EP0517594A1 - Polishing machine with a tensioned finishing belt and an improved work supporting head - Google Patents

Abstract

The machine according to the invention comprises a microabrasive belt (33) tensioned above a planar reference platform (30) between a feeding spool and a receiving spool (50), and a head (32) for supporting a specimen (44) comprising a rigid part (140) and a disc (142) made of a flexible material and capable of holding the face of the specimen (44) to be polished opposite the abrasive belt. Application to the polishing of silicon plates (wafers) containing integrated components, in particular magnetic read and write heads. <IMAGE>

Description

The subject of the present invention is a polishing machine with a tight microabrasive strip and with an improved insert support head.

The invention finds a particular application in the polishing of microelectronic components integrated in semiconductor wafers (in silicon for example). They may, in particular, be magnetic writing and reading heads.

Methods for producing such heads are described in numerous documents and in particular in US-A-4,837,924 and US-A-4,333,229. The first document relates to heads with a so-called "horizontal" structure -car formed from a stack of layers deposited on the upper face of a semiconductor wafer- and the second to heads with a structure called "vertical" -car formed from layers deposited on the edge of such a wafer.

The micro-machining carried out on such wafers consists, in the first case, in leveling (or "planarizing") and in polishing various intermediate sub-assemblies obtained during the production process, in defining an air gap and in bringing the assembly of the head in the general plane of the substrate, also called the flight plan.

In the second case, the purpose of micro-machining is to define an air gap and to adjust the shape of the flight pads.

Although possibly applicable for producing heads of the second category (vertical heads), the machine which is the subject of the present invention is above all intended for polishing assemblies or sub-assemblies corresponding to the first category (horizontal heads) because it is in this case that the technological problems are the most difficult.

Figure 1 shows, by way of example of a polishing piece, a magnetic writing and reading head in a horizontal structure. The assembly represented corresponds to the last stage of production before final polishing. This assembly includes a silicon substrate 10 in which a box has been etched, a magnetic circuit 12 made of iron-nickel alloy, a double coil 14 made of copper, a layer of silica 16 3 to 6 μm thick, a non-magnetic spacer 18 in silica approximately 1µm thick and two upper pole pieces 20 in iron-nickel. The final polishing plan is marked in broken lines and referenced 22.

The material removal relates to the pole pieces 20 and to the protrusions 23 in silica. In order not to alter the magnetic circuit, this removal must not reduce the thickness of the uniform layer of silica by more than 0.3 µm. The final polishing plan defines the flight plan of the head.

Two such heads are generally placed side by side on two parallel strips called "skis", defining two flight plans, in a general catamaran structure.

• la rectification à l'outil diamanté : il s'agit d'un usinage dans lequel on forme un "copeau" semi-continu ou continu par deux mouvements combinés relatifs entre l'outil et la pièce à usiner (un mouvement d'avance et un mouvement de coupe) ;
• le rodage et le polissage : il s'agit d'une abrasion plus ou moins fine (ou écrouissage) et contrôlée de la surface par frottement sur des disques très variés non abrasifs par nature, sur lesquels on apporte un abrasif en pâte ou en solution aqueuse ; une variante consiste à placer, sur un plateau de polissage rotatif, un disque de film abrasif et à arroser celui-ci lors du polissage avec un liquide pour refroidir la pièce et éviter l'encrassement.

Polishing, which consists of removing material in very small quantities, is a well-known operation. It is found in metallography, optics and microelectronics. Either two following techniques are used:

• grinding with a diamond tool: this is a machining in which a semi-continuous or continuous "chip" is formed by two relative combined movements between the tool and the workpiece (a movement of advance and a cutting movement);
• lapping and polishing: this is a more or less fine abrasion (or hardening) and controlled of the surface by friction on very varied discs non abrasive by nature, on which an abrasive is applied in paste or in solution aqueous; a variant consists in placing, on a rotary polishing plate, a disc of abrasive film and in spraying the latter during polishing with a liquid to cool the part and avoid fouling.

• tout d'abord, la plaquette est déformée et déformable,
• par ailleurs, le rodage doit affecter simultanément plusieurs matériaux de duretés très différentes : silice, alumine, alliage alumine/carbure de titane, alliage fer/nickel,
• les pièces à roder sont de surfaces très petites par rapport à la plaquette de silicium,
• enfin, il s'agit d'usiner, dans leur épaisseur, des couches déposées sur une plaquette, et, généralement, il faut polir simultanément 600 excroissances correspondant à 600 têtes magnétiques, en dépassement de quelques microns et cela avec une précision de l'ordre du nanomètre, sans diminuer l'épaisseur de la couche mince qui recouvre la plaquette de plus de 200 à 300nm.

