EP0980741A1 - Method and apparatus for abrasive slurry distribution in mechanical polishing of substrate - Google Patents

Abstract

The apparatus for distribution of abrasive suspension or slurry comprises a reservoir (1) in a recirculation contour (40) with pumps (5,6) and a filter (8), and a flow control system including a pressure sensor (80) and a proportional integral-differential (PID) block (81) electrically connected to pumps. The apparatus also includes a refilling reservoir (42), a reservoir (44) for an inert gas under pressure as e.g. nitrogen (N2) of high purity, and valves (70,71,72) in lines to user terminals. The flow velocity in the distribution system is within the limits 0.2-10 m/s, preferably 0.5-2 m/s or 1-1.2 m/s. The two pumps (5,6) are connected in parallel, each operating at 50 % of power with possibility of 100 % in the case of breakdown of one pump. The distribution system also contains the means for vaporization of deionized water in gas under pressure to prevent blockage of filters. In another embodiment, the apparatus comprises two reservoirs for abrasive suspension connected in parallel, and a control system with the pressure sensor, the PID block, and additional controlled valves and level sensors.

Description

The present invention relates to a device and a abrasive suspension distribution process for the mechanical polishing of substrates.

The use of silica or alumina suspension for polishing the silicon wafers on the back side as well as the front side tends to generalize in the semiconductor, optoelectronics and optics. This naturally poses the problem of distribution of this product, from the delivery keg to several points of use. Such suspensions may also be used to polish a metallic layer already deposited on the silicon wafer; in this cases, these suspensions may have characteristics physical or chemical different from the first.

Physico-chemical characteristics, viscosity, dry extract and pH contribute to making these suspensions easily solidifiable if they are not regularly restless. It follows that the pipes of distribution of these suspensions often get blocked by solidification of these and that the polishing of silicon wafers has defects, characterized by deep scratches, due to the detachment of agglomerates from solidified silica or alumina; the pipes of distribution therefore need to be replaced frequently and all of this ultimately generates operating costs high equipment availability rates weak.

The different methods of distributing abrasive suspensions (or "slurries") in the process of chemical and / or mechanical polishing of silicon wafers can be summarized as follows:

The most conventional method to distribute these abrasive suspensions consists in using pumps connected to pipes that will bring the product to point of use. However, the nature of the suspensions abrasive, in particular because they contain a on the one hand, abrasive particles, and on the other hand, that they have a pH that is either very acidic or very basic, leads to a very rapid degradation of the pumps used which causes very serious maintenance problems: these distribution systems can only operate during a short duration, a large part of the time being used replacement, unclogging and maintenance of the circuit. Even using pumps based on polytetrafluoroethylene, the diaphragms of these pumps must be replaced every two to three months.

It is known from US Patents 5,148,945 and 5,330,072 a chemical vacuum distribution system in which, instead of using pumps, intermediate containers which are evacuated sequentially so as to aspirate the products chemicals and pressurize them to force them to head to the point of use. Although such a system does not use a pump, it is not possible with such system to distribute abrasive suspensions such as those used in particular to remove metals on surfaces. Indeed, such suspensions have an extremely short lifespan before they solidify and are very aggressive.

It is also described in application WO 96/02319 a device which first comprises a measuring container in which are brought in turn the different chemicals used in the abrasive suspension, said chemicals being dosed in this container of measurement, each chemical compound then being, after measure, sent to a mixing container in which in situ mixing all the compounds of the suspension, this mixing container itself being connected to a mixing container negative pressure or vacuum so as to suck up the suspension which is in the container of mix and send it to the different delivery stations of the abrasive suspension.

There is therefore currently a need for devices distributing abrasive suspensions which do not not require significant maintenance and which can be available when the need exists.

The apparatus according to the invention is characterized in that that it has a reservoir in which is placed a abrasive suspension to distribute, a loop of distribution of the suspension connected at its two ends to distribution tanks, means of circulation to circulate the abrasive suspension in the loop, recovery means to recover the suspension abrasive after circulation in the loop, and means for regulation of the means of circulation so as to maintain continuous circulation of the abrasive suspension in the loop. (Preferably at least when the device is in functioning, i.e. in condition of distributing the suspension at points of use).

