FR2501907A1 - Molecular adhesion plane substrate holder - takes any sized substrate held on plate by copolymer elastomer and resin layer pressed in place and removed by fluid pressure - Google Patents

Abstract

The substrate holder (1) is for holding any sized plane substrate on a carrier for e.g. particle beam lithography. The substrate carrier is first prepd. with a surface layer of an elastomer mixed with a tacky substance and conducting material. Such a mixt. may be a polyisoprene polystyrene copolymer and a natural terpene resin. The carrier is prepd. by drilling holes from the plane face and placing it in a holder contg. the mixt. at a temp. above its m.pt. in the horizontal plane until the mixt. flows into the carrier. The mixt. is cooled to normal temp. and a number of the holes are cleaned off the mixt. The holes are blocked off to prevent fluid entry and the substrate placed on the holder. The substrate is pressed onto the holder by reducing the pressure under the holder, the substrate being held in place by molecular attraction. The substrate is removed from the carrier by pumping fluid under pressure through the holes in the holder. The holes are set in a matrix in the centre of the holder and surrounded by an annular groove of dovetail profile.

Description

METHOD OF POSITIONING, HOLDING A PLANAR SUBSTRATE
ON A SUBSTRATE HOLDER PLATE AND WITHDRAWAL OF THIS SUBSTRATE
AS WELL AS THE APPARATUS FOR IMPLEMENTING THE PROCESS.

 ! The present invention relates to a method of positioning and holding a substrate on a substrate-holder plate as well as its removal and the apparatus for implementing this method.

 The invention finds more particularly its application in machines intended for vacuum lithography using a particle beam. The substrates used can be thin silicon wafers or glass plates, for example.

 Under these conditions, it is no longer possible to use permanent vacuum substrate carriers in common use in lithography machines working at atmospheric pressure. To meet the above needs, various methods and devices have been proposed.

 According to a first approach, the substrate holder comprises mechanical members trapping the substrate in a ring or collar which is fixed on the plate of the substrate holder by a system of baillonnettes. This system requires as many rings as the dimensions of substrates capable of being used and can cause deformations in the plane of thin substrates.

 According to a second approach, the substrate is held against the plate of the substrate holder by the creation of an electrostatic force. This electrostatic force is generated by the application of an electric field. It is therefore necessary to have additional electrical equipment. In addition, there is a risk of introducing parasitic fields detrimental to the accuracy of the plots, by the influence of these fields on the trajectory of the charged particle beams.

 The present invention aims to satisfy the needs mentioned above while avoiding the drawbacks of the known art. It allows the substrate to be maintained in any position in space.

 The subject of the invention is therefore a method of positioning and holding a flat substrate on a substrate-carrying plate comprising at least one flat surface intended to receive the substrate, and of removing this substrate mainly characterized in that it includes an initial phase of preparation of the portasubstrate plate comprising the establishment of a surface layer on the flat face of an elartc.m # re 4 material! aged with a tackifying material, a phase of positioning the substrate on the prepared substrate substrate plate and its many. by molecular attraction of the surface layer of the elastomeric material and a phase of withdrawal of the substrate during which the decoilemerit of the substrate is obtained by introducing a fluid under a determined pressure between the substrate and the flat face of the substrate-holder plate.

 Another subject of the invention is an apparatus for implementing this method.

The invention will be better understood and other characteristics and advantages will appear on reading the description below and the appended figures among which
- Figures 1 to 6 illustrate the stages of the initial phase of the method according to the invention during which the substrate holder is treated according to two alternative embodiments;
- Figures 7 and Il illustrate the stages of the phase of the method according to the invention during which the substrate is positioned 'on the substrate holder;
- Figure 9 illustrates a step in the process of the process of the invention during which the substrate is removed;
- Figure 10 illustrates a concrete embodiment of the substrate holder used in the method of the invention;
- Figure II illustrates the application of the invention to vacuum lithography.

