WO2006056557A2

WO2006056557A2 - Electrostatic holding device having a number of supply sources

Info

Publication number: WO2006056557A2
Application number: PCT/EP2005/056095
Authority: WO
Inventors: Yvon Pellegrin
Original assignee: Semco Engineering Sa
Priority date: 2004-11-25
Filing date: 2005-11-21
Publication date: 2006-06-01
Also published as: FR2878371B1; FR2878371A1; EP1820208A2; WO2006056557A3

Abstract

The invention relates to an electrostatic holding device for holding a wafer made of a conductive or semiconductor material, comprising an electrically insulating base (11) on which said wafer (12) is placed. The invention is characterized in that the base (11) has an electrically insulating surface under which at least two electrode pairs (14, 15, 17, 18), at least two independent power supplies (16, 19) each supplying at least one electrode pair by a potential difference of which the polarities are, at different instants, inverted in order to release the accumulated electrostatic charges. The device preferably comprises a programmable means for controlling power supplies, that is configured for controlling the voltages furnished by the power supplies.

Description

ELECTROSTATIC HOLDING DEVICE WITH MULTIPLE POWER SOURCES.

The present invention relates to an electrostatic holding device with several electrical power sources. It applies, in particular, to the maintenance of platelets of conductive or semiconductor materials (in English "wafer") such as silicon while undergoing micro-machining or any other type of treatment such as plasma treatments or deposits in a vacuum chamber for example. The various processing operations throughout the manufacturing process require that the wafer of material be securely held on a support, while controlling its temperature as perfectly as possible. Platelets are generally moved from one station to another by automated means.

It is known to maintain the wafer by flanges bearing on the periphery of the upper surface of the wafer, but these systems have the disadvantage of monopolizing a portion of the wafer that can not be processed and will be lost. In addition, these flanges can damage the wafer during its movements and possibly influence the applied treatment locally.

It is also known electrostatic holding systems whose principle is to place the wafer of semiconductor material on an insulating surface and to have sets of electrodes under this surface. The electrodes are subject to a potential difference. The electric field created by the electrodes then generates a phenomenon called "electrostatic bonding".

The treatments or micro-machining performed on the wafer require very high precision, the wafer must be perfectly maintained and thermalized throughout the treatment cycle. However, when the material constituting the soleplate is subjected to an electric field of the same polarity for a certain time, it tends to accumulate charges in traps which eventually maintain the wafer bonded to the surface even when the external electric field does not will be more applied, but also degrade the material and greatly reduce the life of the sole.

US Pat. No. 5,452,177 describes an electrostatic holding device on an insulating circular surface under which at least six electrodes arranged regularly in pairs, facing the center of the circular surface, are placed. The electrodes are powered by an alternating voltage generator, providing six different outputs, each pair of electrodes being fed cyclically under different polarities. The three pairs of electrodes are powered by signals shifted in phase of 120 degrees so that two pairs of electrodes are fed and thus maintain the wafer at the moment when the third changes polarity. The switching frequencies are of the order of 30 Hz. In order to achieve this result, the system implements means of supplying the electrodes which are very complex and therefore expensive and offer no flexibility. The present invention aims to remedy these disadvantages. The object of the present invention is to propose a novel electrostatic holding device of simplified constitution which is therefore economically advantageous, allowing a great deal of flexibility, while ensuring perfect maintenance of the wafers and avoiding any accumulation of charges.

For this purpose, the device for holding a wafer of conductive or semiconductor material, comprising an electrically insulating soleplate on which said wafer is arranged, characterized in that the soleplate comprises an electrically insulating surface under which at least two pairs are arranged. of electrodes, at least two independent power supplies each feeding at least one pair of electrodes by a difference of potentials whose polarities are, at different times, inverted in order to release the accumulated electrostatic charges.

Thanks to these arrangements, the electrical voltages applied by the power supplies are programmable independently of each other.

According to particular features, the device as briefly described above includes a programmable power supply control means adapted to control the voltages supplied by the power supplies.

Thanks to these provisions, the electrical voltages applied by the power supplies are programmed independently of each other by said control means.

According to particular features, the power supply control means is adapted to reverse the polarities of at least one pair of electrodes at each wafer change. According to particular characteristics, the electrodes have a symmetry which has a character of revolution, with respect to each other, that is to say that there is an angle of rotation, different from 0 ° and 360 °, which has the set of electrodes superimposed on itself.