Preferably, we will use a distribution loop at the point of use consisting of a plurality of loops arranged in parallel with preferably each loop comprising at least one arm common to all loops, at one end of which are connected one ends of the plurality of loops. In a way preferential, this loop will have two common arms connected by their respective first ends to a pluralities of sub-loops connected in parallel, the first common arm being by its second end at abrasive suspension tank and the second common arm being connected by its second end to means of recovery of the abrasive suspension. Preferably also, the length of each pipe (whatever the loop) between the tank and the point of distribution of the suspension will be roughly the same while the length of each pipe between the distribution point and means of recovery of the abrasive suspension will also preferably be much the same.

In order to obtain a continuous circulation and regular abrasive suspension, it is best to provide regulation means on the device according to the invention and these means of regulation will be preferably either means of regulating the pressure of the abrasive suspension in the pipes, i.e. means of adjusting the suspension flow abrasive in said pipes (or possibly two in combination). We will preferably choose a minimum circulation speed in pipes of distribution to avoid deposit of abrasive suspension on the walls of the distribution pipes which would cause blockage of the pipes. It is indeed one of the important contributions of the invention than to avoid this way the plugging of the pipes which is a major problem in the facilities currently known. By circulating so continues the abrasive suspension in the pipes with preferably a speed greater than or equal to about 0.2 m / s and preferably also a lower speed or equal to about 10 m / s, thus avoiding problems obstruction of these pipes, obstructions which often result from hardening and / or coagulation of the abrasive suspension. Preferably, the speed of circulation will be between approximately 0.5 m / s and approximately 2 m / s and more preferably between about 1 and 1.2 m / s.

To circulate this suspension abrasive, preferably use a pump because realized that by using a circulation pump while maintaining continuous circulation of the abrasive suspension, we avoided problems usually encountered in previous pumps, at know the blockages of the filters, obstruction of the pump, etc ... However, and in order to give maximum reliability the apparatus according to the invention, preferably two circulation pumps in parallel, each of the two pumps preferably only operating at approximately (and at plus) 50% of their power which allows in case one of the two pumps fails to continue normal operation of the device with a single pump which will then operate at 100% of its power. Well heard, we will have in this case an alarm which will indicate when one of the pumps is blocked, defective or when the material flow leaving one of the pumps is substantially zero.

According to another embodiment of the invention particularly advantageous, and so as to avoid the use of pumps especially in cases where the abrasive suspension would be very corrosive, circulation of the suspension will be provided by pressurized gas, preferably an inert gas such as nitrogen and / or argon and / or pressurized helium. It was found in this case however, there may still be a blockage of pipes. The Applicant has demonstrated that these blockages, in the case of circulation using gas under pressure, could be avoided by ensuring hygrometry minimum pressure gas used for circulation. For this purpose, we will prefer to spray or nebulize water deionized (having all the characteristics of purity required for electronic applications) in gas inert before injecting it into the pipes, so as to maintain sufficient humidity in the gas and avoid drying of the abrasive suspension and thus clogging of filters. Using a concentration of water in the pressurized gas adapted within the limit of speeds usually used and mentioned above, any clogging of the pipes is avoided.

la figure 1, un premier exemple de réalisation d'un appareil selon l'invention et mettant en oeuvre le procédé selon l'invention,

la figure 2, une variante de réalisation de la figure 1.

The invention will be better understood with the aid of the following embodiments given without limitation in conjunction with the figures which represent:

FIG. 1, a first embodiment of an apparatus according to the invention and implementing the method according to the invention,

FIG. 2, an alternative embodiment of FIG. 1.

In FIG. 1, a tank 1 contains a suspension abrasive 2 removed by a pipe 3 which crosses the tank wall 1 at point 4 and extends to two pumps 5 and 6 placed in parallel on this pipeline whose outputs are reconnected again together on the pipe 7 connected to the filter 8 which leads to line 9. In points 10, 11, ... 12, the pipes 20 are respectively connected, 21, ... 22, of which the other ends respectively 30, 31, ... 32, are interconnected with line 40 of abrasive suspension return to tank 1, this pipe 40 crossing at 41 the wall of said tank to enter suspension 2 contained in this tank 1. A filling tank 42 is arranged in parallel with the tank 1 and connected to it by a line 43 so as to maintain, in all circumstances, a level always above a certain predetermined minimum in tank 1. This tank appendix 42 allows in particular to adjust the viscosity of the abrasive suspension.

