Patent Description:
Horizontal magnetic field Czochralski (HMCZ) methods may be used to produce relatively high quality silicon wafers. Strong convection control under horizontal magnetic field Czochralski enables a relatively smooth axial diameter profile as well as improves the nanotopology due to low dopant striation. The magnetic field may change the condition of the ingot-melt interface due to relatively high velocity and complexity of the melt flow. To ensure the crystal grows with high zero dislocation (ZD) success, the ingot crown is grown under controlled conditions with application of the horizontal magnetic field. Controlled growth conditions include longer process times to allow the crown height to be longer (e.g., <NUM> to <NUM> for <NUM> crystals) and in some instances special crown shapes are used. Neck growth is conventionally performed under horizontal magnetic field to stabilize the melt temperature near the meniscus. Conventional horizontal magnetic field methods result in longer process times.

A need exists for methods for producing a monocrystalline silicon ingot by the horizontal magnetic field Czochralski (HMCz) method which may decrease process time for forming the ingot.

<CIT> describes the production of a silicon single crystal ingot by a magnetic field Czochralski method in which seeding is executed while no magnetic field is applied. Further methods for producing monocrystalline silicon ingots by the Czochralski method are disclosed in <CIT>, <CIT> and <CIT>.

This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the invention, which are described and claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention.

The present invention is directed to a method for producing a monocrystalline silicon ingot having a neck and a main body suspended from the neck by the horizontal magnetic field Czochralski method. An outwardly flaring portion is disposed between the neck and the ingot main body. The monocrystalline silicon ingot is grown in an ingot puller apparatus having a heat shield. A seed crystal is contacted with a silicon melt held within a crucible. The neck is pulled from the silicon melt. A horizontal magnetic field is not applied to the melt while the neck is pulled from the melt. The crucible is rotated at a crucible rotation rate of <NUM> rpm or more while pulling the neck from the melt. A distance between the melt and a bottom of the heat shield is at least <NUM> while pulling the neck from the melt. The outwardly flaring portion is pulled from the melt, wherein a horizontal magnetic field is not applied to the melt while pulling the outwardly flaring portion from the melt. The ingot main body is pulled from the melt. The main body is suspended from the neck, the outwardly flaring portion being disposed between the neck and the ingot main body. A horizontal magnetic field is applied to the melt while the ingot main body is pulled from the melt.

Various refinements exist of the features noted in relation to the above-mentioned present invention. Further features may also be incorporated in the above-mentioned present invention as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to any of the illustrated embodiments of the present invention may be incorporated into the above-described present invention, alone or in any combination.

The present invention provides a method for producing a monocrystalline silicon ingot by the horizontal magnetic field Czochralski method (HMCz). An ingot puller apparatus (or more simply "ingot puller") for growing a monocrystalline silicon ingot is indicated generally at "<NUM>" in <FIG>. The ingot puller apparatus <NUM> includes a crystal puller housing <NUM> that defines a growth chamber <NUM> for pulling a silicon ingot <NUM> from a melt <NUM> of silicon. The ingot puller apparatus <NUM> includes a crucible <NUM> disposed within the growth chamber <NUM> for holding the melt <NUM> of silicon. The crucible <NUM> is supported by a susceptor <NUM>.

The crucible <NUM> includes a floor <NUM> and a sidewall <NUM> that extends upward from the floor <NUM>. The sidewall <NUM> is generally vertical. The floor <NUM> includes the curved portion of the crucible <NUM> that extends below the sidewall <NUM>. Within the crucible <NUM> is a silicon melt <NUM> having a melt surface <NUM> (i.e., melt-ingot interface). The susceptor <NUM> is supported by a shaft <NUM>. The susceptor <NUM>, crucible <NUM>, shaft <NUM> and ingot <NUM> have a common longitudinal axis A or "pull axis" A.

