Patent Application
20080099006
1. Field of the Invention
The invention relates to a method for simultaneously slicing at least two cylindrical workpieces into a multiplicity of wafers by means of a multi wire saw.
2. Background Art
Multi wire saws are used for example for slicing cylindrical mono- or polycrystalline workpieces of semiconductor material, for example silicon, simultaneously into a multiplicity of wafers in one working step. The production of semiconductor wafers from cylindrical semiconductor material, for example single crystal rods, places exacting requirements on the sawing method. The sawing method ideally ensures that each sawed semiconductor wafer should have two surfaces which are as plane as possible and lie parallel to one another. The throughput of the multi wire saw is also of great importance for economic viability.
In order to increase the throughput, it has been proposed for a plurality of workpieces to be simultaneously clamped into the multi wire saw and sliced in one working step. U.S. Pat. No. 6,119,673 describes the simultaneous slicing of a plurality of cylindrical workpieces, which are arranged coaxially behind one another. To this end a conventional multi wire saw is used, a plurality of workpieces each adhesively bonded on a sawing bar being fixed with a certain spacing in a coaxial arrangement on a common mounting plate, clamped with it into the multi wire saw and sliced simultaneously. This creates a number of stacks of wafers, which are still fixed on the mounting plate, corresponding to the number of workpieces. After the slicing, separating plates are placed loosely into the spaces between the stacks of wafers, in order to prevent the wafers of the various stacks from being confused. This is of great importance since the wafers produced from different workpieces will generally be further processed in different ways and/or the workpieces may have different properties, specified by the customer to which the wafers will be delivered. It is therefore necessary to ensure that all wafers produced from a workpiece intended for a certain customer or a certain order are further processed together, but processed separately from wafers produced from other workpieces.
After the various wafer stacks have been demarcated by separating plates, the mounting plate is immersed in a basin of hot water so that the wafers connected to the mounting plate via the sawing bar hang below the mounting plate. The hot water dissolves the cement bond between the wafers and the sawing bars, so that the detached wafers fall into a wafer carrier placed at the bottom of the basin. The various wafer stacks, which are subsequently contained in the wafer carrier, are separated from one another by the previously introduced separating plates.
The method disclosed in U.S. Pat. No. 6,119,673 for demarcating the various stacks of wafers has the disadvantage that the wafer stacks are not secured against lateral tilting (as can be seen in
U.S. Pat. No. 6,802,928 B2 describes a method in which dummy pieces with the same cross section are adhesively bonded onto the end surfaces of the workpiece to be sliced, sliced with the workpiece and then discarded. This is intended to prevent the resulting wafers from fanning out at the two ends of the workpiece during the end phase of the slicing, and therefore to improve the wafer geometry. This method has the crucial disadvantage that some of the gang length, which is limited by the dimensions of the multi wire saw, is used for slicing the “unused” dummy pieces and is therefore not available for the actual production of the desired wafers. Furthermore, the provision, handling and adhesive bonding of dummy pieces is very elaborate. Both lead to a significant reduction in economic viability.
Also in the method described in U.S. Pat. No. 6,119,673 for simultaneously slicing a plurality of workpieces in a multi wire saw, the gang length of the multi wire saw often cannot be utilized optimally since the workpieces to be sliced have very different lengths owing to the way in which they are produced. This problem arises particularly when the workpieces consist of monocrystalline semiconductor material, since the known crystal pulling processes only permit certain usable lengths of the crystals or it is necessary to cut the crystals and produce test specimens at various positions of the crystal in order to control the crystal pulling process. Furthermore, various types of semiconductor wafers with different properties (which for the most part are already defined by the crystal from which the wafers are produced) are usually fabricated in the same plant for a plurality of customers, in which case different delivery deadlines need to be complied with.
It was therefore an object of the invention to improve the utilization of the available gang length of a multi wire saw. It was also an object to avoid damaging the wafers during the insertion of separating plates or the wafer edges during separation from the mounting plate and individualization. These and other objects are achieved by a sawing process in which a plurality of workpieces are sawed simultaneously, the lengths of the individual workpieces selected such that maximum utilization of gang length occurs. The wafers from each workpiece are preferably separated from those of other workpieces and edge damage is also prevented by spacer elements fastened to the wafer carrier.