Connected by the pipe 45 to the tank 1 are also provided means for injecting pressurized gas 44 such as an inert gas, preferably nitrogen, of a purity compatible with the desired application, in particular a purity of the N50 type or better. Between the connections respectively 10.30, 11.31, ... 12.32, there are disposed distributions, respectively B, C, ... D, of abrasive suspension at the points of use, respectively 60, 61, .. .62 via valves 70, 71, ... 72 respectively. We will preferably use pipes of the same diameter in the different loops respectively 20, 21, ... 22, and we will maintain the same distance between point 4 and points B, C, .D respectively (or if the diameters are different , lengths such that the pressure drops are identical in each line). In this way, the pressures P _a , P _b , ... P _c which are respectively the abrasive suspension pressures at points B, C, D, are substantially equal which allows a uniform distribution of the product. The apparatus also comprises, between point 32 and point 41 of line 40, a pressure detector 80 electrically connected to a PID type regulation system (proportional / integral / differential) which is itself electrically connected, respectively by the lines 83 and 84 at the pumps, respectively 5 and 6. The pressure must be maintained at a pressure equal to the set pressure set in the PID, and when the pressure detected by 80 is lower than the pressure displayed, an electrical signal is generated by PID 81 so as to increase the pumping pressure of pumps 5 and 6. When the pressure measured by the pressure detector 80 becomes equal to or greater than the pressure displayed in PID 81, the latter corrects the signal sent by the lines 83 and 84 to pumps 5 and 6 so as to reduce the pumping pressure of pumps 5 and 6 to the value corresponding to the pressure setpoint. Conversely, the PID decreases the pumping pressure of the pumps by sending an adequate electrical signal to pumps 5 and 6 if the pressure detectors detect a pressure higher than the set pressure displayed in the PID 81.

The apparatus may also include a regulation of flow either alone (without the pressure detection system with the PID), or in addition to the detection system pressure. This flow control system consists of in particular to measure the flow rate of the solution of the abrasive suspension and compare the measured value to a setpoint then increase or decrease the flow pumps 5 and 6 by sending an electrical signal corresponding to the said pumps after comparing the flow measured and the displayed flow rate. In Figure 2, is shown an alternative embodiment of the device of the Figure 1 in which in particular at least two are used abrasive suspension storage tanks connected in parallel and which are used alternately as well when a tank is empty and this state is detected by a level detector, for example, we can automatically start the second tank without having need to interrupt abrasive suspension circulation which avoids clotting and clogging problems pipes, which therefore reduces the maintenance of the device. (In this figure 2, the same devices as those of Figure 1 have the same references).

At point 126, the two pipes 101 meet and 119 respectively connected to the suspension tanks abrasive 112 and 115 by means of the controlled opening 103 and 102 respectively, pipes 113 and 118. In FIG. 2, the tank 112 is under pressure from inert gas, for example nitrogen (ultra pure, of “electronic” purity for use suspension for polishing silicon wafers ("Wafer")) under a pressure for example of around 3 bar in the space 114 above the suspension 127. The suspended supply when 112 is empty is done via valve 111 (open for filling, then closed in use - see below) and the pipeline 128 which plunges into 129 in the reserve 130 in the tank back 131. Similarly the lower part of the tank 115 is connected via line 117, valve 116 and the line 120 to line 128 so as to fill 115 when the latter is empty (valve 116 open and 111 closed) and fill in 112 when 111 is open 116 is closed (or fill in neither when 111 and 116 are closed).

The pressurized nitrogen source 121 makes it possible to adjust gas pressures above the suspension via respectively the valves controlled (by a device of control well known to those skilled in the art "Proportional" (P), "Integral" (I), "Differential" (D) or "PID" 81) 108 and 110 allowing set pressures at for example 3 bar (relative) at above 127 (to push the suspension in 113) and at 0 bar (relative) in 115 to allow filling of 115 in suspension. 121 is also connected via the pipeline 131 and the controlled valve 122 (also controlled or not by PID 81) which maintains nitrogen pressure by example of 0.5 bar (relative) above the suspension 130.

Line 137 returning the excess suspension returns to tank 131.

To provide additional supply to the tank 131 to compensate for the consumption of suspension upstream, a large capacity tank 132 is provided which supplies 131 via line 124. This reservoir 132 can be raised to feed others gravity tanks (possibly you can also inject ultra-pure nitrogen pressure at the top through line 125). This mode of transport without pump but by gas pressure avoids the problem of seizure of pumps.