The polishing of semiconductor wafers comprising a very large number of integrated microcomponents poses particular and difficult problems:

• first of all, the insert is deformed and deformable,
• moreover, running-in must simultaneously affect several materials with very different hardnesses: silica, alumina, alumina / titanium carbide alloy, iron / nickel alloy,
• the parts to be lapped have very small surfaces compared to the silicon wafer,
• finally, it involves machining, in their thickness, layers deposited on a wafer, and, generally, it is necessary to simultaneously polish 600 protuberances corresponding to 600 magnetic heads, exceeding a few microns and this with an accuracy of the order of a nanometer, without reducing the thickness of the thin layer which covers the wafer by more than 200 at 300nm.

Known polishing machines do not meet all of these requirements. In particular, the use of an abrasive grain liquid or of an abrasive film bonded to a support is not suitable as may be convinced in connection with FIGS. 2 to 6.

FIG. 2, first of all, illustrates the known principle of lapping with liquid loaded with abrasive grains. The plate 10 and its protuberances 25 are placed opposite a polishing plate 23 and the liquid 24, loaded with abrasive grains, forms a film between the plate and the reference plane. The translational movement of the insert causes abrasion of the growths.

However, complex hydrodynamic phenomena, linked in particular to the formation of vortex movements around the protuberances and to cavitation phenomena, lead to defective polishing, the result of which is illustrated in FIG. 3. In this figure, the part a shows an outgrowth 25a. before polishing, the shape of which corresponds very substantially to that encountered in the case of integrated magnetic heads, as will be seen more clearly below. This profile takes the form 25b after the start of polishing (part b ) and, finally, the form 25c (part c ) at the end of polishing. We see that did not obtain the desired result because the flight plan was reached and peaks remain. In particular, soft materials are more hollow than hard materials.

Another known technique consists in using a microabrasive plastic film bonded to a reference plate. We thus see, in Figure 4, a reference plate 23 on which a microabrasive film 27 has been glued by means of glue dots 28 (aerosols). The adhesive layer has a thickness of approximately 100 μm. The thickness of the sheet is approximately 50 to 75 μm. The assembly therefore has a thickness of approximately 150 to 175 μm.

FIG. 5 shows this abrasive means with a plate 10 and its protrusions 25 to be polished. It is observed that the presence of the protuberances and the relatively large thickness of the polishing layer cause it to crease, by local compression of the sheet and crushing of the glue points. In this case again, the polish finally obtained is not satisfactory. Figures 6 schematically show the profile of a polished pad 29, before polishing (Figure 6a) and after polishing (Figure 6b).

Polishing machines are also known which use a microabrasive film, not glued to a reference surface but stretched over a plate. Such machines are described in documents DE-U-8 717 353 and DOS 26 37 343. These machines comprise a supply coil and a take-up coil between which passes, in a step-by-step movement, the microabrasive strip. This strip passes over a piece of soft material. The polishing piece, which in this case is a base plate, is held by a head animated by a rotational movement, A pneumatic means, placed at the lower part of the machine, allows to press the microabrasive band under the base of the plate, so that it comes to deform the abrasive film and sink into the soft piece. The tension of the strip is obtained by means of pliers or jaws which simultaneously make it possible to advance the strip step by step.

Such a machine is not suitable for polishing semiconductor wafers for many reasons. First of all, sinking into the soft material is inadmissible for the reasons already indicated, namely that the reliefs would be rounded and deformed. It is therefore in fact necessary to work on a perfectly flat reference plate and to work with a very thin microabrasive film so that one can benefit from the flatness of the reference plate.

In addition, if the polishing of the plate feet allows coarse abrasive grains, the polishing of the semiconductor wafers requires much finer grains. But with such grains, there is a phenomenon of sticking of the wafer on the abrasive film, to the point that the separation of the wafer at the end of polishing requires the formation of an air wedge to allow the wafer to take off. . This phenomenon tends to cause wrinkling of the microabrasive band. To avoid this risk, the microabrasive band should be very tightly stretched over its entire width. However, this is impossible with the machine of document DE-U-8 717 353 which only provides for this purpose claws (or jaws) pinching the strip on its sides. With such means, a certain tension would be obtained on the edges of the strip but not in the center and moreover the risks of tearing of the strip would be real.

Furthermore, with the prior machine, it is impossible to continuously scroll the abrasive strip kept under tension. Indeed, the advance of the strip can only be carried out step by step, since the latter is stretched by the claws which pinch it.

The need to use very fine polishing grains and a very tight film, which causes sticking of the wafer, leads to other difficulties which are not resolved by the machines described in DE-U-8 717 353 and D- OS-26 37 343. In fact, in such machines, the polishing peg, in this case a plate, is simply held in a support by a depression caused above the plate. Such a depression, which should be very strong to maintain the semiconductor wafer, would break it.