The invention relates to a first method for simultaneously slicing at least two cylindrical workpieces into a multiplicity of wafers by means of a multi wire saw with a gang length LG, comprising the following steps:
a) selecting a number n≧2 of workpieces from a stock of workpieces with different lengths, so that the inequality
is satisfied and at the same time the right-hand side of the inequality is as large as possible, where Li with i=1 . . . n stands for the lengths of the selected workpieces and Amin stands for a predefined minimum spacing,
b) fixing the n workpieces successively in the longitudinal direction on a mounting plate while respectively maintaining a spacing A≧Amin between the workpieces, which is selected so that the relation
is satisfied,
c) clamping the mounting plate with the workpieces fixed thereon in the multi wire saw, and
d) slicing the n workpieces perpendicularly to their longitudinal axis by means of the multi wire saw.
The invention also relates to a further embodiment for simultaneously slicing at least two cylindrical workpieces into a multiplicity of wafers by means of a multi wire saw, comprising the following steps, with reference to the drawing figures but not limited thereby:
a) selecting a number n≧2 of workpieces from a stock of workpieces with different lengths,
b) fixing the n workpieces successively in the longitudinal direction on a mounting plate 11 while respectively maintaining a spacing between the workpieces,
c) clamping the mounting plate 11 with the workpieces fixed thereon in the multi wire saw,
d) slicing the n workpieces perpendicularly to their longitudinal axis by means of the multi wire saw so as to form n stacks 121, 122, 123 of wafers 12 fixed on the mounting plate 11,
e) introducing the wafers 12 fixed on the mounting plate 11 into a wafer carrier 13, which supports each wafer 12 on at least two points of the wafer circumference that lie away from the mounting plate 11,
f) introducing at least one separating piece 15 into each of the spaces between two neighboring stacks 121, 122, 123 of wafers 12 and fastening the separating piece 15 on the wafer carrier 13,
g) releasing the bond between the wafers 12 and the mounting plate 11,
i) sequentially removing each individual wafer 12 from the wafer carrier 13.
In this method, the workpieces are selected from a stock of workpieces with different lengths so that the gang length LG of the multi wire saw is optimally utilized. Since the capacity of the multi wire saw is therefore exploited better, the productivity is significantly increased.
A conventional multi wire saw is employed in the method according to the invention. The essential components of these multi wire saws include a machine frame, a forward feed device and a sawing tool, which consists of a gang comprising parallel wire sections. The workpiece is generally fixed on a mounting plate and clamped with it in the multi wire saw.
In general, the wire gang of the multi wire saw is formed by a multiplicity of parallel wire sections which are clamped between at least two (and optionally three, four or more) wire guide rolls, the wire guide rolls being mounted so that they can rotate and at least one of the wire guide rolls being driven. The wire sections generally belong to a single finite wire, which is guided spirally around the roll system and is unwound from a stock roll onto a receiver roll. The term gang length refers to the length of the wire gang as measured in the direction parallel to the axes of the wire guide rolls and perpendicularly to the wire sections from the first wire section to the last.
During the sawing process, the forward feed device causes an oppositely directed relative movement of the wire sections and the workpiece. As a consequence of this forward feed movement, the wire, to which a sawing suspension is applied, works to form parallel sawing grooves through the workpiece. The sawing suspension, which is also referred to as a “slurry”, contains hard material particles, for example of silicon carbide, which are suspended in a liquid. A sawing wire with firmly bound hard material particles may also be used. In this case, a sawing suspension does not need to be applied. It is merely necessary to add a liquid cooling lubricant, which protects the wire and the workpiece against overheating and simultaneously transports workpiece swarf away from the cutting grooves.
The cylindrical workpieces may consist of any material which can be processed by means of a multi wire saw, for example poly- or monocrystalline semiconductor material such as silicon. In the case of monocrystalline silicon, the workpieces are generally produced by sawing an essentially cylindrical single silicon crystal into crystal pieces with a length of from several centimeters to several tens of centimeters. The minimum length of a crystal piece is generally 5 cm. The workpieces, for example the crystal pieces consisting of silicon, generally have very different lengths but the same cross section. The term “cylindrical” is not to be interpreted as meaning that the workpieces must have a circular cross section. Rather, the workpieces may have the shape of any generalized cylinder, although application of the invention to workpieces with a circular cross section is preferred. A generalized cylinder is a body which is bounded by a cylinder surface with a closed directrix curve and by two parallel planes, i.e. the base surfaces of the cylinder.
