Apparatus for forming crystalline sheet from a melt

An apparatus for drawing a crystalline sheet from a melt. The apparatus may include a crucible configured to contain the melt and having a dam structure, where the melt comprises an exposed surface having a level defined by a top of the dam structure. The apparatus may further include a support apparatus disposed within the crucible and having an upper surface, wherein the crystalline sheet is maintained flush with the exposed surface of the melt when drawn over the support apparatus, and may include a melt-back heater directing heat through the upper surface of the support apparatus to partially melt the crystalline sheet when the crystalline sheet is drawn over the support apparatus.

FIELD

The present embodiments relate to growth of crystalline material from a melt and more particularly to forming a single crystalline sheet from a melt.

BACKGROUND

Sapphire is the second most widely used synthetic monocrystalline material next to silicon. Sapphire represents one crystalline form of aluminum oxide (Al03) and may be formed by growing crystalline boules of sapphire from a melt, for example. Applications for sapphire include use as light emitting diode (LED) substrates, optical windows, silicon-on-sapphire (SOS), mobile devices, etc. All these applications entail single crystal (monocrystalline) sapphire in the form of thin crystalline sheets. Accordingly, when grown as boules, sapphire sheets or wafers may be formed by slicing the boules after growth to form a sapphire sheet or substrate of a target thickness.

In the case of silicon growth, monocrystalline silicon may be grown by a so-called floating silicon method (FSM). In the FSM method crystalline sheets of silicon are formed from a silicon melt by cooling a portion of the melt surface to crystallize a layer at the melt surface, and by pulling the crystalline layer in a horizontal direction. In this manner, a sheet of monocrystalline silicon may be continuously drawn as a ribbon from the silicon melt. Fortuitously, the density of monocrystalline silicon is less than the density of the silicon melt, causing the growing silicon ribbon to float on the melt surface. This allows the ribbon to be conveniently drawn along the melt surface and separated from the silicon melt at a target location. Other materials systems where monocrystalline sheet formation from a melt may be desirable, including sapphire, exhibit higher density in the crystalline phase than in the liquid phase. Accordingly, techniques for forming sapphire by drawing a horizontally oriented sheet from a melt are lacking. The present disclosure is provided in view of these considerations and other considerations.

SUMMARY

In one embodiment, an apparatus for drawing a crystalline sheet from a melt may include a crucible configured to contain the melt and having a dam structure, where the melt comprises an exposed surface having a level defined by a top of the dam structure. The apparatus may further include a support apparatus disposed within the crucible and having an upper surface, wherein the crystalline sheet is maintained flush with the exposed surface of the melt when drawn over the support apparatus and may include a melt-back heater directing heat through the upper surface of the support apparatus to partially melt the crystalline sheet when the crystalline sheet is drawn over the support apparatus.

In another embodiment, In a further embodiment, an apparatus for growing a crystalline sheet from a melt may include a crucible configured to contain the melt and having a dam structure, the melt having an exposed surface defined by a top of the dam structure. The apparatus may also include a crystallizer disposed above the melt to remove heat in a cooling zone from the exposed surface of the melt, wherein a leading edge of the crystalline sheet forms at the exposed surface in the cooling zone, the crystalline sheet having an initial sheet thickness; and a support apparatus disposed within the crucible downstream of the crystallizer and having an upper surface, wherein the crystalline sheet is maintained flush with the exposed surface of the melt when drawn over the support apparatus.

In a further embodiment, a method of growing a crystalline sheet from a melt may include: forming the crystalline sheet on an exposed surface of a melt in a crucible having a dam structure, the exposed surface defined by a top of the dam structure; measuring a position of a surface of the crystalline sheet; placing an upper surface of a support apparatus at a first level based on the measured first position; and drawing the crystalline sheet along a first direction over the upper surface of the support apparatus wherein the crystalline sheet is maintained flush with the exposed surface.

In a further embodiment, an apparatus for drawing a crystalline sheet from a melt may include a crucible configured to contain the melt and having a dam structure, the melt comprising an exposed surface having a level defined by a top of the dam structure; and a support apparatus disposed within the crucible and having an upper surface, wherein the crystalline sheet is maintained flush with the exposed surface of the melt when drawn over the support apparatus; and a melt-back heater directing heat through the upper surface of the support apparatus to partially melt the crystalline sheet when the crystalline sheet is drawn over the support apparatus. The apparatus may further include a gas jet disposed downstream of the dam structure directing a stream of gas to the melt adjacent the dam structure.

