WO2002092885A1

WO2002092885A1 - Electrically conductive crystal seed chuck assembly

Info

Publication number: WO2002092885A1
Application number: PCT/US2001/015622
Authority: WO
Inventors: Carl F. Cherko
Original assignee: Memc Electronic Materials, Inc.
Priority date: 2001-05-15
Filing date: 2001-05-15
Publication date: 2002-11-21

Abstract

A chuck assembly which is electrically conductive for detecting initial contact between a semiconductor seed crystal and a molten semiconductor source material for growing an ingot according to the Czochralski process. The chuck assembly holds the seed crystal and suspends the seed crystal from an elongate member. The chuck assembly includes an upper chuck body in electrical continuity with the elongate member and a lower chuck body in electrical continuity with the seed crystal. An electrically conductive element extends between conductive portions of the upper and lower chuck bodies for providing electrical continuity between the seed crystal and the elongate member to enable flow of electrical current when the seed crystal touches the melt. The conductive element is preferably a coil spring which resiliently adjusts to variations in spacing between the upper and lower chuck bodies.

Description

ELECTRICALLY CONDUCTIVE CRYSTAL SEED CHUCK ASSEMBLY

Background of the Invention

This invention relates generally to the production of semiconductor ingots, and in particular to a seed chuck assembly which is electrically conductive for detecting contact between a seed crystal and a molten semiconductor source material.

Most semiconductor chips used in electronic devices are fabricated from single crystal silicon prepared by the Czochralski method. In that method, a quantity of molten source material is formed by melting polycrystalline silicon in a quartz crucible in a low pressure, inert gas environment. A seed crystal is lowered into the crucible to a point where its bottom tip touches the surface of molten source material. As source material crystallizes on the seed crystal and grows into an ingot, it is slowly pulled upwardly from the melt. The seed crystal is held in a chuck which is suspended from an end of a cable or a rod above the melt. The cable is typically wrapped around a drum which is rotated to lower the seed crystal or to slowly pull the ingot upwardly from the melt as it grows.

Ideally, the seed crystal should be lowered until its bottom tip just touches the surface of the melt. That point establishes a position reference where the source material begins crystallizing and forming a neck to support the ingot. If lowered too far, the seed crystal is partially submerged and begins to melt, leading to subsequent inaccuracies in neck length and potential degradations in crystal quality. Typically an operator determines the position of the seed crystal relative to the surface of the melt by a visual method through a window, using either a camera or manual observation. Unfortunately, these methods are prone to inaccuracy because visibility inside the crucible is often poor. The window is positioned above the crucible at a vantage point which fails to provide direct line-of-sight to the bottom tip of the seed crystal. Further, visual systems are not compatible with fully-automated machine processes which avoid the cost of direct operator control. A method providing better accuracy in determining when the tip of the seed crystal first contacts the melt is to detect electrical continuity. Specifically, an electrical voltage is applied across the melt and cable. When the seed crystal touches the melt, it completes an electrical circuit passing through the chuck assembly which initiates flow of an electrical current. Detection of a current establishes the position of the seed crystal and chuck relative to the melt surface. The motion of the chuck is stopped so that the neck of the ingot begins forming on the tip of the seed crystal. The method is readily adaptable for automated processes in crystal growing because machine operation may be pre- programmed and it does not require manual intervention.

A drawback to the electrical method is that some chucks are not conductive and thereby prevent flow of electrical current. Some chucks are formed in two parts, including an upper part which attaches to the cable and a lower part which holds the seed crystal. Both parts must be made of materials which can withstand the high temperature environment and which avoid releasing contaminative particles into the melt. Typically the lower part is made of graphite and the upper part is made from a material suitable for thermally insulating the lower end of the cable, such as quartz. Although graphite is an effective electrical conductor, quartz has high electrical resistance which effectively blocks electrical continuity between the cable and the seed crystal unless undesirably high voltages are used. The chuck is therefore practically unable to complete an electrical circuit or serve as a tool for detecting when the seed crystal initially contacts the melt.

Summary of the Invention Among the several objects and features of the present invention may be noted the provision of a chuck assembly for electronically detecting contact between a semiconductor seed crystal and a molten source material; the provision of such an assembly providing electrical continuity between the seed crystal and a supporting cable; the provision of such an assembly which maintains electrical continuity for a range of dimensional variations within the chuck assembly arising from both manufacturing tolerances and thermal expansion; and the provision of such an assembly which may be formed from existing chucks without substantial modification.