 To fix the ideas and by way of nonlimiting example, we will place ourselves in the following in the context of vacuum lithography using a beam of charged particles. In this context of application, the problem to be solved by the method of the invention is the positioning, the maintenance of a flat substrate, for example of a semiconductor wafer, on a substrate holder itself comprising at least one surface. plane, as well as the removal of this substrate.

 In what follows, the elements common to two or more figures will bear the same references and will only be described once.

 According to one of the main characteristics of the invention, the maintenance of the substrate on the substrate-holder plate is obtained by using the phenomenon of molecular attraction. To do this, a thin film of an organic material from the class of elastomers is created on the face intended to receive the substrate. This product can be found in the form of adhesive sheets or be based on a mixture of a resin added with a tackifier, that is to say a product making the mixture adhesive.

 The adhesion of the elastomeric mixture to the metal support must be greater than the adhesion of this mixture relative to the substrate in order to be able to carry out the operation of removing this substrate.

 You would think you could directly use a double-sided adhesive sheet to maintain the substrate on the substrate holder.

However, the layer of adhesive material must have sufficient flatness to avoid any mechanical stress on the substrate. The adhesive layer must then be sufficiently thin and of constant thickness, all conditions difficult to achieve. In addition, when the substrate is removed, it is subjected to non-homogeneous stresses which risk causing deformations or breaks.

 According to the method of the invention, it is preferred to form the layer of elastomeric material by casting the latter by bringing it to the softening temperature so as to coat the upper face of the substrate holder intended to receive the substrate.

 According to a preferred variant which will be described in more detail in relation to FIG. 10, the substrate holder is pierced with a matrix of holes intended to allow the molten elastomer to penetrate. These holes are located in a central area, inscribed in a circle for example. The substrate holder is extended at the periphery by a collar thinner than the central zone and comprising on the upper face an annular groove profiled in dovetail intended to ensure the penetration of the elastomer and increasing the adhesion of the elastomeric film on the upper face of the substrate holder.

 The creation of a surface film on the upper face of the substrate holder as well as the preparation of the substrate holder constitute the stages of a preliminary phase of the process, stages illustrated by FIGS. 1 to 6.

 FIG. 1 schematically illustrates in section a substrate holder plate 1 comprising a central zone in which channels 11 have been formed and an annular peripheral zone extending the plate of the substrate holder 10 in which an annular groove profiled in dovetail 12 has been carried out. Preferably, this substrate holder is made of metal to ensure good evacuation of the charges created by the bombardment of the particles of the beam when the substrate holder is placed in the lithography machine. On the other hand, this substrate carrier must have good thermal conductivity so as to avoid excessive heating of the substrate, properties which most metals combine.

 The substrate holder is then placed in a container 3 illustrated in FIG. 2. This container has an internal cavity with a diameter substantially equal to the outside diameter of the plate of the substrate holder 1. Furthermore, the bottom of the container 36 has an annular flange 32 of complementary shape to the rim 10 of the substrate holder 1 and forming a receptacle for this substrate holder. This flange 32 is intended to distance the substrate holder from the bottom 36 of the container 3 so as to provide a free space 35 between this bottom and the substrate holder. In addition, the walls of the container 31 comprise an annular cavity 30 communicating with the space left free 35, by peripheral channels, two of which are shown in FIG. 2 33 and 34.

 In the annular cavity 30 is arranged the polymeric material. This material is chosen so that it has a sufficiently low softening point, of the order of 2500C, without chemical decomposition and that its bonding power is compatible with the holding force required for the substrates used and with the pressure to exercise to recover the substrate after use, without damage to it. In addition, to be usable under vacuum, it must have a low vapor pressure. Finally, it must not leave any trace on the substrates after use.

 An example of a usable material combining these properties is the polystyrene polystyrene copolymer. In this material must be introduced a tackifier. An example is the natural terpene resin. In addition, to promote the evacuation of electrical charges, parts # icules of conductive material can be introduced into the elastomer so as to make it conductive of electricity.