Finally, a head with rotary movement such as that of the cited documents, would not be suitable for polishing semiconductor wafers since then the center of the wafer would not be polished. A simple rotary movement is only suitable for ring pieces, such as the base of plates.

• la première, liée à l'utilisation d'une bande microabrasive tendue, prévoit tout d'abord que la bande est tendue au-dessus d'un plateau de référence offrant une surface plane rigide ; ensuite elle prévoit des moyens particuliers de tension de la bande entre la bobine débitrice et la bobine réceptrice et cela sans utilisation de griffes, mors ou pinces, ce qui permet de tendre le film sur toute sa largeur et de manière uniforme ; ceci permet en outre de faire défiler continuement la bande pour la renouveler ;
• la seconde disposition est liée aux moyens pour supporter la plaquette ; ces moyens sont tels qu'un mouvement de translation circulaire est obtenu ; ce mouvement parti culier impose à la bande abrasive des efforts dont la direction change en permanence (360° par tour) et qui s'exercent tantôt dans le sens longitudinal (soit dans le sens de déplacement, soit dans le sens contraire) tantôt dans le sens transversal ; ces efforts particuliers, qui auraient tendance à provoquer des plissements de la bande, requièrent que celle-ci soit parfaitement tendue, ce qui ramène au problème précédent ; la plaquette étant par nature déformable et déformée, il est prévu un disque de matière souple placé à l'arrière de la plaquette ; enfin, un système à rotule permet d'avoir un point de rotulage le plus près possible du plan de polissage, ce qui diminue l'apparition de couple parasite dans le mouvement de translation circulaire, qui serait particulièrement néfaste dans le cas de ces efforts intenses communiqués à la plaquette pour la déplacer.

The object of the present invention is precisely to remedy these drawbacks. To this end, the invention provides two types of arrangement:

• the first, linked to the use of a stretched microabrasive strip, firstly provides that the strip is stretched over a reference plate offering a rigid flat surface; then it provides special means of tensioning the strip between the supply reel and the take-up reel without using claws, jaws or pliers, which makes it possible to stretch the film over its entire width and in a uniform manner; this also makes it possible to continuously scroll the strip to renew it;
• the second arrangement is linked to the means for supporting the wafer; these means are such that a circular translational movement is obtained; this particular movement imposes on the abrasive belt forces whose direction is constantly changing (360 ° per revolution) and which are exerted sometimes in the longitudinal direction (either in the direction of movement, or in the opposite direction) sometimes in the transverse direction; these particular efforts, which would tend to cause folding of the strip, require that the latter be perfectly stretched, which brings back to the previous problem; the plate being by nature deformable and deformed, there is provided a flexible material disc placed at the rear of the plate; finally, a ball joint system makes it possible to have a swiveling point as close as possible to the polishing plane, which reduces the appearance of parasitic torque in the circular translation movement, which would be particularly harmful in the case of these intense forces. communicated to the plate to move it.

All these means contribute to the resolution of the problems linked to the polishing of semiconductor wafers. These problems can be summarized as follows: in an abrasive film polishing machine stretched, to keep the wide abrasive film (15 to 45 cm approximately) very thin (25 to 50 microns) stretched without wrinkling (approximately 10 to 50 kg, or 4 to 5 kg per mm² of strip section). For example, for a width of 300 mm and a thickness of 25 µm, the tension will be 35 kg) on a reference plane as long as the width of the film. The abrasive belt thus stretched must be able to be set in motion continuously and very slowly (0.2 to 20 cm per minute) in an adjustable and controlled manner, and smoothly. These two operating conditions (voltage and running speed) must in no case be disturbed by the forces imposed on the strip by the sample to be polished. This, animated by a circular translational movement, must be maintained in an appropriate support head, capable of taking into account the deformation of the sample and avoiding parasitic couples.

The microabrasive sheets (or "strips" or "films") which can be used in the invention can be commercial sheets, such as those sold by the company 3M. The film called "Imperial Lapping Film (ILF)" of thickness 12 or 25 or 35 or 50 or 75 μm may be suitable. This film is available on roll.

• une bande microabrasive tendue au-dessus d'un plateau entre une bobine débitrice et une bobine réceptrice,
• une tête support apte à maintenir un échantillon à polir, face à polir en regard de la bande abrasive,

caractérisée par le fait que :

• le plateau est un plateau offrant une face supérieure rigide et plane,
• la bobine débitrice est équipée de moyens pour exercer sur elle un couple résistant et la bobine réceptrice est commandée par un moteur,
• la tête support d'échantillon comprend une pièce rigide et un disque de matière souple ayant une certaine épaisseur, ce disque étant fixé à ladite pièce rigide et recevant la plaquette à polir, la pièce rigide étant reliée par un roulement à rotule et un axe vertical à des moyens pour déplacer la tête support dans un mouvement de translation circulaire.

