EP0517595A1 - Polishing machine with pressure control - Google Patents

Abstract

The machine of the invention comprises a bracket (50) supporting an electromagnet (56), an articulated arm (70) with a specimen holder head (86) and a pressure sensor (62). Application to the production of magnetic read and write heads. <IMAGE>

Description

The present invention relates to a pressure control polishing machine.

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 to the production of heads of the second category (vertical heads), the machine object of this The invention is above all intended for the polishing of 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, at the last stage of production, just 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 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 of the following two techniques is used:

• grinding with a diamond tool: it it 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 feed movement and a cut 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 aqueous solution ; 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 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 is a question of machining, in their thickness, layers deposited on a wafer, and generally, it is necessary to simultaneously polish 600 protuberances corresponding to 600 magnetic heads, in excess of a few microns, and this with a precision of the order of a nanometer, without reducing the thickness of the thin layer which covers the wafer by more than 200 to 300nm.

Known polishing machines do not meet all of these requirements. One of the reasons for this incapacity is that the pressure exerted on the workpiece is not controlled with sufficient precision throughout the polishing.

It should be understood that this pressure control problem is particularly difficult to solve because if the machine must be able to work with relatively high pressures (from 5 to 40 kg) it must also be able to work with very low values (from 0, 1 to 0.5 kg). Indeed, during the polishing of silicon wafers containing integrated circuits, the start of the polishing operation is carried out on overshoots (or reliefs) sometimes very high and very narrow (as will be better understood in connection with the figures 5 to 8 which will be described later). These overruns are very fragile. It is therefore advisable to treat them with very low pressures. However, at the end of polishing, very high pressures are necessary to avoid any loss of contact between the wafer and the abrasive film and thus obtain the desired perfect polarization.

• un plateau de polissage,
• un moyen apte à maintenir un échantillon, face à polir en regard du plateau de polissage,
• des moyens pour exercer une force d'application de la face à polir sur le plateau de polissage et pour déplacer transversalement l'échantillon à polir par rapport au plateau de polissage,

cette machine étant caractérisée par le fait que les moyens pour exercer la force et déplacer l'échantillon et le plateau de polissage l'un par rapport à l'autre sont perfectionnés pour permettre un contrôle très fin de la force appliquée. Ces moyens sont définis dans la revendication 1. Ils comprennent essentiellement un électroaimant asservi disposé à l'extrémité d'une potence, un bras monté oscillant sur cette potence, l'extrémité du bras supportant une broche et une tête support d'échantillon, cette tête support étant donc disposée sous l'électroaimant.The object of the present invention is precisely to remedy this drawback. To this end, the invention provides a machine which, first of all, and like all polishing machines, comprises:

• a polishing plate,
• a means capable of maintaining a sample, face to be polished opposite the polishing plate,
• means for exerting an application force of the surface to be polished on the polishing plate and for transversely displacing the sample to be polished relative to the polishing plate,

this machine being characterized by the fact that the means for exerting the force and moving the sample and the polishing plate with respect to each other are perfected to allow very fine control of the force applied. These means are defined in claim 1. They essentially comprise a controlled electromagnet disposed at the end of a bracket, an arm mounted oscillating on this bracket, the end of the arm supporting a spindle and a sample support head, this support head therefore being arranged under the electromagnet.

By these means it is possible to perfectly balance the machine so that the pressure exerted on the sample is initially zero, from which it is possible to adjust and control the pressure at any low or high value, by action on the electromagnet.

It will be observed that it is already known to use an electromagnet to regulate a polishing pressure. For example, the Japanese patent abstract vol. 14, No. 107, (M-94) February 27, 1990 (JP-A-1 310 859) describes a polishing machine comprising a sample support block, this block being rotated. This block, by its own weight, creates a certain pressure on the samples. There is also provided, inside the support block, an electromagnet for further increasing the pressure.

Besides that this system is complex due because the electromagnet is placed in a rotating block, it does not allow the pressure to be controlled at very low values, since at least this pressure is that exerted by the whole of the sample support block. This system therefore makes it possible to control only an increase in pressure, compared to a minimum pressure.

There are also balanced arm polishing machines. Document EP-A-0 284 343 describes one, which comprises an inflatable balloon capable of quickly absorbing the variations in pressure exerted on the sample to be polished due to variations in the contours of the polishing tool. But this machine does not allow to control the polishing pressure.