 Referring again to FIG. 2, a strip of this material is placed in the annular space 30 of the container 3 which is in communication with the lower part of the metal support by channels: 32, 34. The quantity of elastomeric material is calculated so that in the liquid state the level of the elastomer is suitable for covering the substrate holder 1. The container 3 is placed in an oven, in the open air or under an inert atmosphere , the temperature of which is adjusted to a value equal to or greater than the softening temperature of the elastomer 2, for example 2500C. The softening process of the elastomer is followed and the container 3 is removed from the oven when the face of the metal support is covered with elastomer, the elastomer rising to the surface through the channels 11 formed in the substrate holder.

 While allowing better adhesion of the elastomeric film to the upper face of the substrate holder, the annular groove with dovetail profile 12 absorbs an excess of elastomer and allows a better equalization of the thickness of the film. elastomer 2 on the upper face of the substrate holder.

 The substrate holder 1 is then extracted from the container 3.

 To ensure good adhesion of the substrate to the surface of the elastomeric film 2, it is necessary to apply sufficient pressure, distributed evenly over the entire surface. Since this pressure cannot be exerted mechanically on the surface of the substrate without deterioration, the pressure of a fluid is used. It is possible, in a preferred variant of the process of the invention, to subject the upper surface of the substrate to atmospheric pressure. To do this, one can unclog a part of the channels 11 drilled in the substrate holder 1 and press the substrates on the substrate holder 1 by creating a depression on its rear face. The distribution of the outlet channels 110 must be as homogeneous as possible. The number of open channels can, for example, be equal to half the total number of channels.

 FIG. 4 illustrates the substrate holder 1 after trimming the upper layer of elastomer 2 at a short distance from the annular groove in the shape of a dovetail 12. A portion of the channels 11 drilled in the metal area is released to create curnrnunicatish channels 110 between the lower face and the upper face of the substrate holder 1. Holes 111 with a diameter 111 are cut in the upper and lower elastomer layers. substrate 1 using a punch guided by a gauge. The parts of the elastomer layers thus precut are then mechanically removed from their housing.

 The preparation phase of the substrate holder 1 comprises an ultimate step consisting in providing the latter with means for limiting the penetration of a fluid in the channels thus released #. Figures 5 and 6 illustrate two variants of limiting means.

 In FIG. 5, a plate 112 carrying calibrated orifices 113 is bonded to the underside of the substrate holder using the lower layer of elastomer. The diameter of the calibrated orifices 113 is of the order of a few tenths of a millimeter in a practical embodiment.

 According to another variant illustrated by FIG. 6, threaded rods 114 are introduced into the free channels of the elastomer 110. The more or less deep introduction of these threaded rods 114 by screwing into the cleared channels 110 defines a mechanical impedance at the penetration of fluids, directly proportional to the penetration depth of the threaded rods, that is to say adjustable.

 Any other arrangement authorizing a calibrated nozzle can be implemented without departing from the scope of the invention.

 After the stages of the preparation phase of the substrate holder 1, it will now be described with reference to FIGS. 7 and 8 the stages of the positioning and holding phase of a substrate on the substrate holder thus prepared.

 For the implementation of the method, in addition to the container 3 described in connection with FIG. 2, the apparatus comprises a second device 5, consisting of a second receptacle forming a receptacle for the substrate holder 1. This container is a box comprising a upper circular opening above which the substrate holder 1 is placed in a horizontal plane. The upper edge of the wall of the housing in contact with the substrate holder is provided with an O-ring 52 so as to ensure the seal between the enclosure defined by the interior of the housing and the substrate holder, and the external environment. at atmospheric pressure. The substrate holder 1 is secured to the housing using any suitable and for example non-limiting fixing means using a ring 51 which is screwed onto the housing 50 by the thread 510. The housing 50 is provided with means of admission of a fluid inside the internal enclosure 54 and means for expelling this fluid 53.