We can also have several other tanks such as 112, 115 in parallel with these.

In FIG. 1, the tank 2 is of sufficient volume to preferably provide autonomy to the equipment of the order of 2.5 / 3 days; it is preferably mechanically agitated continuously and maintained at slight overpressure (1 to 3 mbar) so as to avoid the action of atmospheric CO ₂ on the ammonia contained in certain abrasive suspensions. The tank is made of a plastic material on which the solidifying silica or alumina are not likely to adhere. The withdrawal of the tank is preferably carried out by a bottom valve; the loop is returned by a valve descending below the minimum level of the tank. The tank is filled using a positive displacement pump from shuttle drums. If necessary, a water dilution must be carried out by injecting water and a silica suspension continuously into a static mixer. After each unloading of the shuttle drum 42, the silica supply circuit of the storage tank, pumps and pipe included is purged with deionized water.

Given the abrasive nature of the suspension at circulate, we prefer in practice to operate the two pumps in parallel with half of their speed nominal, each pump preferably being connected to a variable frequency power supply 0-100 Hz controllable by a current (for example, from 4 to 20 mA); the 2 power supplies variable frequency are controlled by a PID 81 regulator at the pressure at the end of the loop just before the valve setting.

We can for example choose ceramic gear pumps. To ensure normal longevity of the seals,
the shaft of each pump should preferably be fitted with 2 seals, the space (bell) between the seals being kept at overpressure relative to the silica by injection of deionized water. The deionized water supply circuit will have at its inlet an orifice calibrated at 100-200 l / h and 10-20 l / h at the outlet. The pressure prevailing in the bell must be constant, if there is no leakage on the interior lining in contact with the silica. On the other hand, if there is a leak, the pressure in the bell between the 2 linings will gradually decrease and a pressure sensor will transmit alarm signals and organize a curative action. Preferably, an automatic purge of the pumps with deionized water will also be provided in the event of a stop.

• usure limitée en milieu abrasif grâce au faible régime de rotation des pompes,
• l'utilisation de 2 pompes permet de maintenir le débit et la pression de la boucle en cas de défaillance d'une à pompe, la seconde augmentant automatiquement son régime pour compenser.
• en cas de soutirage important, la pression au retour à la cuve baissant, les pompes augmentent leur régime pour maintenir le débit constant. Si, par exemple, les équipements soutirent plus que le débit nominal de la boucle, il y a compensation automatique et le débit de la boucle n'est jamais arrêté. Inversement, lorsque les soutirages s'arrêtent, la pression aura tendance à augmenter et le régulateur PID réduira la fréquence de commande des pompes. Les variations de pression dans le réseau de distribution sont aussi de faible amplitude, par exemple, ± 100 g ce qui est important pour le dosage de la solution abrasive sur les équipements de polissage.

These arrangements have the following advantages:

• limited wear in abrasive medium thanks to the low rotation speed of the pumps,
• the use of 2 pumps makes it possible to maintain the flow rate and the pressure of the loop in the event of a pump failure, the second automatically increasing its speed to compensate.
• in the event of heavy withdrawal, the pressure returning to the tank decreases, the pumps increase their speed to keep the flow constant. If, for example, the equipment draws more than the nominal flow rate of the loop, there is automatic compensation and the flow rate of the loop is never stopped. Conversely, when the withdrawals stop, the pressure will tend to increase and the PID regulator will reduce the pump control frequency. The pressure variations in the distribution network are also of small amplitude, for example, ± 100 g which is important for the dosing of the abrasive solution on the polishing equipment.

The loop (in its common arm 7, 9) is equipped with a filter 8 preferably of “polypropylene” type preferably fixing at least 90% of the materials of size greater than about 100 micrometers. Given the character of this suspension, we will have to oversize this filter in order to work with a reduced differential pressure, from 0.1 to 0.2 bar.

• répartir le débit dans les différentes branches du réseau de manière égale,
• le soutirage de chaque équipement (distribution) qui s'effectue par un té en amont de l'orifice ne provoque pas de cette manière de baisse de pression significative.

All circuits derived from the distribution loop have substantially the same pressure drop. Each loop (distribution) is served by a flow tee and a return tee, each tee being fitted with an isolation valve. The “return” isolation valve is equipped with a calibrated orifice which has two functions:

• distribute the flow in the different branches of the network equally,
• the withdrawal of each equipment (distribution) which is carried out by a tee upstream of the orifice does not in this way cause a significant drop in pressure.