In summary, a chuck assembly according to the present invention holds a seed crystal and suspends the seed crystal from an elongate member in a growth chamber as an ingot is grown on the seed crystal from a semiconductor source material according to the Czochralski process. The assembly provides electrical continuity between the seed crystal and the elongate member to enable flow of electrical current therethrough for sensing contact between the seed crystal and the source material. The chuck assembly comprises an upper chuck body constructed for attachment to an end of the elongate member, the upper body having an electrically conductive portion in electrical continuity with the elongate member when the upper body is attached to the elongate member. A lower chuck body is constructed for holding the seed crystal, the lower body having an electrically conductive portion in electrical continuity with the seed crystal when the lower body is holding the seed crystal. The upper body and lower body are constructed for connection in an attached configuration to suspend the seed crystal from the elongate member. An electrically conductive element extends between the conductive portions of the upper and lower chuck bodies at the attached configuration for providing electrical continuity between the seed crystal and the elongate member.

Other objects and features of the present invention will be in part apparent and in part pointed out hereinafter.

Brief Description of the Drawings

FIG. 1 is a schematic cross sectional view of a crystal puller including a chuck assembly of the present invention;

FIG. 2 is an elevational view of the chuck assembly with an upper part broken away;

FIG. 3 is an exploded view of the chuck assembly; FIG. 4 is an enlarged, fragmentary section of the chuck assembly of Fig. 2; and FIG. 5 is an enlarged sectional view of a helical spring of the chuck assembly.

Corresponding reference characters indicate corresponding parts throughout the views of the drawings.

Detailed Description of the Preferred Embodiment

Referring now to the drawings and in particular to Fig. 1 , a crystal puller for producing monocrystalline semiconductor ingots according to the Czochralski method is indicated generally at 10. The crystal puller 10 includes a growth chamber 12 and a crucible 14 for initially holding solid semiconductor source material, such as polycrystalline silicon, which is heated to a temperature above the melting point of the material to form a liquid melt 16.

A seed chuck assembly, indicated generally at 20, is constructed for holding a seed crystal 22 and is suspended in the growth chamber 12 from an elongate member 24 suitable for supporting an ingot 26. The elongate member 24 is preferably a flexible cable, as shown in Fig. 1 , although it can be a rigid rod or other support. The cable 24 is attached to a puller apparatus which includes a winch (generally indicated at 28) for raising or lowering the chuck 20. The winch 28 includes a drum 30 on which the cable is wound, a pulley 32, and a motor (not shown). The winch 28 is located in a housing 34 above the growth chamber. The chuck 20 is lowered until a bottom tip of the seed crystal 22 comes into contact with the molten semiconductor source material 16. As the single crystal ingot 26 grows on the seed crystal 22, the chuck 20 is raised to lift the ingot. The crucible 14 rests on a turntable used to rotate the crucible about a vertical axis. The turntable can also raise the crucible within the growth chamber 12 to maintain the surface of the molten source material 16 at the same level relative to the growth chamber 12 as the ingot 26 is grown and source material is removed from the melt.

Referring now to Fig. 2, the chuck assembly 20 includes an upper chuck body 40 and a lower chuck body 42 which are joined together in an attached configuration and suspended from the cable 24. The upper and lower chuck bodies 40, 42 are generally cylindrical with approximately equal diameters. It is understood that chucks with other shapes and relative dimensions do not depart from the scope of this invention. The upper and lower chuck bodies 40, 42 are longitudinally aligned together in the attached configuration and commonly aligned in vertical orientation with the cable 24. The seed crystal 22 is a monocrystalline silicon rod which is releasably retained in a bottom end of the lower chuck body 42 so that when production of the ingot 26 is complete, it can be easily separated from the chuck for further processing. A retention device (not shown) for retaining the seed crystal 22 in the chuck is typically a latch pin which is interengaged with a corresponding notch formed in the seed crystal. A socket 48 (Fig. 3) is formed in the upper chuck body 40 for receiving a corresponding post 50 formed on the lower chuck body 42. The post 50 extends from the center of an upper side 52 of the lower chuck body 42. The post 50 is preferably cylindrical in shape and sized with a length and diameter suitable for aligning and firmly joining the lower chuck body 42 with the upper chuck body 40. The socket 48 is located generally in the center of a lower side

54 of the upper chuck body 40. In the attached configuration, the post 50 is received in the socket 48, and the upper and lower chuck bodies 40, 42 are coaxial. An annular ridge 56 extends around the perimeter of the upper side 52 of the lower chuck body 42. The ridge 56 comprises an engagement surface which engages the lower side 54 of the upper chuck body 40 in the attached configuration.