According to particular features, the electrodes have a spiral shape. In the case where the supply voltages of the electrodes vary in time, it is well known that very slight vibrations of the wafer occur. These vibrations correspond to the temporal variations of the equilibrium of the pressures involved. In some cases, displacements of the plate are even observed. These observed vibrations have an amplitude which, all things being equal, depends on the dimensions of the surfaces subjected to pressure variations. The inventors have found that a spatial alternation of the electrodes as close as possible decreases the range of motion. The spiral shape makes it possible to cover any surface, with a nesting of the different electrodes limited only by the constraints of implementation. In addition, this spiral arrangement requires only electrical contact per unit electrode, which simplifies the implementation, reduces costs and greatly increases the reliability of the sole. Among the other advantages inherent to the spiral shape, the electrostatic pressures exerted at the periphery edges of the plates depend on all the electrodes and not just one as in the case of an electrode in the form of concentric rings. The bevelled end of each electrode locally ensures an optimized maintenance of the periphery of the wafer. Finally, in certain applications, this spiral device makes it possible to use the same electrostatic sole for different wafer diameters. Indeed, if the wafer is mechanically centered with respect to the center of the spiral, the intersections of the surfaces of the different electrodes with any disk diameter will always be equal to each other, which is very important for the proper functioning of the electrostatic sole.

Thanks to these arrangements, the electrodes exert the maintenance of the wafer on a large angle of the wafer, which can reach 360 °, or even several turns. In addition, the spiral shape tolerates better positioning defects of the wafer.

According to particular features, the electrodes are annular. Thus, the holding pressure of the wafer is uniform over its entire periphery.

According to particular features, the arrangement of the electrodes is symmetrical or concentric with respect to the center of the sole. According to particular characteristics, the plane surfaces of the two electrodes forming a pair have the same area. Thanks to these provisions, the electrostatic pressures exerted are equal on the electrodes of the same pair.

According to particular features, the plane surfaces of the two electrodes of the same pair covered by any wafer dimensions have the same effective area. According to particular characteristics, the absolute value of the average, over the maintenance period of a wafer, of the potential differences to which the electrodes are subjected is at least ten times lower than the maximum absolute value of these differences in potentials, such that the duration of maintenance of the wafer on the soleplate may be greater than twenty-four hours without the duration of detachment of the wafer becoming greater than two seconds. Preferably, the factor of ten mentioned above is increased to at least one hundred.

According to particular characteristics, the electrodes are made by serigraphy of thick layers on a base plate. According to particular features, the electrically insulating surface is made by serigraphy of thick layers on a base plate.

According to particular features, the electrically insulating surface made by screen printing thick layers on a base plate is coated with a high performance dielectric layer deposited by CVD under low pressure. Other advantages and characteristics will appear on reading the following description of embodiments of the invention given as non-limiting examples and illustrated by the accompanying drawings in which:

- Figure 1 shows schematically, in section, a particular embodiment of the holding device; - Figure 2 shows schematically, in top view, a particular embodiment of electrodes of the device shown in Figure 1;

- Figure 3 shows schematically, in top view, another particular embodiment of electrodes of the holding device;

FIGS. 4 and 5 show, schematically, in plan view, other possible configurations of the electrodes and

FIG. 6 represents an exemplary chronogram of variations of the voltages applied to the electrodes of the holding device as illustrated in FIGS. 1 to 5.

As can be seen in Figure 1, the holding device 10 is composed of a sole 1 1 of electrically insulating material on which a plate 12 to maintain contact with the upper surface 13 of the sole 1 1. Electrodes 14, 15, 17 and 18 are arranged beneath this surface 13. According to a particular embodiment, the sole plate 1 1 consists of a base plate 22 on which the electrodes 14, 15, 17 and 18, then the assembly is covered with a layer of dielectric material or insulating surface 23. The electrodes 14, 15, 17 and 18 and the dielectric material layer 23 may be made by thick film screen printing. according to techniques known to those skilled in the art. The base plate 22 may be made of any type of dielectric material that is electrically insulating.

According to a particular embodiment of the invention, the base plate 22 is made of virgin alumina. The layer of dielectric material 23 covering the electrodes 14, 15, 17 and 18 may also be made by any type of dielectric material, for example based on ceramic.

In particular embodiments, the electrodes 14, 15, 17 and 18 are made by serigraphy of thick layers on the base plate 22. Also, in particular embodiments, the dielectric layer 23 is made by screen printing of thick layers on the base plate 22.

The dielectric layer 23 made by serigraphy of thick layers can be advantageously completed by a surface treatment. More specifically, the electrically insulating surface or dielectric layer 23 made by serigraphy of thick layers on a base plate 22 is coated with a high performance dielectric layer deposited by low pressure deposition.