A pulling mechanism <NUM> is provided within the ingot puller apparatus <NUM> for growing and pulling an ingot <NUM> from the melt <NUM>. Pulling mechanism <NUM> includes a pulling cable <NUM>, a seed holder or chuck <NUM> coupled to one end of the pulling cable <NUM>, and a seed crystal <NUM> coupled to the seed holder or chuck <NUM> for initiating crystal growth. One end of the pulling cable <NUM> is connected to a pulley (not shown) or a drum (not shown), or any other suitable type of lifting mechanism, for example, a shaft, and the other end is connected to the chuck <NUM> that holds the seed crystal <NUM>. In operation, the seed crystal <NUM> is lowered to contact the melt <NUM>. The pulling mechanism <NUM> is operated to cause the seed crystal <NUM> to rise. This causes a single crystal ingot <NUM> to be pulled from the melt <NUM>.

During heating and crystal pulling, a crucible drive unit <NUM> (e.g., a motor) rotates the crucible <NUM> and susceptor <NUM>. A lift mechanism <NUM> raises and lowers the crucible <NUM> along the pull axis A during the growth process. As the ingot grows, the silicon melt <NUM> is consumed and the height of the melt in the crucible <NUM> decreases. The crucible <NUM> and susceptor <NUM> may be raised to maintain the melt surface <NUM> at or near the same position relative to the ingot puller apparatus <NUM>.

A crystal drive unit (not shown) may also rotate the pulling cable <NUM> and ingot <NUM> in a direction opposite the direction in which the crucible drive unit <NUM> rotates the crucible <NUM> (e.g., counter-rotation). In embodiments using iso-rotation, the crystal drive unit may rotate the pulling cable <NUM> in the same direction in which the crucible drive unit <NUM> rotates the crucible <NUM>. In addition, the crystal drive unit raises and lowers the ingot <NUM> relative to the melt surface <NUM> as desired during the growth process.

The ingot puller apparatus <NUM> may include an inert gas system to introduce and withdraw an inert gas such as argon from the growth chamber <NUM>. The ingot puller apparatus <NUM> may also include a dopant feed system (not shown) for introducing dopant into the melt <NUM>.

According to the Czochralski single crystal growth process, a quantity of polycrystalline silicon, or polysilicon, is charged to the crucible <NUM> (e.g., charge of <NUM> or more). A variety of sources of polycrystalline silicon may be used including, for example, granular polycrystalline silicon produced by thermal decomposition of silane or a halosilane in a fluidized bed reactor or polycrystalline silicon produced in a Siemens reactor. Once polycrystalline silicon is added to the crucible to form a charge, the charge is heated to a temperature above about the melting temperature of silicon (e.g., about <NUM>) to melt the charge. In some embodiments, the charge (i.e., the resulting melt) is heated to a temperature of at least about <NUM>, at least about <NUM> or even at least about <NUM>. The ingot puller apparatus <NUM> includes bottom insulation <NUM> and side insulation <NUM> to retain heat in the puller apparatus <NUM>. In the illustrated embodiment, the ingot puller apparatus <NUM> includes a bottom heater <NUM> disposed below the crucible floor <NUM>. The crucible <NUM> may be moved to be in relatively close proximity to the bottom heater <NUM> to melt the polycrystalline silicon charged to the crucible <NUM>.

To form the ingot, the seed crystal <NUM> is contacted with the surface <NUM> of the melt <NUM>. The pulling mechanism <NUM> is operated to pull the seed crystal <NUM> from the melt <NUM>. The ingot <NUM> includes a crown portion <NUM> in which the ingot transitions and tapers outward from the seed crystal <NUM> to reach a target diameter. The ingot <NUM> includes a constant diameter portion <NUM> or cylindrical "main body" of the crystal which is grown by increasing the pull rate. The main body <NUM> of the ingot <NUM> has a relatively constant diameter. The ingot <NUM> includes a tail or end-cone (not shown) in which the ingot tapers in diameter after the main body <NUM>. When the diameter becomes small enough, the ingot <NUM> is then separated from the melt <NUM>. The ingot <NUM> has a central longitudinal axis A that extends through the crown portion <NUM> and a terminal end of the ingot <NUM>.