If we wanted to combine these two documents, this would lead to a machine in which the sample holder head itself would be equipped with an electromagnet, that being placed under the end of the balanced arm. The articulated arm should therefore support this electromagnet which would pose serious mechanical problems. Furthermore, an increase in pressure by action on the electromagnet would actually have the effect of compressing the inflatable balloon, which would cancel the desired effect, the role of the balloon being precisely to compensate for variations in pressure.

Thus, the combination of these two documents is ineffective and in any case does not lead to the machine of the present invention where the electromagnet is placed above the support head, at the end of a rigid bracket, the oscillating mounted arm supporting only the means for moving the support head. Thus, a pressure as low as desired can be obtained and controlled.

• 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 montre la structure générale d'une machine à polir selon l'invention,
• les figures 3a et 3b montrent en détail une machine conforme à l'invention avec ses moyens pour exercer une pression contrôlée sur la tête support et pour déplacer cette tête,
• la figure 4 montre quatre positions de ces différents moyens et illustre le principe de mise en place de la tête support avec contrôle de la pression,
• la figure 5 montre un sous-ensemble intermédiaire dans la réalisation d'une tête magnétique d'écriture et de lecture,
• la figure 6 montre le profil de ce sous-ensemble avant et après polissage,
• la figure 7 montre la tête magnétique terminée,
• la figure 8 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 shows the general structure of a polishing machine according to the invention,
• FIGS. 3a and 3b show in detail a machine in accordance with the invention with its means for exerting controlled pressure on the support head and for moving this head,
• FIG. 4 shows four positions of these different means and illustrates the principle of placing the support head with pressure control,
• FIG. 5 shows an intermediate sub-assembly in the production of a magnetic writing and reading head,
• FIG. 6 shows the profile of this sub-assembly before and after polishing,
• FIG. 7 shows the completed magnetic head,
• Figure 8 shows the profile of this head before and after polishing.

In the example which will be described, the sample is assumed to be held by a support piece disposed above the polishing plate. In this case, the force is applied to the sample via the support head and the polishing plate is fixed. However, the invention is equally applicable to a machine where the polishing sample is located below the polishing tool on a sample support table, the polishing tool being held by the support head on which pressure is exerted.

• un plateau de polissage fixe 30,
• des moyens 34 pour exercer une force F sur la tête support 32 afin d'appliquer l'échantillon 44 à polir sur le plateau de polissage 30 et pour déplacer transversalement la tête de support 32 par rapport au plateau fixe 30, ce moyen pouvant comprendre un excentrique 37.

The machine shown in FIG. 2 schematically comprises:

• a fixed polishing plate 30,
• means 34 for exerting a force F on the support head 32 in order to apply the sample 44 to be polished on the polishing plate 30 and for transversely moving the support head 32 relative to the fixed plate 30, this means possibly comprising a eccentric 37.

The invention relates essentially to the means represented schematically by the block 34.

The machine shown in Figures 3a (side view) and 3b (end view) comprises a rigid bracket 50 sliding on vertical columns 52 and 54. An electromagnet 56 is powered by means 58. This electromagnet is fixed at one end of the stem 50, and is provided with a movable axial rod 60. This rod is equipped with a pressure sensor 62 having an outlet 64 connected to a pressure display means 65 and to the supply means 58.

The pressure sensor 62 (or the display means 65) is connected to a comparator 67 also receiving a reference signal from a generator 69. The output of the comparator delivers an error signal equal to the difference, in magnitude and as a sign, between the pressure measured by the sensor and the pressure setpoint. The electromagnet 56 is then controlled so that the error signal is canceled.

An arm 70 is mounted oscillating around two horizontal axes 72-74 integral with the bracket 50. A spindle 80 supports, at its lower part, an adjustable eccentric 82 provided with a means 84 for hooking the sample support head 86. This spindle is equipped, at its upper part, with a stop 90, for example a ball stop. The pin 80 is fixed to the end of the arm, under the electromagnet 56. The axial rod 60 of this electromagnet and its pressure sensor 62 come to bear on the stop 90.

A motor 91 is arranged at the other end of the arm 70 and forms a counterweight. A balancing mass 92, adjustable in position, is disposed near the motor 91 and is capable of balancing the arm 70 in the position of the latter.

A means 93 for transmitting movement, for example a toothed belt, is arranged between the motor 91 and the eccentric 82 to control the rotation of the eccentric 82.