 FIG. 7 illustrates the first step of positioning the substrate 4 on the substrate holder 1. The intake means 54 are open, using a member represented schematically by a screw 540 in FIG. 7. By the means inlet 54 a fluid, for example air, is admitted under slight overpressure relative to atmospheric pressure in the enclosure underlying the substrate holder 1. A slight overpressure is created in the space between the face upper of the substrate holder 1, that is to say the polymer layer 2 and the substrate 4. In this way, the substrate 4 can be positioned precisely using mechanical manipulation means of the art known, not shown in Figure 7, the nozzle 114 of the variant of Figure 6 has been shown. Because it creates a significant pressure drop during the passage of the fluid, it avoids the pressure drop in the enclosure underlying the substrate holder 1. This nozzle also allows a homogeneous and calibrated admission break of the fluid in the openings and allows the creation of an air cushion under the substrate 4 in good conditions. However, this step is not compulsory in the process of the invention although it allows easier positioning of the substrate 4 on the polymer layer 2. According to a simplified process, it can be carried out directly in the step which will now be described.

 FIG. 8 illustrates the final step of positioning the substrate 4 on the substrate holder 1. To do this, the means for extracting and expelling fluid 53 are connected to a pump; for example of the vane pump type, and a vacuum is created in the enclosure underlying the substrate holder 1.

The intake means are closed using the member 540 and the extraction means open using a member 530 shown schematically in Figure 8 by a screw. The means 530 and 540 are actually valves or other similar devices, well known to those skilled in the art. A partial vacuum of the order of 103 Pascal is created in the enclosure. The substrate 4, the upper face of which is in contact with the atmosphere, is pressed onto the substrate carrier and thus maintained by the molecular attraction created by the elastomer layer 2. Then the admission means 54 are again open and the air admitted into the underlying enclosure. The substrate 4 on its substrate holder 1 is placed in its place of use, for example in a vacuum lithography machine, this in any position in space. The pressure of "bonding" of the substrate on its support is of the order of 104 Pascal (100 glcm2). It remains so until a back pressure has not been applied. The differences in flatness observed on several # ilicium substrates do not exceed a deflection of 10 11 m.

 After use, the substrate holder 1 is replaced on the housing 50 under the same conditions as what has been described in relation to FIGS. 7 and 8, this for the removal of the substrate 4 from the e-substrate port 1.

 Figure 9 illustrates this operation. Fluidic pressure is gradually applied to the underside of the substrate 4 by introducing a gaseous fluid into the enclosure underlying the substrate holder 1 in a manner analogous to the operation described in relation to FIG. 7. The means of intake are open and the fluid introduced into the enclosure. The nozzle makes it possible to limit the passage of the fluid in the channels 110 as it was previously recalled. The pressure of the admitted gas is gradually increased to approximately 5.104 Pascal (500 g / cm2). The substrate peels off and floats on the gas cushion thus created, it is then easy to recover.

 It goes without saying that the operations of the preliminary phase illustrated in FIGS. 1 to 6 only take place once and that the substrate holder 1 is recovered after the operation of FIG. 9 for subsequent operations.

 Figure 10 illustrates in more detail a concrete embodiment of a substrate holder 1 for implementing the method of the invention. This substrate holder is in the form of a metal plate extended by an annular rim 10. In this rim was hollowed out a circular channel in the form of a groove with a dovetail profile 12.

In the central zone inscribed for example in a circle 115, it was created according to a homogeneous (for example matrix) distribution of the channels 11.

Still by way of illustration, the outside diameter of the substrate holder
can be 10 inches (about 25 cm), the central area 115 can have a
diameter between 3 and 6 inches (respectively about 8 and 15 cm) and
there are 50 channels, to accommodate substrates of common dimensions. The thickness of the tray in its central zone is approximately 10 mm. Each channel has a 2.5 mm diameter hole with a 5mm pitch. After the final stage of the preliminary phase of preparation of the substrate carrier, half of the channels 11 can be released from the elastomer, ie 25.