This surface treatment consists of one or more thin layers (a few fractions to a few tens of nanometers) of high performance dielectric material obtained by deposition technologies such as those used in the manufacture of semiconductors (LPCVD, PECVD , CVD, Plasmas, PVD, ...). These materials are, for example, oxides or nitrides of silicon, aluminum or rare earth whose essential role is to intervene only very weakly in the electrostatic bonding properties of the sole, given their very small relative thickness, but to provide a specific characteristic of mechanical anti-wear protection and / or physico-chemical protection and / or modification of the components related to the effects of electric fields such as those of the type described by the Fowler-Nordheim theory. The choice of material is directly dependent on the result required by the application and its conditions.

The wafer 12 is disposed flat on the upper surface 13 of the base plate, that is to say, in the embodiment described above, on the thin layer of the highest dielectric material among the thin layers. which encapsulate the insulating surface or dielectric layer 23.

According to a particular embodiment of the electrodes 14, 15, 17 and 18, illustrated in FIG. 2, the electrodes 14, 15, 17 and 18 each have an annular shape and are arranged, under the surface 13, parallel to the wafer. 12. In this configuration, the electrodes are concentric rings of different diameters whose center corresponds to the center of the sole 1 1. The annular shape of the electrodes is one of the preferred forms since the wafer 12 is of generally circular shape, which allows to maintain it on its entire periphery.

However, for maintaining parts of different shapes, for example rectangular, the invention can implement electrodes of different shapes, for example rectangular. The wafer 12 is disposed on the upper surface 13 of the sole 1 1 so that its center corresponds to the center of the ring shapes of the electrodes 14, 15, 17 and 18. In order to obtain a good distribution of the electric field, preferentially, the surfaces of the rings forming the electrodes 14, 15, 17 and 18 have the same area. It is observed that the central electrode 15 can be made in the form of a ring or a disc. The electrodes 14 and 15 are subjected to a potential difference by the power supply 16 supplying a DC voltage, in pieces, for example during regular or non-regular time intervals, of +

1 000 volts, then -500 volts, then 0 volts, ... The field lines created between the wafer 12 and the two electrodes 14 and 15 allow the electrostatic bonding of the wafer 12 on the upper surface 13 of the sole 1 1. The bonding pressure is proportional to the square of the potential difference between the two electrodes 14 and 15.

The electrodes 17 and 18 are subjected to a potential difference by the electrical supply 19 supplying a DC voltage, in pieces, for example during regular time intervals or not, of - 800 volts, then 0 volts, then + 600 Volts, ... Field lines created between the wafer 12 and the two electrodes 17 and 18 allow the electrostatic bonding of the wafer 12 on the upper surface 13 of the sole 1 1. The bonding pressure is proportional to the square of the potential difference between the two electrodes 17 and 18.

The programmable power supply control means 30 is, for example, constituted by a PLC or a microprocessor card. It controls the power supplies 16 and 19 and, in particular, the voltages supplied by these power supplies, independently.

The only constraints applied by the control means 30 are:

- permanently, at least one power supply provides a potential difference for electrostatic bonding and

over a long period of time, the average of the voltages applied to each electrode is substantially zero, that is to say that the absolute value of the mean, over the period of maintenance of a wafer, of the potential differences to which the electrodes is at least ten times, preferably a hundred times, smaller than the maximum absolute value of these potential differences. Thus, even when the duration of maintenance of the wafer on the soleplate is greater than twenty-four hours without the duration of detachment of the wafer becoming greater than two seconds.

Indeed, when they are subjected to an intense electric field, the materials constituting the sole 1 1 tend to accumulate electrostatic charges that may hinder the detachment of the wafer 12 even when the electrodes are no longer fed. This accumulation of electrostatic charges is proportional to the supply time of the device as well as to the value of the voltage.

The wafer 12, resting on the surface 13, is generally raised by rods 20 distributed over its surface so that the wafer 12 is gripped by a manipulator arm at the end of its treatment. The rods 20 move in vertical translation in holes 21 through the sole 1 1 under the action of a cylinder, for example. Also, the rods 20 would damage the wafer if it remained stuck to the surface 13 under the effect of an accumulation of electrostatic charges. It is accepted that a duration of less than two seconds between the moment of the cut-off of the supply of the electrodes - that is to say the moment when the potential difference between these electrodes is zero - and the integral take-off of the wafer previously maintained is satisfactory. This duration is of the order of magnitude of the RC time constants involved in the system and does not involve stickiness remanence due to accumulated charges. If this duration is greater than two seconds, it is very likely that this duration is random from one wafer to another, making the automation of handling very complicated or impossible and in any case very risky for the wafer as it has been mentioned above.