In a further embodiment, an apparatus for drawing a crystalline sheet from a melt may include a crucible configured to contain the melt and having a dam structure, the melt comprising an exposed surface having a level defined by a top of the dam structure; and include a gas jet disposed downstream of the dam structure directing a stream of gas to the melt adjacent the dam structure.

DETAILED DESCRIPTION

The present embodiments provide apparatus and techniques for forming monocrystalline sheets from a melt. Various embodiments may be advantageously employed for growing monocrystalline sheets by drawing the sheets in a horizontal manner along the surface of the melt. In various embodiments, monocrystalline sheets of material such as sapphire may be grown where the monocrystalline sheets have a greater density than the density of the melt.

Turning now to the figures,FIG. 1Adepicts an apparatus100for growing a crystalline sheet from a melt.FIG. 2Ashows a side view of portions of an apparatus200, where the apparatus200may be a variant of the apparatus100.FIG. 2Bshows an end view of the apparatus200. In various embodiments the apparatus100may include a crucible102configured to contain a melt106. The crucible may be heated by various heaters, shown as heaters104. The heaters104may be used to generate and maintain a melt, as well as for other purposes as discussed below. The apparatus100may include a crystallizer108disposed above the melt106to remove heat in a cooling zone120. In particular the heat may be removed from an (upper) exposed surface124of the melt, wherein a leading edge122of a crystalline sheet112is formed, as shown inFIG. 2A.

The apparatus100may further include a heat intensifier110, disposed under the leading edge122. The apparatus100may also include a dam structure116. As discussed in more detail below the dam structure116may define a level of the exposed surface124of the melt106. The apparatus100may further include a support apparatus114disposed within the crucible102and downstream of the crystallizer108. As discussed below, the support apparatus114or similar support apparatus may include an upper surface whose level is variable along the Y-axis of the Cartesian coordinate system shown. The support apparatus114may be used to support the crystalline sheet112as well as direct heat to the crystalline sheet112. In various embodiments for growing a crystalline sheet of sapphire, the crucible102, heat intensifier110, and support apparatus114may be made from refractory metal such as molybdenum or tungsten.

As further shown inFIG. 1Bthe crystalline sheet112may be drawn by a puller (not shown) to the right in the figure, where the crystalline sheet112is separated from the melt106as the crystalline sheet112is pulled over the dam structure116. The apparatus100may also include a gas jet118where the gas jet118aids in separation of the crystalline sheet112from the melt106by directing gas to the underside of the crystalline sheet112. The gas jet118may be disposed downstream of the dam structure116, directing a stream of gas121to the melt106adjacent the dam structure116.

The gas jet118may be particularly useful when the melt106is alumina (sapphire), where contact angle between the melt106and dam structure116may be lower. For example, in the case of a Si melt, the contact angle between the silicon melt and a quartz dam structure is 87 degrees, allowing a meniscus119to remain stable over a wide range of contact angles, while stability can be maintained by merely pulling upward slightly on the sheet. For embodiments where the melt106is sapphire and the dam structure116is made from Mo, the contact angle between the melt106and dam structure116is ˜30 degrees. This lower contact angle makes maintain stability more difficult by merely lifting on the crystalline sheet112. The gas jet118may generate a first curvature of the meniscus119between the melt106and dam structure116different than a second curvature of the meniscus119between the melt106and support apparatus116when the gas jet118is not present.

In embodiments where the crystalline sheet112has a higher density than the melt106, the support apparatus114may maintain the crystalline sheet112at a level, such as flush with the exposed surface124, as shown inFIG. 2A. In other words, the support apparatus114may prevent the crystalline sheet112from sagging below the exposed surface124as the crystalline sheet is drawn away from the crystallizer108.

In various embodiments the apparatus100or apparatus200may be employed for faceted edge growth of a monocrystalline sheet from a melt. Faceted edge growth may refer to the process of drawing a monocrystalline sheet from a melt where a leading edge of a monocrystalline sheet is formed along specific direction with respect to a melt surface.