The ingot puller apparatus <NUM> includes a side heater <NUM> and a susceptor <NUM> that encircles the crucible <NUM> to maintain the temperature of the melt <NUM> during crystal growth. The side heater <NUM> is disposed radially outward to the crucible sidewall <NUM> as the crucible <NUM> travels up and down the pull axis A. The side heater <NUM> and bottom heater <NUM> may be any type of heater that allows the side heater <NUM> and bottom heater <NUM> to operate as described herein. In some embodiments, the heaters <NUM>, <NUM> are resistance heaters. The side heater <NUM> and bottom heater <NUM> may be controlled by a control system (not shown) so that the temperature of the melt <NUM> is controlled throughout the pulling process.

In accordance with the present invention, the ingot puller apparatus <NUM> also includes a reflector assembly <NUM> (or simply "reflector" or "heat shield") disposed within the growth chamber <NUM> and above the melt <NUM> which shrouds the ingot <NUM> during ingot growth. The heat shield <NUM> may be partially disposed within the crucible <NUM> during crystal growth. The heat shield <NUM> defines a central passage <NUM> for receiving the ingot <NUM> as the ingot is pulled by the pulling mechanism <NUM>.

The reflector <NUM> is, in general, a heat shield adapted to retain heat underneath itself and above the melt <NUM>. In this regard, any reflector design and material of construction (e.g., graphite or gray quartz) known in the art may be used without limitation. The reflector <NUM> has a bottom <NUM> (<FIG>) and the bottom <NUM> of the reflector <NUM> is separated from the surface of the melt by a distance Hr during ingot growth.

A single crystal silicon ingot <NUM> produced in accordance with the present invention and, generally, the Czochralski method is shown in <FIG>. The ingot <NUM> includes a neck <NUM>, an outwardly flaring portion <NUM> (synonymously "crown" or "cone"), a shoulder <NUM> and a constant diameter main body <NUM>. The neck <NUM> is attached to the seed crystal <NUM> that was contacted with the melt and withdrawn to form the ingot <NUM>. The main body <NUM> is suspended from the neck <NUM>. The neck <NUM> terminates once the cone portion <NUM> of the ingot <NUM> begins to form.

The constant diameter portion <NUM> of the ingot <NUM> has a circumferential edge <NUM>, a central axis A that is parallel to the circumferential edge <NUM> and a radius R that extends from the central axis A to the circumferential edge <NUM>. The central axis A also passes through the cone <NUM> and neck <NUM>. The diameter of the main ingot body <NUM> may vary and, in some embodiments, the diameter may be about <NUM>, about <NUM>, about <NUM>, greater than about <NUM>, about <NUM> or even greater than about <NUM>.

The single crystal silicon ingot <NUM> may generally have any resistivity. The single crystal silicon ingot <NUM> may be doped or undoped.

In accordance with the present invention, the ingot main body <NUM> is grown while applying a horizontal magnetic field to the crucible <NUM> and melt <NUM> in accordance with known horizontal magnetic field Czochralski techniques (HMCz). <FIG> show a horizontal magnetic field applied to a crucible <NUM> and melt <NUM> during ingot growth. A cylindrical coordinate system for the crucible <NUM> includes a radial direction R, an angular direction Θ, and an axial direction Z. The melt <NUM> may contain one or more convective flow cells <NUM>, <NUM> induced by heating of the crucible <NUM> and rotation of the crucible <NUM> and/or ingot <NUM> in the angular direction Θ. The structure and interaction of these one or more convective flow cells <NUM>, <NUM> may be modulated via regulation of one or more process parameters and/or the application of a horizontal magnetic field as described in detail herein below.