Means 94, 95, for example jacks, are provided for vertically moving the bracket and all the means attached thereto, along the vertical columns 52, 54 until the sample is brought near the polishing plate 30.

Figure 4 provides a better understanding of how this machine works.

At rest, the gallows is at the top stop; the swinging arm is then slightly inclined (figure a); the ball stop is supported on the rod of the electromagnet; the pressure display 65 indicates substantially zero.

- Hang the sample holder 86 on the eccentric; the swinging arm 70 swings down; we find the equilibrium position (sensor at zero) by moving the balancing mass 92 (figure b).

The stem descends along axes 52-54 and is locked in the low position by the jacks (figure c).

In this configuration, the oscillating arm 70 is always inclined very slightly upwards and the sample holder is approximately one millimeter from the abrasive plane 30.

The electromagnet 56 is then supplied with an adjustable and stable DC voltage, until the sample comes into contact with the polishing plane. Then the load is increased to the desired force value (figure d). Polishing can then be carried out by controlling the motor 91.

This device makes it possible to adjust and control the polishing pressure from an almost zero value to a maximum value fixed by the electromagnet chosen from 0 to 100 kg for example.

• "Procédé de réalisation d'un outil de polissage et outil obtenu par ce procédé",
• "Machine de polissage à bande microabrasive tendue et à tête support perfectionnée",
• "Machine de polissage à table porte-échantillon perfectionnée".

The machine which has just been described can advantageously be combined with the provisions provided for in three other European patent applications filed by this Applicant, on the same day as the filing of this application, and which are entitled respectively:

• "Process for producing a polishing tool and tool obtained by this process",
• "Polishing machine with stretched microabrasive band and improved support head",
• "Table polishing machine improved sample holder ".

With the polishing machine which has just been described, equipped with an improved wafer support head, and with a microabrasive sheet stretched over the polishing plate, the Applicant has obtained remarkable results which are illustrated in FIGS. 5 to 8.

FIG. 5 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. 5 essentially comprises a silicon substrate 100, two box edges 102 of silica, two vertical pads 104 of 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 6. On the 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 presents the shape of part b of FIG. 6. 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 ten 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. 7 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. 8, we see the profile of this sub-assembly before running in. The units are the same as for FIG. 6a: 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 8 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 (3)

Polishing machine including: - a polishing plate (30), a means (32) able to hold the polishing samples (44), facing the polishing opposite the polishing plate (30), means (34) for exerting an application force of the face to be polished on the polishing plate (30) and for transversely moving the sample to be polished relative to the fixed polishing plate (30), this machine being characterized in that said means for exerting the application force and for moving the sample relative to the polishing plate comprise: - a rigid bracket (50) sliding on vertical columns (52, 54), - an electromagnet (56) and its supply means (58), the electromagnet being fixed to one end of the bracket (50), this electromagnet being provided with a movable axial rod (60), this rod being equipped with a pressure sensor (62) having an outlet (64) connected to the supply means (58) of the electromagnet (56), a comparator (67) with two inputs connected respectively to the pressure sensor (62) and to a setpoint generator (69), and to an output connected to the supply circuit (58) of the electromagnet (56), an arm (70) mounted oscillating around two horizontal axes (72-74) integral with the bracket (50), - A spindle (80) supporting, at its lower part, an adjustable eccentric (82) provided with a means (84) for hooking a support head and equipped, at its upper part, with a stop (90) , this pin (80) being fixed to one end of the arm, under the electromagnet (56) fixed to the bracket (50), the axial rod (60) thereof and its pressure sensor (62) coming to bear on the stop (90) of the spindle (80), - a motor (91) disposed at the other end of the arm (70) and forming a counterweight, a balancing mass (92) adjustable in position and disposed near the motor (91) capable of balancing the arm (70) in a substantially horizontal position, a means (93) for transmitting movement between the motor (91) and the eccentric (82) for controlling the rotation of the eccentric (82) supported by the spindle (80), - Means (94-95) for vertically moving the bracket and all the means attached thereto along the vertical columns (52, 54) until bringing the sample (86) into contact with the polishing plate (30 ). Machine according to claim 1, characterized in that the support head supports the sample to be polished, the polishing plate being located under the support head. Machine according to claim 1, characterized in that the support head supports the polishing plate, the sample to be polished being located under the support head.

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