents the aforementioned short circuit between the wire and the work piece.

Therefore, in a preferred embodiment, the present invention comprises the step of periodically applying a current between about 0 and 350 amps to the wire, wherein the current is about 2.0 and 2. It is provided for an on-time of between 0.5 ms and then an off-time of about 14 and 16 ms. The open circuit voltage is preferably about 350 volts.

In a special example of the invention, a Sonic with an optional high power circuit (PCD kit)
A Model 280L EDM machine was used to slice 4H silicon carbide wafers from silicon carbide bulk single crystals. The on-time was set to 2 ms and the off-time was set to 14 ms. Peak current (IP) is 3
The amperage was set to 50 amps, the average current (MA) was set to about 50 milliamps, and the open circuit voltage was set to 350 volts. The cutting wire was a GISCO® wire with a diameter of about 0.254 mm. The wire is formed from a base material or substrate consisting of copper and zinc that coats the copper and is approximately 9,139 kg / cm.
^{It has} a tensile strength of ² (130,000 psi), an elongation of less than 2%, and a conductivity (IACS) of 20%.

After cutting the wafer according to the invention,
Such wafers are further conditioned by procedures well known to those skilled in the art. The subsequent handling of the wafer is
Wafer is lapped and polished (p
and the surface of the wafer is adjusted by etching the wafer. Generally, it is epitaxially grown. Chemical etching techniques, such as the plasma technique as described in US Pat. No. 4,946,547, assigned to the assignee of the present application and incorporated herein by reference in its entirety, are suitable. There is.

Generally speaking, the present invention provides a method of making a silicon carbide 4H wafer useful as a device precursor, the method being well-doped to be electrically conductive. The step of growing the bulk single crystal, the step of slicing such a bulk single crystal with an EDM wire, and the step of finishing the surface of the sliced wafer. An effective method for growing bulk single crystals of silicon carbide is Davis.
U.S. Pat. No. 4,866,005, which is assigned to the assignee of the present application and is hereby incorporated by reference in its entirety.

The method may further include growing an epitaxial layer of 4H silicon carbide on the finished surface of the wafer. As will be appreciated by those skilled in the art, the epitaxial layer can be the same type of conductivity as the wafer, or the opposite type of conductivity, and the epitaxial layer can be used to form junctions and other device elements. Can form the basis of yet another epitaxial layer for forming the. An exemplary method for epitaxial growth of silicon carbide is disclosed in US Pat. No. 4,912,06.
No. 3, No. 4,912, 064, No. 5,011,549
No. 5,119,540.

Accordingly, in yet another aspect, the invention also includes wafers made according to the methods of the invention described herein.

Generally speaking, the present invention provides very flat and parallel wafers, also results in very little kerf loss (loss of material due to cutting) and, in addition, a machine that results in compression of the crystal structure. Does not use physical power. For example, when cutting silicon carbide with a diamond saw, the kerf loss is about 40 mils, while when cutting with EDM it drops to 12 mils. E of the present invention
DM equipment is more accurate and versatile than diamond saw cutting (EDM cuts regardless of crystal orientation, diamond saw does not),
It can be easily assembled and can be automated. Finally, the present invention does not exert excessive pressure and heat on the material being sliced, thus preventing stress strains, thermal effects and other surface damage.

Although specific language is used herein to disclose representative preferred embodiments of the invention, such term is used in a generic sense merely for purposes of explanation. Thus, the scope of the invention is not set forth for purposes of limitation, but is set forth in the following claims.

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Claims (12)

[Claims] 1. A method of manufacturing a silicon carbide wafer from a silicon carbide boule, comprising: positioning a wire electrode adjacent to a boule of 4H silicon carbide that is sufficiently doped to be electrically conductive. Maintaining a flow of water between the boule and the wire; periodically applying to the wire electrode a current sufficient to cause a discharge arc between the boule and the wire; Periodically moving the wire across the boule, thereby slicing a 4H silicon carbide wafer from the boule. 2. The method of claim 1, further comprising removing material dislodged by the discharge between the wire and the boule from between the wire and the boule. 3. The method of claim 1, wherein the step of periodically applying a current to the wire electrode comprises the step of applying a current between about 0 and 350 milliamps. 4. The method of claim 1, wherein the step of periodically applying a current to the wire electrode comprises the step of applying a current in a cycle pattern, wherein the cycle pattern comprises about 2.0 and 2. A method characterized in that the current is applied for a time of between 5 ms and then the current is stopped for a time of between about 14 and 16 ms. 5. A method of manufacturing a portion of silicon carbide from a bulk crystal of silicon carbide, wherein a boule of 4H silicon carbide sufficiently doped to be electrically conductive is used with an electrode tool and a dielectric fluid. , A method comprising the step of electrical discharge machining. 6. The method of claim 5, wherein the electrode tool comprises a wire and the dielectric fluid comprises deionized water. 7. The method of claim 4, wherein the step of electrical discharge machining a boule comprises slicing a 4H silicon carbide wafer from the boule. 8. A method of manufacturing a 4H silicon carbide wafer that can be used as a device precursor, comprising: growing a bulk single crystal of 4H silicon carbide sufficiently doped to be electrically conductive; Slicing a wafer from the bulk single crystal using an EDM wire. 9. The method according to claim 1, claim 7, or claim 8, wherein the sliced wafer is lapped, the lapped wafer is polished, and the polished wafer is etched for epitaxial growth. A method further comprising the step of finishing the surface of the wafer by providing a surface. 10. The method of claim 9, further comprising growing an epitaxial layer of 4H silicon carbide on the finished surface of the wafer. 11. The method of claim 8, wherein the step of slicing the wafer comprises: positioning a wire electrode adjacent to a bulk single crystal of 4H silicon carbide; and between the boule and the wire. Maintaining a flow of deionized water, periodically applying a current sufficient to cause a discharge arc between the bulk single crystal and the wire to the wire electrode, and periodically applying the current. Moving the wire across the bulk single crystal while providing. 12. A 4H silicon carbide wafer produced according to the method of claim 1, claim 5 or claim 8.