<FIG> is a diagram illustrating a horizontal magnetic field applied to the crucible <NUM> and melt <NUM> in the ingot puller apparatus <NUM>. As shown, the crucible <NUM> contains the silicon melt <NUM> from which the ingot <NUM> is grown. The transition between the melt and the crystal is generally referred to as the crystal-melt interface (alternatively the melt-crystal, solid-melt or melt-solid interface) and is typically non-linear, for example concave, convex or gull-winged relative to the melt surface. Two magnetic poles <NUM> are placed in opposition to generate a magnetic field generally perpendicular to the crystal-growth direction and generally parallel to the melt surface <NUM>.

The magnetic poles <NUM> may be a conventional electromagnet, a superconductor electromagnet, or any other suitable magnet for producing a horizontal magnetic field of the desired strength. Application of a horizontal magnetic field gives rise to Lorentz force along the axial direction, in a direction opposite of fluid motion, with opposing forces driving melt convection. The convection in the melt is thus suppressed, and the axial temperature gradient in the crystal near the interface increases. The melt-crystal interface then moves upward to the crystal side to accommodate the increased axial temperature gradient in the crystal near the interface and the contribution from the melt convection in the crucible decreases. The horizontal configuration has the advantage of efficiency in damping a convective flow at the melt surface <NUM>.

The position of the magnetic poles <NUM> relative to the melt surface <NUM> may be varied to adjust the position of the maximum gauss plane (MGP) relative to the melt surface <NUM>. The magnetic poles <NUM> may be cooled (e.g., water cooled) and/or may include a ferrous shield to reduce stray magnetic fields and to enhance the strength of the field produced.

The loss of zero dislocation (ZD) structure (quantified by LZD rate) is generally higher during silicon crystal Cz growth in a horizontal magnetic (HMCZ) field versus growth in a Cusp (or vertical) magnetic field. However, the LZD rate in HMCZ can be lowered with shorter process time by not applying a magnetic field (e.g., a horizontal magnetic field or any type of magnetic field) during neck <NUM> growth and/or at least a portion of crown <NUM> growth or by applying a relatively weak horizontal magnetic field (e.g., less than <NUM> gauss) during neck <NUM> growth and/or at least a portion of crown <NUM> growth.

According to the present invention, a horizontal magnetic field is not applied to the melt while the neck is pulled from the melt. In accordance with embodiments of the present invention, once the seed crystal <NUM> contacts the silicon melt <NUM> and the neck <NUM> begins grown, a magnetic field is not applied to the melt <NUM> (i.e., the magnetic poles <NUM> are not powered) or a relatively weak magnetic field such as <NUM> gauss or less is applied. Once the neck <NUM> is formed, the outwardly flaring crown <NUM> begins to be pulled from the melt <NUM>. According to the present invention, a horizontal magnetic field is not applied to the melt while pulling the outwardly flaring portion from the melt. In some embodiments, the entire crown <NUM> is formed without a magnetic field being applied to the melt or a relatively weak magnetic field such as <NUM> gauss or less is applied. In some embodiments, when the crown diameter is less than <NUM>, a magnetic field is not applied to the melt or a magnetic field is applied with the magnetic flux density being less than <NUM> gauss. In some embodiments, the magnetic field flux is ramped up (i.e., increase) during crown growth.

Once the desired ingot diameter is reached, the shoulder <NUM> is formed to transition toward forming the constant diameter portion <NUM> of the ingot <NUM>. A horizontal magnetic field is applied to the melt <NUM> during growth of the constant diameter portion <NUM>. For example, a horizontal magnetic field having a magnetic flux density of at least <NUM> gauss (e.g., <NUM> gauss to <NUM> gauss or <NUM> gauss to <NUM> gauss).