In embodiments of monocrystalline sapphire growth, the leading edge of a crystalline sheet may be formed using the “A” plane facet of the sapphire crystal structure, forming a 60 degree angle with respect to the crystalline sheet surface (another “A” plane). InFIG. 2A, this crystalline surface may be represented by the surface222of crystalline sheet112. When drawn as a horizontal sheet, the surface222may lie parallel to the exposed surface124of the melt106, where the exposed surface124of the melt106lies in the horizontal plane (X-Z) plane. A monocrystalline sheet may thus be formed by drawing the sheet along a horizontal plane while the surface of the sheet is maintained parallel to the horizontal plane.

In various embodiments, the apparatus100may facilitate formation of the crystalline sheet112by providing intensive cooling to remove heat from the solid/liquid interface where the crystalline sheet112forms near the leading edge122. In various embodiments the crystallizer108may provide this intensive cooling by facilitating a combination of radiative, gas conductive, and gas convective processes. In particular embodiments, the crystallizer108may be a cold block positioned in close proximity to the exposed surface124. The crystallizer108may include gas gets in some embodiments to direct cooling gas at the exposed surface124.

In embodiments where the melt106is aluminum oxide, cooling jets may optionally be omitted since gas conduction from the exposed surface124and radiation may be sufficient to generate adequate cooling to promote crystallization. This is because, unlike silicon (melting temperature 1412° C.), a sapphire melt (2050° C.) is held at a much higher temperature with respect to a crystallizer and radiation cooling is much stronger (Qradiation˜T4), where the crystallizer108may be a water cooled block.

In addition to the intense cooling provided by crystallizer108, localized heat flow may be provided from the melt by the heat intensifier110in order to stabilize the leading edge growth of crystalline sheet112. In embodiments of faceted edge growth for forming sapphire sheets, intense heat flow in the melt106may be readily accomplished due to the insulating nature of a sapphire (aluminum oxide) melt. For example, the thermal diffusivity of sapphire is 4.5×10−7m2/s compared to 2.6×10−5m2/s for Si. Moreover, as noted above, the crucible102and heat intensifier110may be composed of a refractory metal, where the refractory metal is thermally conductive. For example, in some embodiments where tungsten is used for crucible102and heat intensifier110, the thermal conductivity of such components may be 202 W/mK, representative of the thermal conductivity of tungsten metal.

Accordingly, because of the relatively high thermal conductivity of crucible and heat intensifier materials (refractory metals), as well as the relatively low thermal diffusivity of the melt, faceted edge growth of sapphire may in principle be readily accomplished using apparatus100or apparatus200. A challenge for forming high quality crystalline sheet of sapphire or other crystalline materials where density in the solid crystalline phase is higher than in the liquid phase (melt) is the aforementioned sagging of the crystalline sheet below an exposed surface of a melt. In various embodiments, the support apparatus114may be used to address this problem by proving support for a crystalline sheet downstream of the crystallizer108, such as the crystalline sheet112.

As illustrated inFIG. 1A, the upper surface of the support apparatus114may be positioned just below the exposed surface124. This positioning may aid in maintaining the crystalline sheet112flush with the exposed surface124as the crystalline sheet112is drawn towards the right in the figure. In various embodiments the crucible102may have a depth along the Y direction adequate to contain a melt depth for melt106of 5 mm to 25 mm. The embodiments are not limited in this context. In some embodiments, the crystalline sheet112may be formed to an initial sheet thickness near the leading edge122of approximately 0.5 mm to 3 mm. The embodiments are not limited in this context. Accordingly the upper surface of the support apparatus114may be positioned approximately 0.5 mm to 3 mm below the exposed surface of the melt, to maintain the surface222of the crystalline sheet flush with the exposed surface124of the melt106.

In accordance with different applications a target thickness of the crystalline sheet112may be different than the initial sheet thickness of the crystalline sheet112. Moreover, crystalline sheet thickness may vary from run to run for the same nominal process conditions. As detailed below, the upper surface of the support apparatus114may be accordingly be positioned or repositioned to lie sufficiently close to exposed surface124of the melt106to support the crystalline sheet112so the crystalline sheet112may remain flush with the exposed surface124of the melt. In other words, the surface222of crystalline sheet may lie at the same level as the level of exposed surface124when the crystalline sheet is drawn over the support apparatus114.