Specifically, the present invention therefore relates to a polishing machine which, like certain known machines, comprises:

• a microabrasive strip stretched over a plate between a supply reel and a take-up reel,
• a support head capable of holding a sample to be polished, facing to be polished opposite the abrasive belt,

characterized by the fact that:

• the plate is a plate offering a rigid and flat upper face,
• the supply reel is equipped with means for exerting a resistive torque on it and the take-up reel is controlled by a motor,
• the sample support head comprises a rigid part and a disk of flexible material having a certain thickness, this disk being fixed to said rigid part and receiving the polishing pad, the rigid part being connected by a spherical bearing and a vertical axis to means for moving the support head in a circular translational movement.

• la figure 1, déjà décrite, montre un exemple de pièce à polir correspondant à une tête magnétique d'écriture et de lecture,
• la figure 2, déjà décrite, illustre le principe connu du rodage avec film liquide chargé de grains abrasifs,
• la figure 3, déjà décrite, montre le profil d'un motif au cours d'un polissage effectué par le principe précédent,
• la figure 4, déjà décrite, illustre le principe connu du rodage par film plastique microabrasif collé sur un plateau de référence,
• la figure 5, déjà décrite, montre la déformation subie lors d'un polissage par une feuille microabrasive collée,
• la figure 6, déjà décrite, montre le profil d'un patin poli par la technique des figures 4 et 5,
• la figure 7 montre la disposition générale de la machine de l'invention avec une feuille microabrasive tendue et un support de plaquette à translation circulaire,
• les figures 8a et 8b (respectivement vue de dessus et vue de côté) montrent une machine de polissage conforme à l'invention,
• la figure 9 montre trois positions (a, b, c) de la tête support et illustre la répartition de la force exercée,
• la figure 10 est une courbe montrant les variations de l'enfoncement de la plaquette dans le disque souple en fonction de la force exercée sur la tête support,
• la figure 11 montre, en coupe, un exemple de réalisation de la tête support,
• la figure 12 montre un détail d'un anneau périphérique,
• la figure 13 illustre un mode de réalisation de l'anneau périphérique,
• la figure 14 montre un sous-ensemble intermédiaire dans la réalisation d'une tête magnétique d'écriture et de lecture,
• la figure 15 montre le profil de ce sous-ensemble avant et après polissage,
• la figure 16 montre la tête magnétique terminée,
• la figure 17 montre le profil de cette tête avant et après polissage.

In any case, the characteristics and advantages of the invention will appear better in the light of the description which follows. This description relates to exemplary embodiments given by way of explanation and in no way limitative and refers to the attached drawings, in which:

• FIG. 1, already described, shows an example of a polishing piece corresponding to a magnetic writing and reading head,
• FIG. 2, already described, illustrates the known principle of lapping with a liquid film loaded with abrasive grains,
• FIG. 3, already described, shows the profile of a pattern during polishing carried out by the previous principle,
• FIG. 4, already described, illustrates the known principle of lapping by microabrasive plastic film bonded to a reference plate,
• FIG. 5, already described, shows the deformation undergone during polishing by a glued microabrasive sheet,
• FIG. 6, already described, shows the profile of a skate polished by the technique of FIGS. 4 and 5,
• FIG. 7 shows the general arrangement of the machine of the invention with a stretched microabrasive sheet and a plate support with circular translation,
• FIGS. 8a and 8b (respectively top view and side view) show a polishing machine according to the invention,
• FIG. 9 shows three positions (a, b, c) of the support head and illustrates the distribution of the force exerted,
• FIG. 10 is a curve showing the variations in the insertion of the plate into the flexible disk as a function of the force exerted on the support head,
• FIG. 11 shows, in section, an exemplary embodiment of the support head,
• FIG. 12 shows a detail of a peripheral ring,
• FIG. 13 illustrates an embodiment of the peripheral ring,
• FIG. 14 shows an intermediate sub-assembly in the production of a magnetic writing and reading head,
• FIG. 15 shows the profile of this sub-assembly before and after polishing,
• FIG. 16 shows the completed magnetic head,
• Figure 17 shows the profile of this head before and after polishing.