A connecting pin 60 secures the upper and lower chuck bodies 40, 42 together in the attached configuration. A transverse passage 62 extends across the entire upper chuck body 40, passing through the socket 48. The post 50 has a corresponding transverse passage 64 extending through the post.

When the post is received in the socket, the transverse passages 62, 64 of the upper and lower bodies are aligned and the connecting pin 60 inserted through the passages to secure the upper and lower chuck bodies together.

A passageway 66, located in the upper chuck body 40 and extending from the socket 48 to an opening 68 on the upper side, is suitable for receiving the cable 24. The passageway 66 has a relatively wider lower portion toward the socket 48, a relatively narrower upper portion toward the opening 68, and a conical neck 76 where a diameter of the passageway transitions between the wider and narrower portions. The upper chuck body 40 includes a cable coupling 78 receivable in the upper body. The neck 76 is configured for receiving the cable coupling 78 for suspending the chuck assembly 20 from the cable 24. The coupling 78 has conically shaped shoulders which are sized and shaped for engagement against the neck 76 to support the weight of the chuck 20 and ingot 26. A spherical tip end 82 of the cable 24 is receivable in the coupling 78 such that the chuck may self-align and hang vertical on the end of the cable during the crystal pulling operation. The upper and lower bodies 40, 42 are made of materials selected to withstand the high temperature environment and avoid release of particles that would contaminate the melt 16, potentially causing crystal dislocations during ingot growth. The lower body 42 is preferably made of graphite for its high temperature integrity. Graphite is an effective electrical conductor, so that substantially the entire lower chuck body is conductive. The cable 24 and cable coupling 78 are each made of a refractory metal, such as tungsten, which is electrically conductive. These metals are subject to creep failure or oxidation after repeated use at high temperature. In order to extend reliability and usable lifetime of these parts, the upper body 40 is made of a suitable material which provides thermal protection, such as quartz, to insulate the cable 24 and the cable coupling 78. Although quartz provides low thermal conductivity to protect the cable 24, it unfortunately also exhibits low electrical conductivity (high resistance). Therefore the cable coupling 78 is the only conductive portion of the upper chuck body 40. The coupling 78 is in direct contact with the cable 24 to provide electrical continuity. It is understood that chuck bodies with different conductive portion(s) do not depart from the scope of this invention.

In order to provide an accurate electronic indication of the seed crystal 22 contacting the silicon melt 16, an electric potential source 86 (Fig. 1) is provided. The potential source 86 is connected either directly or indirectly to the cable 24 and to the melt 16, creating an electric potential between the two. An electric circuit is completed when the seed crystal 22 contacts the melt 16, producing current which is detected by the crystal puller 10 for use in operating the puller to grow the semiconductor ingot. The quartz upper body 40 would block conduction of electrical current through the chuck 20 to the seed crystal 22. Although the cable 24, coupling 78, lower body 42, seed crystal 22, and melt 16 are each formed of electrically conductive or semiconductive materials, the quartz upper body 40, if needed to complete the circuit, would block that conduction.

As shown in Fig. 4, the chuck assembly 20 according to the present invention includes a conductive element, indicated generally at 90, extending between a conductive portion of the upper body 40 and a conductive portion of the lower body 42. Preferably, the conductive element 90 is a helical or coil type spring which extends from the cable coupling 78 to the post 50. Without the conductive element 90, the chuck 20 would not readily provide electrical continuity between the cable 24 and the seed crystal 22 because there would be no conductive path through the chuck assembly. In the attached configuration of the chuck 20, there is a space or gap between the top of the post 50 of the lower chuck body and the cable coupling 78. The spring 90 spans that gap and is made from a conductive material so as to provide the chuck assembly with electrical continuity from the cable 24 to the seed crystal 22. The spring 90 extends into the passageway 66. At its upper end the spring 90 engages the cable coupling 78, and at its lower end the spring engages the post 50 in a depression 92 typically formed in the end of the post. It is understood that any conductive element in any shape or form, including a flexible wire or rigid member, does not depart from the scope of this invention. Further, the conductive element may extend between any conductive portions or components of the chuck assembly 20 which provide electrical continuity. The conductive element 90 is preferably a spring because it is resiliently compliant in an axial direction, and can vary in length between its ends while applying force at the ends to ensure effective electrical contact. Length variations of the spring 90 compensate for differing internal dimensions of the chuck due to accumulated manufacturing tolerances of the components and/or differing rates of thermal expansion. For example, the quartz upper body 40 has a lower coefficient of thermal expansion than the tungsten cable 24, the tungsten cable coupling 78 and the graphite lower body 42. As the assembly is heated and cooled, differential thermal expansion produces variation in spacing between the upper and lower bodies. The spring 90 is resiliently compliant, providing continuous positive contact with the cable coupling 78 and the post 50 of the lower body 42 to bypass the quartz upper body 40 and maintain a low electrical resistance path.