As further suggested inFIG. 2, as the crystalline sheet112is drawn to the right, heaters such as heaters104may be employed to adjust heat flow the crystalline sheet112to modify the sheet thickness of the crystalline sheet112as well as to improve the morphology of the crystalline sheet112.

In various embodiments, the support apparatus114may have multiple portions, where the multiple portions facilitate changing the level of the upper surface of the support apparatus114. Turning toFIG. 2A,FIG. 2B, andFIG. 3, there is shown a support apparatus202, where the support apparatus202may be a variant of the support apparatus114. In this configuration, the support apparatus202may include a fixed portion204, where the fixed portion204extends from the bottom of the crucible102. The support apparatus202may further include an adjustable portion206, where the adjustable portion206defines an upper surface214of the support apparatus202.

As suggested inFIG. 3, the adjustable portion206may be movable at least along the Z-axis. For example, the adjustable portion206may be movable from a first position P1to a second position P2as shown inFIG. 3. This movability may accommodate process changes such as changes in sheet thickness of the crystalline sheet112. As shown inFIG. 2A, the level of the exposed surface124of the melt106may defined by the top210of the dam structure116. In order to facilitate the ability to continuously draw the crystalline sheet112from the melt106, the level of the exposed surface124of the melt106may be maintained at approximately the level of the top210of the dam structure116. If the level of the exposed surface124slightly exceeds the level of the top210of the dam structure116, excess melt may spill over the dam structure116. Accordingly, by maintaining sufficient amount of melt106within the crucible102the level of the exposed surface124may be defined by the top210. In this circumstance, in order to maintain the surface222of crystalline sheet112flush with the exposed surface124, variations in thickness of the crystalline sheet112may be accommodated by the support apparatus202in the following manner.

In a first scenario the crystalline sheet112may have a thickness of 2 mm. In this circumstance the adjustable portion206may be positioned at position P1as indicated by the dashed structure, wherein the upper surface214is disposed at a first level L1with respect to other parts of the crucible102such as the top210of the dam structure116. In the first scenario given a 2 mm thickness for crystalline sheet112, the surface222of crystalline sheet112may be flush with the exposed surface124of the melt106. In a second scenario also illustrated inFIG. 3, the support apparatus202may be positioned at position P2, as indicated by the solid structure. In the second scenario the upper surface214is disposed at a second level L2with respect to the top210of the dam structure116, and accordingly with respect to the exposed surface124. In the second scenario, the upper surface214is positioned closer to the exposed surface124. This configuration may be useful if the crystalline sheet112is thinner than in the first scenario. For example, the crystalline sheet112may have a thickness of 1 mm in the second scenario. Accordingly, to maintain the surface222of crystalline sheet112flush with the exposed surface124, the level L2of upper surface214of the support apparatus202is closer to the exposed surface124to adjust for the lesser thickness of crystalline sheet112as compared to in the first scenario. More generally, by merely moving the adjustable portion206along a direction parallel to the direction of drawing of the crystalline sheet (Z-axis), the level of the upper surface214may be adjusted upwardly or downwardly with respect to the exposed surface of the melt as needed.

In addition to providing support for the crystalline sheet112, the support apparatus202may direct heat toward the exposed surface124and in particular toward the crystalline sheet112while the crystalline sheet112is drawn over the support apparatus202. For example, a melt-back heater, shown as heater212, may be disposed under the support apparatus202, providing heat flowing through the support apparatus202to the crystalline sheet112. In other embodiments a melt-back heater may be disposed within the support apparatus202. As discussed below, the heat provided by support apparatus202may be used to melt back a portion of the crystalline sheet112as the crystalline sheet112is drawn to the right. In this manner, the roughness of the crystalline sheet112may be improved and the thickness of the crystalline sheet112may be adjusted to a target sheet thickness before the crystalline sheet112is separated from the melt106. To facilitate transfer of heat to the crystalline sheet112, in various embodiments, the adjustable portion206may be maintained in contact with the fixed portion204. The adjustable portion206and fixed portion204may be maintained in contact with one another as the adjustable portion is moved along the Z-axis, for example.