In step a) of the first method according to the invention, a number n≧2 of workpieces is selected from an available stock of workpieces preferably with the same cross section. The stock of workpieces comprises a multiplicity of workpieces with different lengths, although this does not preclude the existence of a plurality of workpieces with the same length. The workpieces are selected so that Inequality (1) is satisfied. This means that the sum of the lengths Li of the selected workpieces i plus an established minimum spacing Amin between each pair of workpieces, which is maintained when fixing the workpieces on a mounting plate, does not exceed the gang length LG. The minimum spacing is freely definable, and may even be zero. It is preferably close to zero, since a larger minimum spacing automatically leads to inferior utilization of the gang length of the multi wire saw. Taking this condition into account, the workpieces are selected from the stock such that the right-hand side of Inequality (1) is as large as possible, so that the gang length is utilized as well as possible when slicing the workpieces.
is satisfied, where Lmin stands for a predefined minimum length which is less than the gang length LG. According to this embodiment, the length should not be less than this minimum length when selecting the workpieces. The minimum length Lmin is preferably established in relation to the gang length LG so that Lmin≧0.7·LG, preferably Lmin≧0.75·LG and particularly preferably Lmin≧0.8·LG, Lmin≧0.85·LG, Lmin≧0.9·LG or Lmin≧0.95·LG.
Since very large stocks of workpieces are usually available, it is expedient and therefore preferable to carry out selection of the workpieces by means of a computer, which has access to the lengths of all workpieces in the stock. For example, the computer may be connected to an EDP-supported stock management system in which all stock input and output processes together with the properties (length and type) of the workpieces are recorded, and which therefore knows the current stock status at any time. A program, in which all rules for the selection of the workpieces are implemented, runs on the computer.
In step b), the n selected workpieces are fixed successively with respect to their longitudinal direction on a mounting plate while respectively maintaining a spacing A≧Amin between the workpieces, which is selected so that Inequality (2) is satisfied. The spacing A must thus on the one hand correspond at least to the predefined minimum spacing Amin between two workpieces, but on the other hand it should not be selected to be so large that the sum of the lengths Li of the workpieces plus the spacings A between the workpieces exceeds the gang length LG. The expression “successively with respect to their longitudinal direction” does not necessarily imply a coaxial arrangement of the workpieces, although this is preferable. The workpieces may nevertheless be arranged so that their longitudinal axes do not lie on the same straight line. “Successively” is merely intended to express the fact that the base surfaces, rather than the lateral surfaces, of two neighboring cylindrical workpieces face one another.
The workpieces are preferably not fixed directly on the mounting plate, but are instead first fastened on a so-called sawing bar or sawing base. The workpiece is generally fastened on the sawing bar by adhesive bonding. Preferably, each workpiece is adhesively bonded individually onto its own sawing bar. The sawing bars with the workpieces fastened on them are subsequently fastened on the mounting plate, for example by adhesive bonding or screwing.
Subsequently, the mounting plate with the workpieces fixed on it is clamped in the multi wire saw in step c) and the workpieces are sliced simultaneously and essentially perpendicularly to their longitudinal axis into wafers in step d). The gang length of the multi wire saw is optimally utilized in this case owing to the selection of the workpieces made in step a), which increases the throughput and therefore the economic viability.
In a preferred embodiment of the first method according to the invention, the delivery deadlines arranged with various customers are taken into account when selecting the workpieces in step a). Workpieces that can be used for the production of wafers, for which an earlier delivery deadline is arranged, are preferably selected in step a).
It is also conceivable to provide that Inequality (1) in step a) no longer categorically needs to be satisfied when the time until a delivery deadline is less than a predefined minimum time. In this case, complying with the delivery deadline takes priority over optimal utilization of the gang length.
Another preferred option consists in always first selecting a workpiece which is required in order to fulfill the still unprocessed order with the earliest delivery deadline. Further workpieces are subsequently selected so that the gang length is used in the best possible way.
As described above, the stock of workpieces is produced for example by slicing crystals perpendicularly to their longitudinal axis into at least two workpieces with a length Li, which are added to the stock. The length of the workpieces should not exceed the gang length LG of the multi wire saw used in step d). In another preferred embodiment of the first method according to the invention, the specifications established in the individual orders for the warp of the wafers is already taken into account when producing the stock of workpieces from a stock of cylindrical crystals. The parameter “warp” is defined in the SEMI standard M1-1105. In general a maximum value for the warp of the wafer, which should not be exceeded, is specified for each order from the customer. This maximum value differs from customer to customer and from order to order. There are therefore always orders with a warp specification which is easy to satisfy, and orders with a demanding warp specification. In order to fulfill in particular the latter orders while complying with specification, according to the preferred embodiment, a crystal which is assigned to an order with a low maximum value for the warp is sliced into workpieces which are as long as possible. The length Li of the workpieces in relation to the gang length LG of the multi wire saw used in step d) preferably satisfies the relation LG/2<Li≦LG in this case.