• un plateau de polissage fixe 30,
• une tête support d'échantillon 32, comprenant une pièce rigide 140 et un disque de matière souple 142 ayant une certaine épaisseur (e). Le diamètre du disque souple est sensiblement le diamètre maximum englo bant la zone de la plaquette couverte par les excroissances à polir. Le disque 142 est fixé sur la pièce rigide 140 et reçoit la plaquette à polir 44. Celle-ci se trouve ainsi enfoncée partiellement dans l'épaisseur du disque 142 en cours de polissage par l'effet de la force exercée sur la tête support ; la matière souple du disque 142 peut être un élastomère,
• des moyens 34 pour exercer une force F sur la tête support 32 afin d'appliquer l'échantillon à polir 44 sur le plateau de polissage 30 et pour déplacer la tête par rapport au plateau, ce moyen pouvant comprendre un excentrique 37.

The machine shown in FIG. 7 schematically comprises:

• a fixed polishing plate 30,
• a sample support head 32, comprising a rigid part 140 and a disk of flexible material 142 having a certain thickness (e). The diameter of the flexible disc is substantially the maximum diameter encompassing the area of the insert covered by the protrusions to be polished. The disc 142 is fixed to the rigid part 140 and receives the polishing pad 44. This is thus partially pressed into the thickness of the disc 142 during polishing by the effect of the force exerted on the support head; the flexible material of the disc 142 may be an elastomer,
• means 34 for exerting a force F on the support head 32 in order to apply the polishing sample 44 to the polishing plate 30 and to move the head relative to the plate, this means possibly comprising an eccentric 37.

The polishing means comprises a sheet or microabrasive strip 33 stretched and pressed against a reference plate 30. The sheet 33 is stretched by means 35, 35 ′ arranged on either side of the plate 30.

FIGS. 8a and 8b illustrate a particular embodiment of the means 35, 35 ′ able to tension the abrasive sheet properly and to allow the slow scrolling of it above the board. In these figures, the machine is shown in top view on part a and in side view on part b .

For the sake of simplification, the machine shown does not include a support for polishing pads, an eccentric, a rotating spindle, etc. Figures 8a and 8b stick to the reference plate and the various means for stretching on this plate the microabrasive strip and scrolling it.

As shown, the machine comprises a first coil 40 and a second coil 50 arranged on either side of the reference plate 30. On these coils is wound a microabrasive strip 33, which is thus stretched between the two coils.

The first coil 40 is a supply coil equipped with means for exerting a resistant torque; the second coil 50 is a take-up coil controlled by a motor. The microabrasive strip 33 can thus pass from the first reel 40 to the second 50, by passing over the reference plate 30, which allows the renewal of the abrasive surface.

The two coils 40, 50 are arranged under the upper face of the reference plate 30, two drums 41, 51 being arranged between the coils and the reference plate 30. The microabrasive strip 33 passes over these drums 41, 51 at the outlet of the supply reel 40 and at the entry into the take-up reel 50. These drums are preferably arranged a little below the upper face of the plate 30 so that the microabrasive strip 33 makes a slight angle ϑ with the horizontal at its entry and exit of the tray, which improves its contact with it.

In the illustrated variant, the supply coil 40 is connected to a frame 60 by two ball bearings 41, 42 and two slides 43, 44 whose ends come to bear on two pressure sensors 45, 46 linked to the frame by two adjustable stops 47, 48. The adjustment of the stops makes it possible to balance the tension of the strip over its entire width.

The means for exerting a resistive torque on the supply reel 40 can be constituted, in a first variant, by an annular motor 62 mounted directly on one of the bearings 41 or 42, at the end of the slide 43. Means 64 for controlling the this engine are also provided. In a second variant, these means consist of a motor 66 separate from the supply coil 40 and by a transmission belt 68 between this motor 66 and the supply coil 40. The stretched strand 68a of the belt 68 is in a plane perpendicular to the slides 43, 44. Means 64 for controlling this motor are also provided.

Furthermore, the two pressure sensors 45, 46, disposed at the ends of the two slides 43, 44, are connected to the control means 64 of the motors 62 or 66 exerting a resistant torque on the supply reel 40.

For its part, the take-up reel 50 is controlled in rotation by a geared motor 70. This reel can be connected to the geared motor 70 by a means 72 for interrupting the transmission, such as a mechanical coupling or an electromagnetic clutch.

We will now describe in more detail the structure and functions of the sample support head which cooperates with the stretched microabrasive strip. to allow polishing under the conditions set out above.

As shown in FIG. 7, the sample-holder head comprises a flexible disc 142 whose role is illustrated in FIG. 9.

In this figure, we see, on part a , the support head released from the polishing pad 144, which has been shown with a very exaggerated deformation to clearly show the functions which will be fulfilled by the flexible disc 142. The reliefs to polish are referenced 143.

The force F applied vertically to the rigid part 140 has the effect of pressing the assembly onto the polishing plane 130, the raised patterns 143 coming to bear on this plane (part b ). However, due to the initial deformation of the plate, the bearing force of these reliefs on the polishing plane 130 is unevenly distributed: there are thus forces F1, at the periphery, relatively large and forces F2, at the center, relatively weak in the example illustrated.