The diameter of the calibrated orifices is a function of number of parameters, such as the flow of the loop, loop pressure, number of devices to be serve, the viscosity of the suspension to be distributed, elements that the skilled person will be able to choose without difficulties in ensuring flow velocities, preferably between 0.2 and 0.5 m / s to minimize abrasion of the pipes.

The distribution loop preferably includes a non-controlled control valve (not shown on the figure) resistant to abrasion and upstream of it, a flow meter 90 and a pressure sensor 80.

The flow rate is adjusted using the control valve not subject to the selected value., the pressure setpoint is displayed on the PID 81 controller whose parameters are adjusted to obtain satisfactory operation of the loop, especially during small variations in pressure during withdrawal, start-up, and / or start-up operating on 1 pump.

It should be noted that the system described above can also work in particular with pneumatic pumps membranes or bellows, commonly used in industry semiconductors, provided that the working gas is injected by a pilot-operated valve or a device of the type mass flow controller, controlled by a setpoint pressure given by a pressure regulator. In this case, operation with 2 pumps parallel to half flow nominal will be maintained so as to obtain a rate of high availability.

Claims (17)

Device for distributing abrasive suspensions, characterized in that it comprises: a reservoir (1) containing an abrasive suspension (2), a distribution loop (3, 7, 9, 40) of the abrasive suspension (2) connected to the reservoir (1), circulation means (5, 6) for circulating the abrasive suspension in the loop and ensuring its return to the tank, recovery means (1) for recovering the abrasive suspension after circulation in the loop, regulating means (81) of the circulation means so as to maintain a continuous circulation of the suspension. Apparatus according to claim 1, characterized in what the point-of-use distribution loop is consisting of a plurality of loops (20, 21, 22, ..) connected in parallel. Apparatus according to claim 2, characterized in that the loop has at least one common arm (3, 7, 9) at all loops at one end of which are connected to one end of the plurality of loops. Apparatus according to claim 3, characterized in that the loop has two common arms, a first arm (3, 7, 9) connected at one end to the tank (1), and a second arm (40) connected to a first end recovery means (1). Apparatus according to one of claims 1 to 4, characterized in that the length of each pipe between the tank (1) and the distribution point of the abrasive solution is much the same. Apparatus according to one of claims 1 to 5, characterized in that the lengths of each pipe between the distribution point (60, 61, 62) and the means recovery (9) is substantially the same. Apparatus according to one of claims 1 to 6, characterized in that it includes means of regulation (80, 81) of the circulation of the abrasive solution in the loop by checking the pressure in the pipes distribution of said solution. Apparatus according to one of claims 1 to 7, characterized in that it includes means of regulation the flow of the abrasive solution in the different arms of the distribution loop. Apparatus according to one of claims 1 to 8, characterized in that the abrasive suspension has a speed minimum circulation in the different pipes to avoid suspension deposits on the walls of said distribution pipes. Apparatus according to one of claims 1 to 9, characterized in that the speed of distribution of the solution in the pipes is greater than or equal to about 0.2 m / s. Apparatus according to one of claims 1 to 10, characterized in that the circulation speed of the abrasive suspension in the pipes is lower or equal to about 10 m / s. Apparatus according to one of claims 1 to 11, characterized in that the delivery speed of abrasive suspensions is preferably between 0.5 and 2 m / s. Apparatus according to claim 12, characterized in that the speed of distribution of suspensions abrasive is between about 1 m / s and 1.2 m / s. Apparatus according to one of claims 1 to 13, characterized in that the means of circulation of abrasive suspensions consist of at least one pump (5, 6). Apparatus according to one of claims 1 to 14, characterized in that the means of circulation of abrasive suspensions consist of two pumps (5, 6) mounted in parallel, each of them operating at not more than 50% of its power in order to allow a normal operation of the suspension distribution abrasive if one of the two pumps breaks down. Apparatus according to one of claims 1 to 15, characterized in that the circulation means (44) are consisting of a gas under pressure, in particular a gas inert such as nitrogen. The apparatus of claim 16, wherein the means of circulation consist of a gas under pressure characterized in that it also includes means to create a deionized water nebulization in pressurized gas to prevent clogging of filters in the abrasive suspension distribution system.

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