Referring now to Fig. 5, the spring 90 has a symmetrical helix arrangement with about ten turns 94 about a central axis C. The turns 94 have a trapezoid shaped cross section for purposes of manufacturing, as described below. It is understood that the spring 90 may have any number of turns or any cross sectional shape without departing from the scope of this invention.

The spring 90 is axially compressed in the attached configuration to provide the force required to ensure positive contact. For example, a chuck 20 sized for holding a twenty millimeter square seed crystal 22 has a spring 90 with a free, uncompressed height H (Fig. 5) of 46.0 mm and a minimum, compressed height of 36.8 mm where all of the turns 94 are in engagement with each other. A nominal height, where the spring 90 is installed in the chuck at the attached configuration, is about halfway between, or 41.4 mm. At this point, the spring 90 applies a force (4.9 N or 1.1 lbs. for this example spring) against the cable coupling 78 and the lower chuck body 42 to ensure effective contact. Further, the spring can axially compress or expand from the nominal height, preferably at least

+/- 5 mm, and maintain effective contact. A preferred external diameter D1 for this spring is 25.0 mm with an internal diameter D2 of 17.0 mm. At the uncompressed height, the spring 90 has a spacing between corresponding points on adjacent turns 94, or pitch P, of about 4.5 mm with a minimum spacing S between turns of about 0.9 mm. It is understood the spring may have any dimensions without departing from the scope of the present invention.

The spring 90 is made of an electrically conductive material which can withstand the thermal environment of the growth chamber 12, avoid contaminating the melt 16, and resiliently maintain application of force. Few materials maintain elasticity at these temperatures while avoiding creep failure after repetitive uses. Preferably, the spring 90 is formed from a dense, finegrained, isomolded graphite. A particular grade of graphite is selected to have maximum strength for resiliently withstanding the forces and bending moments within a spring as it compresses and expands. The material must have sufficient strength such that at the solid compressed height, maximum torsion stress in the turns 94 is below a maximum allowable torsion strength of the material. The material is isomolded (i.e., having equal properties in all directions) to support the highly three-dimensional stress pattern prevalent in a spring. One exemplary type material is SFG-2 grade graphite supplied by Poco Graphite, Inc. of Decatur, Texas. This graphite has the following approximate properties: compressive strength 28,000 psi (1.93 x 10⁸ N/m²), flexural strength 18,000 psi (1.24 x 10⁸ N/m²), tensile strength 13,000 psi (0.90 x 10⁸ N/m²), torsion strength 6,500 psi