The application of a greater force on the rigid part 140 has the effect of causing the wafer 144 to penetrate into the flexible disc 142 (part c ). The depression conforms to the initial deformation of the insert and makes it possible to compensate for the latter. The force F3 exerted by each relief on the polishing plane is then substantially the same over the entire surface of the polishing plane.

• amener la face principale de la plaquette à épouser la géométrie du plan de réfé rence, quelle que soit la déformation initiale et les défauts d'épaisseur de la plaquette,
• obtenir sur chaque excroissance une pression suffisante pour que l'enlèvement de matière soit effectif et optimum pour une vitesse de déplacement donnée.

Under these conditions, we understand that the force exerted on the wafer fulfills two functions:

• bring the main face of the wafer to match the geometry of the reference plane, whatever the initial deformation and the thickness defects of the brochure,
• obtain sufficient pressure on each protuberance for the removal of material to be effective and optimum for a given speed of movement.

Once the plate has been applied to the microabrasive film, the support is moved relative to the lapping plane, in a circular translation (rotation of the center of the plate around a point located in the lapping plane, the plate always keeping the same orientation). Thus, each protuberance has the same linear speed, whatever its position on the plate.

In the configuration described above, each projection receives a load proportional to its height. Then, after partial leveling, all the growths receive an identical load. We can then consider that the contact is correct at each overshoot. On the other hand, when the height of the overhangs decreases, the distance separating the main plane of the insert and the running-in plane decreases; as the contact between two planes is never perfect, the phenomena due to the viscosity of the air appear and tend to cause a partial detachment of the plate. So we must decrease the speed of movement and / or increase the pressure on the wafer support.

The removal of material according to the invention excludes the use of any coolant or particle drainage liquid. The work is therefore carried out "dry". If necessary, a vacuum can be created in the work area or the air can be replaced by a light gas such as helium.

Determining the characteristics of the flexible disc to be used according to the invention first passes through that of the minimum force Po to be exerted on the wafer to bring the geometry of the front face to match the reference plane.

In the case of a homogeneous deformation of the plate in the form of a spherical cap, it is a question of making the plate bend so that the stress resulting from the force cancels the arrow "f".

où :

• E est le module d'élasticité (ou module de YOUNG) du matériau constituant la plaquette,
• Po est une charge ponctuelle appliquée au centre de la plaquette (sommet du bombé), la plaquette étant en appui sur sa circonférence,
• e est l'épaisseur moyenne de la plaquette,
• r est le rayon de la plaquette.

The laws of resistance of materials give for Po: $$\text{Po = (3πE e³f) / 5 r²}$$

or :

• E is the elastic modulus (or YOUNG modulus) of the material constituting the wafer,
• Po is a point load applied to the center of the plate (top of the crown), the plate being supported on its circumference,
• e is the average thickness of the wafer,
• r is the radius of the insert.

This load Po applied to the wafer will be distributed in a completely heterogeneous manner. Indeed, this load will be concentrated in the middle, the edges of the plate barely coming into contact with the reference plane without transmission of forces.

In the case of complex deformations, a good approximation consists in taking into account the most difficult relief to bring in contact with the reference plane, by using the preceding formula. This determination amounts to comparing the ratios f / r² in an area of radius "r" affected by this arrow. Once the maximum ratio has been determined, the effort necessary to recover this deformation is reduced to the entire surface of the flexible disk.

The next step is to determine the difference in the distribution of admissible effort. It is a compromise between maximum homogeneity and the maximum value of admissible pressure by the abrasive, for given reliefs (risk of deterioration of the abrasive surface or reliefs). It is generally considered that 5 to 10% difference is acceptable. We will take as maximum effort a value P1 equal to 10 to 20 times the value Po calculated as indicated above.

Finally only, we can determine the characteristics of the disc. The curve in Figure 10 shows the depression (on the ordinate) as a function of the pressure (on the abscissa), the load being assumed to be distributed over a unit surface.

Line A does not take into account the finite thickness of the disc (in other words, it assumes an infinite thickness). Curve B takes this thickness into account. A finite thickness leads to a "hooking" of the material constituting the disc (in general an elastomer).

The load P1 gives the value of the pressure on the unit surface chosen to draw the curve. This value is plotted on the curve to obtain the corresponding arrow, ie "f1".

The penetration of the flexible material is variable depending on the thickness of the wafer. The load P1 leads to a local pressure proportional to the thickness of the plate at a given point.

The value of the maximum difference over the thickness of the plates, Δ e is transferred to the ordinate axis by centering it on f1. This gives the maximum variation ΔP1 due to the difference Δe around P1.