To reduce the incidence of dislocations in the neck <NUM> during neck growth, one or more growth conditions are controlled. The crucible <NUM> is rotated at a crucible rotation rate of <NUM> rpm or more while pulling the neck <NUM> from the melt <NUM>. In addition, the distance (Hr) between the bottom <NUM> of the reflector <NUM> and the surface <NUM> of the melt <NUM> is at least <NUM> while pulling the neck <NUM> from the melt <NUM>. Alternatively or in addition, the pull rate of the neck may be at least <NUM>/min or from <NUM>/min to <NUM>/min. Alternatively or in addition, a relatively long crown may be used such as a crown having a height of at least <NUM>.

Compared with conventional methods for growing a monocrystalline ingot by the horizontal magnetic Czochralski method, methods of the present invention have several advantages. By not applying the horizontal magnetic field during neck growth, the process time to produce the ingot may be reduced. By increasing the crucible rotation rate (to <NUM> rpm or more), the convective flow of the melt may be increased. By increasing the distance (Hr) between the reflector and the melt during neck growth (to <NUM> or more), the neck temperature increases which reduces dislocations in the neck and process time may be reduced.

The methods of the present invention are further illustrated by the following Examples. These Examples should not be viewed in a limiting sense.

Ingots were grown by the horizontal magnetic field Czochralski process (HMCz). A set of ingots were grown with a horizontal magnetic field being applied during crown growth ("Group A"). A second set of ingots were also grown with a horizontal magnetic field being applied during crown growth but at a faster pull speed ("Group B"). A third set of ingots were grown without a magnetic field being applied during crown growth (but applied during growth of the main body of the ingot) ("Group C").

The time to grow the crown for each set of ingots is shown in <FIG>. As shown in <FIG>, the ingots in which a horizontal magnetic field was applied during crown growth took <NUM> times as long to grow than the ingot in which a horizontal magnetic field was not applied during crown growth. The longer run time may be attributed to the larger crown height in ingots in which a horizontal magnetic field was applied during crown growth.

<FIG> shows the range of neck pull speeds in which zero dislocation was achieved for a variety of reflector distances ("Hr group_1") and a variety of magnet conditions during neck growth (cusp field being applied during neck growth ("Cusp On") or no magnetic field being applied during neck growth ("HMCZ off")). As shown in <FIG>, in the conditions in which the horizontal magnetic field was not applied, faster neck pull rates were used to ensure zero-dislocation growth.

Ingots were grown by HMCz with a magnetic field not being applied during neck growth. The ingots were grown over a variety of reflector-melt distances ("Neck Hr"). As shown in <FIG>, as Hr becomes more shallow, the melt flow nearby the neck-melt interface is less able to remove dislocations. The hotter conditions with wide Hr enhances the movement of dislocations in the neck.

As used herein, the term "about," when used in conjunction with ranges of dimensions, concentrations, temperatures or other physical or chemical properties or characteristics, is meant to cover variations that may exist in the upper and/or lower limits of the ranges of the properties or characteristics, including, for example, variations resulting from rounding, measurement methodology or other statistical variation.

Claim 1:
A method for producing a monocrystalline silicon ingot having a neck, a main body suspended from the neck, and an outwardly flaring portion disposed between the neck and the ingot main body, by the horizontal magnetic field Czochralski method, the monocrystalline silicon ingot being grown in an ingot puller apparatus having a heat shield, the method comprising:
contacting a seed crystal with a silicon melt held within a crucible;
pulling the neck from the silicon melt, wherein:
a horizontal magnetic field is not applied to the melt while the neck is pulled from the melt;
the crucible is rotated at a crucible rotation rate of <NUM> rpm or more while pulling the neck from the melt; and
a distance between the melt and a bottom of the heat shield is at least <NUM> while pulling the neck from the melt;
pulling the outwardly flaring portion from the melt, wherein a horizontal magnetic field is not applied to the melt while pulling the outwardly flaring portion from the melt; and
pulling the ingot main body from the melt, the main body being suspended from the neck, the outwardly flaring portion being disposed between the neck and the ingot main body, wherein a horizontal magnetic field is applied to the melt while the ingot main body is pulled from the melt.