In the case of a device comprising two electrodes and for processes requiring a maintenance of relatively short duration, for which the electric polarization does not have time to produce significant accumulation of charges, alternatively, the polarities of the two electrodes each pad change 12. Thus, the charges accumulated by the sole 11 can be evacuated. For this purpose, in this variant, the control means 30 applies to each power supply 16 or 19 a polarity change synchronized with the machining or processing cycle. Thus, at each end of the cycle, for example, the polarities are reversed.

For longer processing times or requiring a greater electrostatic bonding pressure, the present invention uses several pairs of electrodes powered at different polarities so that at any time at least one pair of electrode maintained the plate 12. According to a particular embodiment of the electrodes, illustrated in FIG. 2, the electrodes 14, 15, 17 and 18 are made in the form of four concentric rings operating in pairs.

The pairs of electrodes referred to above are only one example to illustrate the operation of the device object of the present invention. In variants, the electrode 14 and the electrode 15 are separated by the electrode 17 and / or the electrode 18.

According to this principle of inversion of the polarities at different times, the sole 1 1 can remain powered indefinitely without accumulation of charges.

The switching times of the electrodes may be variable depending on the voltage of the power supply. As an example for a silicon wafer maintained at a voltage of 1000 volts, the optimum switching time is a few tens of seconds. It is obvious that this time is variable and can be reduced to a few seconds or less, however it is important to avoid excessive switching that would damage the power supply components. The components and the embodiment of the power supplies 16 and 19 need not be described in detail since they are well known to those skilled in the art. By way of example, the switching operations can be performed by low-voltage relays controlled by a programmable logic controller.

The number of rings forming the electrodes is absolutely not limited to four and their number may be much greater without departing from the scope of the present invention.

The configuration of the electrodes can also be performed in many other forms, for example the shapes illustrated in FIGS. 3, 4 and 5. The symmetry and the equal areas are the preferred characteristics of the possible forms of the electrodes: in the embodiments shown the plane surfaces of the two pair electrodes have the same area.

FIG. 3 shows another particular embodiment of the electrodes 14, 15, 17 and 18, which here take the references 34, 35, 37 and 38 in the general shape of a spiral. In the embodiment illustrated in FIG. 3, each spiral covers an angle of

900 °, two turns and a half. The electrodes all cover the same area.

It can be observed that, as a variant, the arrangement of four spiral electrodes has a symmetry of revolution, that is to say that there exists a rotation angle, different from 0 ° and from 360 °, in this case 90 °, which makes the set of electrodes superimpose itself. The electrodes therefore all have the same area. The pairs of electrodes referred to above are only one example to illustrate the operation of the device object of the present invention. In variants, the electrode 34 and the electrode 35 are side by side.

In FIG. 4, the electrodes 44, 45, 47, 48 are four disk portions operating in pairs facing each other. The number of portions, and consequently the number of feeds, forming the electrodes is variable according to the constraints in the sole and the distribution of desired bonding pressures. Thus, for treatments requiring a low adhesive pressure, the electrodes can be four in number as shown in FIG. 4. For greater pressures, the number of electrode pairs can be multiplied as shown in FIG. example by continuously turning on three of the four pairs of electrodes.

FIG. 6 shows the voltages applied by the power supplies during time intervals of different durations (only the voltages applied to the electrodes 14 and 17 are represented, the voltages applied to the electrodes 15 and 18 being opposite, respectively, of the voltages applied to the electrodes 14 and 17). It is observed that the representation of the durations of the time intervals is not to scale.

From t0 to t1, for 10 seconds, the electrode 14 is supplied with a positive voltage at +1000 volts and the electrode 15 at negative -1000 volts, the electrodes 17 and 18 being at a zero potential; From t1 to t2, for 300 seconds, the electrode 14 is supplied with positive to +1000

Volts and the negative electrode at -1000 volts, the electrode 17 is supplied with negative at -500 volts and the electrode 18 is supplied with positive at +500 volts;

From t2 to t3, for 5 seconds, the electrodes 14 and 15 are at a zero potential, the electrode 17 is supplied with negative at -500 volts and the electrode 18 is supplied with positive at +500 volts;

From t3 to t4, for 180 seconds, the electrode 14 is supplied with a negative voltage at -400 volts and the electrode at +400 Volts positive, the electrode 17 is supplied with a negative power at -800 volts and the electrode 18 is energized positive at + 800 volts.