It is then verified that P1 remains compatible with the 5 to 10% homogeneity chosen.

• augmenter l'épaisseur du disque souple, si on se trouve proche de la zone horizontale de la courbe,
• augmenter sa souplesse et donc rechercher une nouvelle courbe, si l'on se trouve déjà loin du talonnement.

If this value is exceeded, we can:

• increase the thickness of the flexible disc, if you are close to the horizontal zone of the curve,
• increase its flexibility and therefore look for a new curve, if you are already far from the tailgating.

We can observe that it is not desirable to work at the bottom of the curve, the contact between flexible disk and plate is not guaranteed in all points.

• épaisseur variable du disque souple ;
• mauvaise planéité du support sur lequel est collé le disque ;
• mauvais collage du disque sur son support.

Variations in the insertion of the flexible disc can have other origins:

• variable thickness of the flexible disc;
• poor flatness of the support on which the disc is stuck;
• bad bonding of the disc on its support.

These differences must be kept within the limit of 5 to 10% already taken into account.

The mechanisms mentioned above regarding the determination of the minimum pressure and the flexible support, intervene in the regulating role of the flexible disc. In fact, if the growths to be leveled have variable heights, it is the tallest ones which will initially receive the greatest part of the force P1. The plate will undergo in this zone an arrow which will be compensated at the level of the flexible material, by an additional depression, which will result in an increase in the pressure in this zone. The point will therefore be broken in more quickly than the others.

Different embodiments of the wafer support will now be described in conjunction with FIGS. 11 to 13.

The support shown in Figure 11, first of all, comprises a rigid body in two parts 150-152 on which the flexible disc 142 comes to take support, and a device 158 allowing three rotations along three perpendicular axes, two of these rotations, used to correctly position and orient the wafer 44 on the reference plane, which may be partial (or of limited amplitude), the third being complete according to an axis perpendicular to the reference plane. The device 158 allows the connection with a vertical axis 160. This device is preferably a spherical bearing or a needle bearing associated with a spherical bearing. The rigid body 150 is surrounded by a peripheral ring 162 in which a recess 163 has been machined. The height of the step 163 is less than the thickness of the plate and its diameter is slightly greater than that of the plate. The plate 44 comes to bear in this recess 163. The ring piece 162 is connected to the rigid body 150 by posts 164 and springs 166.

The vertical force applied to the axis 160 does not pass through the peripheral ring 162 but through the ball joint 158, the rigid body 150 and the disc 142. The ring 162 only serves to drive the plate 44 in the movement of circular translation necessary for polishing, movement produced by the horizontal force driving the support (produced for example by the eccentric 37 of FIG. 7).

The rigid body 150-152 is pierced with a channel 170 connected by a tube 172 to a vacuum machine not shown. This arrangement keeps the wafer 44 in place during the phases where the support is not pressed against the polishing plane.

FIG. 12 shows a detail of the peripheral ring 162, with its recess 163 receiving the plate 44. In the illustrated variant, it is the ring 162, to which a circular groove 161 is added, which is pierced with a channel 174 connected by a tube 176 very flexible to a vacuum machine not shown. This variant corresponds to polishing requiring greater torque forces than in the case of FIG. 11.

In the variant illustrated in FIG. 13, the peripheral ring consists of a thin ring 180 cut for example from a sheet of steel, this thin ring being rigid in its plane but flexible in the perpendicular direction. This thin ring 180 is coated in a very flexible material 182, for example silicone. Such an annular part is sufficiently rigid in the horizontal plane to transmit the cutting forces, while being flexible enough vertically to match the defects of the insert.

With the polishing machine which has just been described, the Applicant has obtained remarkable results illustrated in FIGS. 14 to 17.

FIG. 14 shows, in section, a sub-assembly corresponding to a magnetic writing and reading head in a horizontal structure, of the kind which has already been mentioned in connection with FIG. 1. The sub-assembly of FIG. 14 essentially comprises a silicon substrate 100, two edges of the casing 102 in silica, two vertical pads 104 in iron-nickel. This involves polishing this sub-assembly according to a plane 106 before continuing the operations of forming the upper pole piece.

Before polishing, the profile of the sub-assembly is shown in part a of Figure 15. In abscissa, the entire recorded interval measures 1.2 mm (the units indicated are therefore in micrometers). On the ordinate, the units are in hundreds of nanometers. We can clearly see, on this statement, the two edges of the box and, in the center, the two vertical iron-nickel studs.

After polishing, the profile has the shape of part b of FIG. 15. The whole of the recorded interval measures 4 mm (which means that the statement relates to the entire "ski" carrying the head). On the ordinate, the scale is in tens of nanometers. The residual overshoot in the natural curvature of the "ski" is less than or equal to 30 nm (this curvature being a fraction of the deformation of the substrate).