In order to adjust the level of the upper surface214and to maintain contact between fixed portion204and adjustable portion206while the adjustable portion is moved, the adjustable portion may be slidably movable, as suggested inFIG. 3. The fixed portion204and adjustable portion206may have wedge shapes as shown. The fixed portion204may have an upper surface defining a first contact surface224, where the first contact surface forms a first angle of inclination θ with respect to horizontal. The adjustable portion206may have a lower surface defining a second contact surface226, where the second contact surface226is disposed at the first angle of inclination θ with respect to the upper surface214. In this manner, when the first contact surface224and second contact surface226are maintained in contact with one another, the upper surface214is parallel to the horizontal. In some embodiments, the adjustable portion206may be made of a different material from the fixed portion204to facilitate movement of the adjustable portion206and avoid sticking or welding between the adjustable portion206and fixed portion204.

Returning toFIG. 1AandFIG. 2A, in various embodiments the level of the crystalline sheet112may be monitored. For example, the crucible102may be disposed in a furnace140. The furnace140may include a window (not shown) where the window facilitates monitoring of the crystalline sheet112, for example, by visual inspection. In some embodiments, the pull rate of the crystalline sheet112may be on the order of 1 mm/s or less. When changes in the level of the crystalline sheet112are observed, the support apparatus202may adjust the level of the upper surface214of adjustable portion206, for example, in order to maintain the crystalline sheet at a target level, such as flush with the exposed surface124. Turning also toFIG. 3, whileFIG. 3explicitly illustrates adjustment of the level of the crystalline sheet112near the support apparatus202, the adjustment of the level of upper surface214may also have the effect of adjusting the level of the leading edge122. In this manner the support apparatus202may be used to maintain the level of the leading edge122at a target position such as flush with the exposed surface124. As further shown inFIG. 2Athe apparatus200may include a monitor215for measuring sheet thickness or the position of the surface222of the crystalline sheet112with respect to the exposed surface124. The monitor may be disposed upstream of the support apparatus202as shown. The information regarding crystalline sheet112measured by the monitor215may be used to adjust the level of the upper surface214of the support apparatus in order to maintain the surface222of crystalline sheet112at a desired level.

FIG. 4depicts an exemplary heat flow profile400provided by embodiments of support apparatus202. As illustrated, the heat flow may be concentrated above the adjustable portion206. This concentration of heat flow may be employed to melt back at least a portion of the crystalline sheet112. By providing sufficient heat flux, such as approximately 4 W/cm2to 5 W/cm2to the crystalline sheet112, the underside of the crystalline sheet may be melted as the crystalline sheet is drawn over the support apparatus202, as shown inFIG. 3. This melting of the underside of the crystalline sheet112may provide a constant fluid interface between the crystalline sheet112and the support apparatus202, providing a lubrication analogous to movement of a skate over the surface of ice. In this manner the adjustable portion206of support apparatus202, by virtue of conducting heat supplied by a meltback heater, may provide a low friction bearing surface for the crystalline sheet112when the crystalline sheet112is drawn over the support apparatus202.

FIG. 5AandFIG. 5Bdepict two different scenarios during processing of a crystalline sheet using the support apparatus202in accordance with embodiments of the disclosure. In particular, a crystalline sheet112is shown in transverse cross-section perpendicular to the pull direction, where the pull direction lies along the Z-axis as illustrated further, for example, atFIG. 2A. Referring also toFIG. 3, there is shown a first scenario inFIG. 5Awhere a region R of the crystalline sheet112is disposed over the point P3of the support apparatus202. As the crystalline sheet112is drawn to the right the region R also moves to the right. In the second scenario shown inFIG. 5B, the region R is disposed over the point P4of the support apparatus202. Accordingly, the scenario ofFIG. 5Brepresents the state of the region R of the crystalline sheet112at a later point in time as compared to the scenario ofFIG. 5A.