 The substrate holder 1 on which the substrate 4 has been arranged according to the method of the invention, can be placed in any device in which a partial or high vacuum has been created, this in any position in space. A particularly interesting example is a lithography machine 6 illustrated diagrammatically in FIG. 11. Only the main organs have been described in this figure. This lithography machine 6 comprises a bearing marble 60 on which rests a translation table 63 with two degrees of freedom, that is to say which can move along two orthogonal reference axes X and Y. This table is set in motion using motors of which only one, 64, has been illustrated in FIG. 11. The assembly is surmounted by an enclosure 61 in which the vacuum is produced using a pump, for example a cryogenic type pump 65. A loading system 62 comprising an airlock is provided on the upper part of the enclosure 61. With the help of this system, the substrate holder 1 is introduced into the enclosure and rendered integral with the XY table, 63. This substrate holder 1 is placed above a column 66, this term having to be understood in the most general sense of an optical system for particulate beams. Finally, systems for measuring the position of the table are also provided and generally include a laser source 67 associated with an interferometer 68. With the exception of the substrate holder 1 produced according to the method of the invention, and the provisions adopted for the maintenance of the substrate 4 on this substrate holder, the elements of FIG. 11 are common with those of the lithography machines of the known art and do not require further description.

Claims (12)

 1. A method of positioning, of maintaining a flat substrate (4) on a substrate-carrying plate (1) comprising at least one flat surface intended to receive the substrate and of removing this substrate, characterized in that it comprises an initial phase of preparation of the substrate-holding plate (1) comprising the establishment of a surface layer on the flat face of an elastomeric material (2) mixed with a tackifying material, a phase of positioning of the substrate (4) on the substrate-holder plate (1) thus prepared and its maintenance by molecular attraction of the surface layer of the elastomeric material (2) and a phase of withdrawal of the substrate (4) during which the separation of the substrate (4) is obtained by introducing a fluid under a determined pressure between the substrate (4) and the planar face of the substrate holder plate (1).  2. Method according to claim 1, characterized in that the initial phase of preparation of the substrate-holder plate (1) # comprises the following steps:  - drilling of channels (11) flush with the surface of the flat face by regularly distributed outlet orifices ;;  - introduction of the substrate holder plate (1) into a container (3) comprising the elastomeric material (2) brought to a temperature equal to or greater than the softening temperature of this material, the inlet orifices of the channels being brought into communication with the elastomeric material and the substrate-holder plate (1) being arranged in space so that the flat face is included in a horizontal plane, to allow the elastomeric material to penetrate into the channels and form a uniform layer by spreading the elastomeric material (2) in the softening state, opening at the outlet of the channels (11) on the flat face;  - return to room temperature of the elastomeric material;  - drilling holes (111) in the surface layer of elastomeric material (2) centered on the outlet orifices of a determined number of channels (110) and removing the elastomeric material filling these channels;  - And arrangement on the inlet orifices of these means channels (113 or 114) limiting the penetration of a fluid in these channels.  3. Method according to claims 1 and 2, characterized in that the positioning phase of the substrate (4) comprises a first step during which the substrate is deposited on the substrate holder plate (1), the plate being disposed in the space so that the flat face is included in a horizontal plane and placed in communication with a medium at atmospheric pressure, and a second step during which the inlet orifices of the channels (110) are placed in communication with a medium fluid at a pressure below atmospheric pressure, so as to create, through limiting means (113 or 114), a depression in the space underlying the substrate (4) and to exert a force of attraction on the substrate towards the surface layer of elastomeric material (2) to obtain the maintenance of the substrate (4) by molecular attraction.  4. Method according to claim 3, characterized in that during the first step, a gas cushion is created in the space between the substrate (4) and the planar surface of the substrate holder plate (1) in blowing the gas through limiting means (113 or 114).  