FIG. 16 shows the head after the operations for forming the non-magnetic spacer 110 and the upper pole pieces 112 made of iron-nickel. Reliefs 114 appear in the center of the head. The final polishing plan is referenced 116.

On part a of FIG. 17, we see the profile of this sub-assembly before running in. The units are the same as in FIG. 15a: 1.2 mm for the entire abscissa axis and hundreds of nanometers on the ordinate. The three peaks corresponding to the three reliefs of the pole pieces are clearly visible.

Part b of Figure 17 shows the reading after polishing. On the abscissa, the units are still in micrometers and on the ordinate, in tens of nanometers. No residual overshoot is detected, we only measure the natural curvature of the pad (this curvature being a fraction of the deformation of the substrate).

Claims (13)

Polishing machine including: - a microabrasive strip (33) stretched over a plate between a supply reel (40) and a take-up reel, - a support head capable of holding a polishing sample (44), facing polishing facing the abrasive belt, characterized by the fact that: - the plate is a plate offering a rigid and flat upper face, the supply reel (40) is equipped with means (62, 66) for exerting a resistive torque on it and the take-up reel (50) is controlled by a motor, - the sample support head (32) comprises a rigid piece (40) and a flexible material disc (42) having a certain thickness, this disc being fixed to said rigid piece (40) and receiving the polishing pad (44 ), the rigid part (40) being connected by a spherical bearing (58) and a vertical axis (60) to means (34) for moving the support head (32) in a circular translational movement. Machine according to claim 1, characterized in that the supply reel (40) is connected to a frame (60) by two spherical bearings (41, 42) and two slides (43, 44) whose ends come to bear on two pressure sensors (45, 46) linked to the frame by two adjustable stops (47, 48), the adjustment of the stops making it possible to balance the tension of the strip over its entire width. Machine according to claim 2, characterized in that the means for exerting a resistive torque on the supply reel (40) consist of an annular motor (62) mounted directly on one of the spherical bearings (41, 42), at the end slide (43), and by means (64) for controlling this motor. Machine according to claim 2, characterized in that the means for exerting a resistive torque on the supply reel (40) consist of a motor (66) separate from the supply reel (40) and by a transmission belt (68) between this motor (66) and the supply coil (40), this belt having a stretched strand (68a) in a plane perpendicular to the slides (43, 44) and by means (64) for controlling this motor. Machine according to claim 3 or 4, characterized in that the two pressure sensors (45, 46) arranged at the ends of the two slides (43, 44) are connected to the control means (64) of the motor (62, 66) exerting a resistant torque on the supply reel (40). Machine according to claim 1, characterized in that the take-up reel (50) is controlled in rotation by a geared motor (70). Machine according to claim 6, characterized in that the take-up reel (50) is connected to the gearmotor (70) by means of interruption (72) of the transmission, such as a mechanical coupling or an electromagnetic clutch. Machine according to claim 1, characterized in that the supply reel (40) and the take-up reel (50) are arranged under the upper face of the reference plate (30), two drums (41, 51) being arranged between the reels and the reference plate (30), the microabrasive strip (33) passing over these drums (41, 51) at its outlet from the supply reel (40) and at its entry onto the take-up reel (50). Machine according to claim 8, characterized in that the two drums (41, 51) are arranged under the upper face of the reference plate (30), the microabrasive strip (33) entering and leaving the plate of the plate an angle (ϑ) with respect to the horizontal. Machine according to claim 1, characterized in that the rigid part (150-152) of the support head is surrounded by a peripheral ring (162) having a groove (163) of a diameter slightly greater than the diameter of the insert (44) and of height slightly less than the thickness of the plate, the latter (44) then coming to bear in the groove (163), this ring (162) being connected to the rigid part (150) of the support wafer by means (164) capable of transmitting to the ring (162), therefore to the wafer (44), the forces associated with the transverse displacement of the support head relative to the microabrasive film, the pressure exerted on the support head transmitted to the wafer by the rigid part (150-152) and the flexible material disc (142). Machine according to claim 10, characterized in that the peripheral ring (162) consists of a thin planar ring (180), rigid in its plane but flexible perpendicular to this plane, and in a flexible way (182) molded around said thin ring (180). Machine according to claim 1, characterized in that the rigid part (150) of the support head is pierced with a channel (170) also passing through the disk made of flexible material (142), this channel (170) being connected by a tubing (172) to a vacuum machine. Machine according to claim 10, characterized in that the peripheral ring (162) is pierced with a channel (174) opening into the groove (163) where the plate (44) bears, this channel (174) being connected by flexible tubing (176) to a vacuum machine.

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