As also shown inFIG. 5Aa melt region502may be disposed between the crystalline sheet112and adjustable portion206. The melt region502may form at least in part due to heat transferred to the crystalline sheet112. As further shown inFIG. 5Athe crystalline sheet112may initially exhibit a larger roughness on the underside of crystalline sheet112adjacent the melt region502. The support apparatus202may advantageously reduce the roughness of crystalline sheet112by providing adaptive melt-back, meaning preferential melt back of thicker regions of the crystalline sheet. This adaptive melt back may lead to a better thickness uniformity of crystalline sheet112, where roughness is reduced as shown inFIG. 5B. In particular, in embodiments where the melt region502is an alumina melt and the crystalline sheet112is sapphire, the melt region502has a relatively high thermal resistivity. Accordingly, in regions506where the crystalline sheet112is relatively thicker, the portions504of the melt region502between the regions506and adjustable portion206is relatively thinner. Accordingly, the regions506present locations of low thermal resistance, allowing heat508to be funneled into the crystalline sheet112, resulting in larger melt-back. Conversely, below regions510of crystalline sheet112where thickness is less, an adjacent melt region, shown as melt region512, has greater thickness. This greater melt thickness presents a higher thermal resistance, blocking the flow of heat, and resulting in a lesser degree of melt-back of the crystalline sheet in melt regions512. This adaptive melt-back process may lead to a smoother crystalline sheet, as well as a thinner crystalline sheet, as shown inFIG. 5B.

An advantage provided by directing heat through the support apparatus202to melt back a crystalline sheet112as shown inFIGS. 5A and 5Bis the more precise control of heat applied to a crystalline sheet when the crystalline sheet and support apparatus are in such close proximity. For example, the average thickness of the melt region502may be less than one mm in some examples. This may be especially advantageous when employed to process sapphire sheets. The differences in properties of sapphire and silicon (sapphire: kinematic viscosity 1.88×10−5m2/s, thermal diffusivity 4.62×10−7m2/s, and thermal expansion coefficient 3.6×10−4/K; silicon: kinematic viscosity 3.14×10−7m2/s, thermal diffusivity 2.61×10−5m2/s, and thermal expansion coefficient 1.0×10−4/K) results in the Rayleigh number being much higher for sapphire, resulting in the likelihood of creating natural convection cells in regions of a melt having a depth of approximately 1 cm. The Rayleigh number Ra is defined as

Ra=g⁢⁢βv⁢⁢α⁢(Tb-Tu)⁢L3,(1)
where g is the gravitational constant, β is the thermal expansion coefficient, ν is the kinematic viscosity, α is the thermal diffusivity, Tb, Tuare the temperatures of the upper and lower boundaries, and L is the depth of the melt. Computational fluid dynamics modeling of alumina melt behavior in an apparatus arranged similarly to apparatus100shows the development of convective cells within “deep” regions of the melt where the melt extends from the crucible bottom to the exposed surface, for approximately 15 mm. Additionally, convective cell formation is suppressed in the regions where the melt is thin, such as over a support apparatus114where the melt thickness may vary from approximately 3 mm to less than one millimeter. Thus much greater control and predictability of the heat flux in provided in certain areas, such as at the melt back area above the support apparatus114.

FIG. 6depicts an exemplary process flow600. At block602, the operation is performed of forming a crystalline sheet on an exposed surface of a melt in a crucible having a dam structure. In various embodiments, the exposed surface of the melt may be defined by a top of the dam structure.

At block604, a first position of a surface of the crystalline sheet is measured. The first position may be measured with respect to the exposed surface. The first position may be measured downstream of a crystallizer. At block606, an upper surface of a support apparatus is placed at a first height based upon the measured first position. The support apparatus may be disposed downstream of a location where the measurement of the first sheet thickness takes place. At block608, the crystalline sheet is drawn over the upper surface of the support apparatus wherein the crystalline sheet is maintained flush with the exposed surface.

Advantages of the present embodiments include the ability to both thin a crystalline sheet and increase the thickness uniformity of the crystalline sheet using a support apparatus having a novel design. Other advantages include the ability to maintain a crystalline sheet flush with a melt surface, facilitating the ability to draw crystalline sheets from a melt in a horizontal direction even when the density of the crystalline sheet is greater than the density of the melt.

The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Furthermore, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize the usefulness of the embodiments of the present disclosure is not limited thereto and the present embodiments may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.