(0.45 x 10⁸ N/m²), and modulus of elasticity 2,100,000 psi (1.45 x 10¹⁰ N/m²). It is understood that the spring 90 can be made of any electrically conductive material, particularly a refractory metal such as molybdenum, tungsten, or tantalum, without departing from the scope of the invention. The spring 90 is manufactured by machining a one piece, solid rod or bar of graphite material. Graphite is generally brittle and it cannot be drawn and formed as are metals when manufacturing conventional, metallic helical springs. A high strength graphite such as SFG-2 can be machined without fracturing and it provides a better surface finish and integrity than refractory metals or less dense grades of graphite. The bar of graphite is machined to a cylinder having the desired external spring diameter D1. Material between turns 94 is next removed using a V-shaped threading tool (not shown). To produce a spring with the previously described example dimensions, the tool has an included angle A (Fig. 5) of about 20 degrees and a depth slightly greater than 4 mm. The internal diameter D2 is next drilled and bored out to the desired dimension, and ends of the spring are machined to a square face. The result is a spring 90 with turns 94 having a trapezoidal cross section. It is understood that the turns 94 may have any angle or cross sectional shape without departing from the scope of this invention. In operation, the chuck assembly 20 facilitates accurate detection of initial contact between the seed crystal 22 and the source melt 16. A voltage is applied across the system by the potential source 86 and the chuck is lowered. When the bottom tip of the seed crystal 22 initially contacts the melt 16, it completes an electrical circuit and current begins flowing. The circuit extends sequentially through the cable 24, coupling 78, spring 90, post 50 and lower chuck body 42, seed crystal 22, and melt 16. Detection of current establishes an accurate position of the seed crystal 22 relative to the surface of the melt 16, and establishes a reference position which may be subsequently used during the ingot pulling process. Provision of an electric signal that the seed crystal has contacted the melt facilitates automation of crystal puller operation. The seed crystal 22 is positioned at the initial contact point and the neck (not shown) and ingot 26 begin crystallizing on the seed. The spring 90 is formed of graphite which is capable of withstanding the high temperature environment over repeated uses, and is constructed to resiliently apply axial force against the coupling 78 and lower chuck body 42 to maintain effective electrical contact as internal dimensions of the chuck vary. A second application of the electrical melt sensing system and chuck assembly 20 is detection of final contact with the melt 16. After the ingot 26 is fully grown and it is being pulled upwardly in the growth chamber 12 by the winch 28, the exact time and position when a tail (not shown) of the ingot is lifted out from the melt 16 is detected by the termination of electrical current. The spring 90 may be readily installed in existing, conventional chucks without modification. The spring is merely inserted in the socket 48 and passageway 66 of the upper chuck body 40 when the chuck is being assembled to the attached configuration. When the post 50 of the lower chuck body 42 is received in the socket 48 and the connecting pin 60 installed, the spring 90 is captured between the upper and lower chuck bodies and is an engagement with conductive portions of the upper body and lower body. The spring is axially compressed to the nominal height and may resiliently adjust to variations in spacing between the upper and lower chuck bodies while continually applying force to maintain effective contact. The chuck assembly 20 provides effective electrical continuity. If desired, the invention facilitates fully automated crystal puller operation without active participation by an operator, because it may be pre-programmed and avoids reliance upon visual methods. In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results obtained.

When introducing elements of the present invention or the preferred embodiment(s) thereof, the articles "a", "an", "the" and "said" are intended to mean that there are one or more of the elements. The terms "comprising",

"including" and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements.

As various changes could be made in the above without departing from the scope of the invention, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

WHAT IS CLAIMED IS:

1. A chuck assembly for holding a seed crystal and suspending the seed crystal from an elongate member in a growth chamber as an ingot is grown on the seed crystal from a semiconductor source material according to the Czochralski process, the assembly providing electrical continuity between the seed crystal and the elongate member to enable flow of electrical current therethrough for sensing contact between the seed crystal and the source material, the chuck assembly comprising: an upper chuck body constructed for attachment to an end of said elongate member, the upper body having an electrically conductive portion in electrical continuity with the elongate member when the upper body is attached to the elongate member; a lower chuck body constructed for holding said seed crystal, the lower body having an electrically conductive portion in electrical continuity with the seed crystal when the lower body is holding the seed crystal; wherein the upper body and lower body are constructed for connection in an attached configuration to suspend the seed crystal from the elongate member; and an electrically conductive element extending between the conductive portions of the upper and lower chuck bodies at said attached configuration for providing electrical continuity between the seed crystal and the elongate member.

2. A chuck assembly as set forth in claim 1 wherein said conductive element is captured between the upper and lower chuck bodies at the attached configuration, the conductive element being in engagement with the conductive portion of the upper body and the conductive portion of the lower body.

3. A chuck assembly as set forth in claim 2 wherein said conductive element is constructed to resiliently adjust to variations in a spacing between the electrically conductive portions of the upper and lower chuck bodies.

4. A chuck assembly as set forth in claim 3 wherein said conductive element comprises a coil spring.

5. A chuck assembly as set forth in claim 4 wherein said spring is made of graphite.

6. A chuck assembly as set forth in claim 4 wherein said spring is in one piece and has a plurality of helical turns.

7. A chuck assembly as set forth in claim 6 wherein said spring at the attached configuration is axially compressed.

8. A chuck assembly as set forth in claim 6 wherein said turns of the spring each have a trapezoidal shape cross section.

9. A chuck assembly as set forth in claim 2 wherein said lower chuck body is formed substantially entirely from a material that is electrically conductive such that its conductive portion comprises substantially the entire lower chuck body.

10. A chuck assembly as set forth in claim 2 wherein said conductive portion of the upper chuck body comprises a coupling receivable in the upper chuck body and constructed for attaching to an end of the elongate member.