From t4 to t5, for 6 seconds, the electrode 14 is supplied negative at -400 volts and the electrode positive at +400 volts, the electrodes 17 and 18 being at zero potential.

From t5 to t6, for 240 seconds, the electrode 14 is supplied with a negative voltage at -400 volts and the electrode with a positive voltage of +400 volts, the electrode 17 is supplied with a positive voltage at + 800 volts and the electrode 18 is powered negative at - 800 volts. From t6 to t7, for 8 seconds, the electrodes 14 and 15 are at a zero potential, the electrode 17 is supplied with a positive voltage at + 800 volts and the electrode 18 is supplied with negative power at -800 volts.

From t7 to tfin, for 300 seconds, the electrode 14 is supplied positive to + 600 volts and the negative electrode to - 600 volts, the electrode 17 is supplied with positive to + 600 volts and the electrode 18 is powered negative at - 600 volts.

Although, in the description, only piecewise continuous voltages have been represented, the present invention is not limited to the implementation of such voltages but extends to implementations of voltages of any shape, for example alternatives.

The regular or non-regular time intervals thus continue throughout the processing or machining phase of one or more platelets 12.

As can be seen the potential differences provided by the power supplies 16 and 19, under the control of the control means 30 are independent, within the limits of compliance with the two constraints explained above.

In the case where several soles are implemented for the treatment of several plates, simultaneously or asynchronously, each sole is associated with at least two pairs of electrodes, each pair of electrodes being connected to a controlled power supply, all the power supplies that can be controlled by the same control means 30.

Claims

CLAIMS:

1 / Electrostatic holding device for a wafer of conductive or semiconductor material, comprising an electrically insulating soleplate (1 1) on which said wafer (12) is arranged, characterized in that the soleplate comprises an electrically insulating surface (23). ) under which at least two pairs of electrodes (14, 15, 17, 18, 34, 35, 37, 38) are arranged, at least two independent power supplies (16, 19) each feeding at least one pair of electrodes by a difference of potentials whose polarities are, at different times, inverted in order to release the accumulated electrostatic charges.

2 / Apparatus according to claim 1, characterized in that it comprises a control means (30) of programmable power supplies adapted to control the voltages provided by the independent power supplies (16, 19).

3 / Apparatus according to claim 2 characterized in that the power supply control means (30) is adapted to invert the polarities of at least one pair of electrodes at each wafer change.

4 / Apparatus according to any one of claims 1 or 3, characterized in that the electrodes (14, 15, 17, 18, 34, 35, 37, 38) have a symmetry which has a character of revolution, some by reports to others, that is to say that there is an angle of rotation, different from 0 ° and 360 °, which is superimposed the set of electrodes to itself.

5 / Apparatus according to any one of claims 1 to 4, characterized in that the electrodes (34, 35, 37, 38) have a spiral shape.

6 / Apparatus according to any one of claims 1 to 5, characterized in that the electrodes (14, 15, 17, 18) are annular.

7 / Apparatus according to any one of claims 1 to 6, characterized in that the arrangement of the electrodes (14, 15, 17, 18) is symmetrical or concentric with respect to the center of the sole (1 1).

8 / Apparatus according to any one of claims 1 to 7, characterized in that the planar surfaces of the two pairwise electrodes (14, 15, 17, 18, 34, 35, 37, 38) have the same area.

9 / Apparatus according to any one of claims 1 to 8, characterized in that the plane surfaces of the two electrodes (34, 35, 37, 38) covered by any wafer dimensions have the same effective area.

10 / Apparatus according to any one of claims 1 to 9, characterized in that the absolute value of the average, over the period of maintenance of a wafer, the potential differences to which the electrodes are subjected is at least ten times more the maximum absolute value of these potential differences, so that the duration of maintenance of the wafer on the soleplate can be greater than twenty-four hours without the duration of detachment of the wafer becoming greater than two seconds.

1 1 / Apparatus according to any one of claims 1 to 10, characterized in that the electrodes (14, 15, 17, 18, 34, 35, 36, 37) are made by serigraphy of thick layers on a base plate (22).

12 / Apparatus according to any one of claims 1 to 1 1, characterized in that the electrically insulating surface (23) is made by serigraphy of thick layers on a base plate (22).

13 / Apparatus according to claim 12, characterized in that the electrically insulating surface (23) made by screen printing thick layers on a base plate (22) is coated with a high performance dielectric layer deposited by CVD under low pressure.