With reference to the example of silicon wafers with a diameter of 300 mm,
If this rule were to be applied for all orders, the effect would be that too many workpieces with a large length are added to the stock and, for the selection in step a), too few workpieces are available which can be fastened together with the long workpieces in step b) on a common mounting plate and sliced in one working step into wafers in step d). Although such a measure would improve the warp achieved on average, at the same time the capacity of the multi wire saw would no longer be utilized optimally. According to this embodiment, therefore, crystals which are assigned to an order with a high maximum value for the warp (which is relatively easy to achieve) are sliced into comparatively short workpieces. The length Li of these workpieces in relation to the gang length LG of the multi wire saw used in step d) preferably satisfies the relation Li<LG/2. For orders with a warp specification which is not very demanding, it is unnecessary to produce workpieces which are as long as possible. At the same time, this measure ensures that a sufficient number of short pieces are always available, which can be combined in step a) with the long workpieces for the orders with a demanding warp specification, and can be processed together with them in the further steps in order to utilize the gang length of the multi wire saw optimally.
This embodiment thus makes it possible to produce a multiplicity of wafers which have a narrow distribution of the geometrical parameter “warp” at a comparatively low level, for orders with a demanding warp specification. At the same time, an improvement of the warp is deliberately obviated for the other orders in order to optimally utilize the gang length of the multi wire saw.
The second embodiment according to the invention will be described in detail below with the aid of
In contrast to the method described in U.S. Pat. No. 6,119,673, the invention safeguards against confusion by means of separating pieces 15 fixable firmly on the wafer carrier 13, which in step f) are preferably inserted preferably laterally between the wafer stacks 121, 122, 123 and then fixed on the wafer carrier 13. The wafer stacks 121, 122, 123 stabilized in this way are optionally subjected to cleaning. The bond between the wafers 12 and the mounting plate 11 is subsequently released, while the separating pieces 15 support the wafer stacks 121, 122, 123 against lateral tilting.
This method avoids mixing or confusion of wafers 12 which have been produced from different workpieces and are intended for different orders. Furthermore, the stacks 121, 122, 123 of wafers 12 are protected reliably in steps g) and i) against lateral tilting and therefore damage to the sensitive wafer edge.
In step a), at least two workpieces are selected from a stock of workpieces. The selection is preferably carried out as described for step a) of the first method according to the invention. In this case, the spacing Amin in step a) is selected so that it corresponds at least to the thickness of the separating pieces 15, optionally plus the thickness of the separating plates 17 (if such separating plates are used), so that they can be introduced into the space. Steps b) to d) are also preferably carried out as in the first method according to the invention.
In step e), the wafers 12 fixed on the mounting plate 11 are put into a wafer carrier 13 which supports each wafer on at least two points of the wafer circumference that lie away from the mounting plate (
In step f) (
In step g), the bond between the wafers 12 and the mounting plate 11 is released. In the preferred embodiment represented in the figures, the wafer carrier 13 with the wafers 12 fixed on the mounting plate 11 via the sawing bars 141, 142, 143 is put into a basin 16 filled with a liquid, as represented in
Between the steps g) and i), an additional step h) is preferably carried out in which at least one separating plate 17 is introduced into each of the spaces between two neighboring stacks 121, 122, 123 of wafers 12, in addition to the separating piece 15 fastened there (
The separating plates 17 are preferably made of a material which is geometrically stable and can withstand the prevailing temperatures and the chemicals coming in contact with it.
In step i), the wafers are removed individually from the wafer carrier 13, for example by means of a vacuum suction device 181. In order to obtain the lateral access to the wafers 12 required for their removal, at least one of the end pieces 132 of the wafer carrier 13 may comprise a suitable opening (for example a vertical slot) through which the vacuum suction device can be moved laterally onto the wafers 12. Alternatively, at least one of the end pieces 132 may be designed in two parts, in which case the upper part can be taken off. This is represented in
In the case of automatic individual removal by a robot 182 (
While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2006 050 330.9 | Oct 2006 | DE | national |