5. Method according to claims 1 and 2, characterized in that the phase of withdrawal of the substrate (4) comprises a step during which the substrate holder plate (1) being arranged in space so that the flat face is included in a horizontal plane and placed in communication with a medium at atmospheric pressure, the inlet ports of the channels (110) are placed in communication with a fluid medium at a pressure higher than atmospheric pressure so as to apply a set peel forces with homogeneous distribution on the underside of the substrate (4).  6. Apparatus for implementing the method according to any one of claims 1 to 5, characterized in that it comprises:  - a substrate-holder plate (1), the body of which consists of a disc comprising communication channels (11) between its two main faces, the outlet orifices of which on the face intended to receive the substrate are flush with the surface d 'A central area, the distribution of these outlet openings being homogeneous;  - A device allowing the establishment of a surface layer of elastomer (2) on at least the face of the plate (1) intended to receive the substrate (4) constituted by a first container (3) at the bottom of which is deposited the substrate holder plate (1) on a receptacle member (32) intended to create an enclosure (35) underlying the plate, enclosure in communication (33,34) with a reservoir (30) containing the polymeric material ( 2);  - And a device allowing the extraction of the substrate holder (1) constituted by a second container (5) comprising an opening on which the substrate holder plate (1) can be arranged using fixing means and sealing (51), the enclosure being connected to reversible pumping means (53,54,530,540) intended to modify the pressure prevailing inside the container  7. Apparatus according to claim 6, characterized in that the channel outlet orifices (11) formed in the substrate-holder plate (1) have a matrix distribution and are inscribed in a circle (115) of diameter substantially equal to the most large bone dimensions of substrates to be positioned on the substrate holder plate and in that it is made on the face intended to receive the substrate (4) a peripheral annular groove (12) with dovetail profile.  8. Apparatus according to claim 6, characterized in that the body of the substrate holder plate (1) is made of metal.  9. Apparatus according to claim 6, characterized in that the first container (3) is a container with a cylindrical wall and the receptable member (32) consists of an annular boss disposed on the bottom of the container, and in that the cylindrical wall of the container comprises an annular cavity (30) forming a reservoir for the elastomeric material and communicating with the bottom of the container through peripheral channels (33,34).  10. Apparatus according to claim 6, characterized in that the second container (5) is a container with a cylindrical wall, the end of which is provided with an O-ring forming sealing means on which the substrate holder plate rests ( 1) and in that the external face of the cylindrical wall is provided with a thread (510) so that the substrate (4) can be held by a ring (51) forming the fixing means and screwing onto said thread, the ring (51) being provided with an orifice for access to the external face of the plate of the substrate holder (1) of dimensions greater than the largest dimensions of substrates (4) to be positioned.  11. Substrate-holder plate (1) comprising a layer of elastomeric material (2) on at least one of its faces put in place using the apparatus according to any one of claims 6 to 10 and provided with channels (110) allowing the free circulation of a fluid through the surface layer of the elastomeric material (2) and of the body of the plate, plate characterized in that it is provided on the face opposite to that carrying the layer of elastomer of a plate comprising openings (113) positioned opposite the inlet orifices of the channels (110) and having a surface area smaller than that of these orifices, to form means for limiting the penetration of a fluid in said channels .  12. Substrate-holder plate (1) comprising a layer of elastomeric material (2) on at least one of its faces put in place using the apparatus according to any one of claims 6 to 10 and provided with channels (110) allowing the free circulation of a fluid through the surface layer of the elastomeric material (2) and the body of the plate, characterized in that threaded rods (114) are introduced into said channels (110) on a predetermined depth from the face opposite to the face carrying the elastomer layer (2), so as to form means for limiting the penetration of a fluid in said channels.