IL121010A - Semiconductor cleaning apparatus - Google Patents

Abstract

Semiconductor cleaning apparatus comprising: (i) a heatable reaction core (102); (ii) a core collector electrode (120) mounted to said reaction core and connectable to a first, core voltage source (Vc); and (iii) a wafer transfer device (130) for transferring at least one semiconductor wafer (122) to be cleaned to within said reaction core, said wafer transfer device being connectable to at least a second, wafer voltage source having a voltage level greater than the voltage level of said core voltage source. 3139 ה' בכסלו התשס" ג - November 10, 2002 3140 ה' בכסלו התשס" ג - November 10, 2002

Description

SEMICONDUCTOR CLEANING APPARATUS nsrm1? ^ πρ^Ί ιρτιη A.Tally Eitan - Zeev Pearl, D. Latzer & Co. Law Offices P-1200-IL FIELD OF THE INVENTION The present invention relates to methods and apparatus for cleaning semiconductor wafers generally and to such methods and apparatus utilizing thermoemission in particular.

BACKGROUND OF THE INVENTION The steps of the production of semiconductor silicon integrated circuits must be very clean, because even small amounts of undesirable contaminating impurities can cause complete degradation or malfunction of integrated circuits. Thus, semiconductor silicon wafers must be cleaned between processing steps.

There exist many cleaning processes used in silicon semiconductor-production. Wet cleaning processes usually remove contaminants from the silicon wafer surface with special chemical solutions as part of a separate production step. Dry cleaning processes usually remove contaminants by etching within some gas or gas mixture or within a plasma environment. Gettering processes utilize the tendency for contaminants to move towards special traps inside the silicon wafer (areas with a high density of such traps are called "getters") and to stay stable within the traps.

However, wet and dry cleaning processes can only remove contaminants from the silicon wafer surface. Gettering processes can be used only for some types of contaminants (such as iron, copper, nickel etc.).

European Patent Publication EP-A-0749153, assigned to the common owners of the present application, describes a cleaning method which utilizes an electric field in the presence of heat to move positively charged impurity ions from the semiconductor wafer surface and above it towards a negatively charged electrode (called the "collector).

The apparatus of European Patent Publication EP-A-0749153 is schematically illustrated in Fig. 1 to which reference is now briefly made. The electric field is created between the electrodes, one of which, labeled 10, is attached to the wafer 12 to be cleaned. The other electrode (the "collector"), labeled 14, is placed at a distance therefrom. Wafer electrode 10 is connected to a voltage source output Vb and the electrode 14 is connected to another source output Vc, where the potential Vc is more electronegative than the potential Vb. Thus, an electric field is created to move positive ions, labeled 16, from the wafer area toward the collector electrode 14, where positive ions will be absorbed (captured). Typically, the apparatus is placed in a vacuum or in presence of a gas mixture and heated to initiate ion emission from the wafer surface. EP Publication EP-A-0749153, also describes utilizing a plasma between the two electrodes 10 and 14 to strengthen the electric field over the wafer surface.

SUMMARY OF THE PRESENT INVENTION It is an object of the present invention to provide methods and apparatus for batch cleaning of semiconductor wafers using thermoemission. These methods and apparatus can be utilized, in particular, within high temperature ovens, reactors or rapid thermal tools such as are common in the semiconductor manufacturing process.

There is therefore provided, in accordance with a preferred embodiment of the present invention, a collector electrode formed of semiconductor wafer. The semiconductor wafer can have an unpolished surface, a rough surface or an oxidized surface.

There is also provided, in accordance with a preferred embodiment of the present invention, a wafer transfer device or boat having a first slotted portion for receiving a first group of semiconductor wafers alternating with a second slotted portion for receiving a second group of semiconductor wafers. The first slotted portion is connectable to a first voltage source and the second slotted portion is connectable . to a second voltage source. The second voltage source is more electronegative than the first one. Typically, the first group of semiconductor wafers have impurities therein which are to be removed and the second group of semiconductor wafers are to receive the impurities.

Additionally, in accordance with a preferred embodiment of the present invention, the first semiconductor wafers are to be cleaned and the second semiconductor wafers are collector electrodes.

There is further provided, in accordance with a preferred embodiment of the present invention, a semiconductor cleaning unit including a heatable reaction core, a core collector electrode and a wafer transfer device. The core collector electrode is mounted to the reaction core and is connectable to a first, core voltage source Va. The wafer transfer device transfers at least one semiconductor wafer to be cleaned to within the reaction core. The wafer transfer device is connectable to at least a second, wafer voltage source Vb. Va is more electronegative than Vb.

Moreover, in accordance with a preferred embodiment of the present invention, the core collector electrode is mounted to the outside of the reaction core.

Further, in accordance with a preferred embodiment of the present invention, the wafer transfer device has a first slotted part which receives a first group of semiconductor wafers alternating with a second slotted portion which receives a second group of semiconductor wafers. The first slotted portion is connectable to the wafer voltage source Vb and the second slotted portion is connectable to a third, collector voltage source Vc. The collector voltage source Vc is more electronegative than Vb.

Still further, in accordance with a preferred embodiment of the present invention, the first semiconductor wafers are to be cleaned and the second semiconductor wafers are collector electrodes.

There is alternatively provided, in accordance with a preferred embodiment of the present invention, a semiconductor cleaning unit which includes a heatable reaction core and a wafer transfer device. The latter transfers a plurality of semiconductor wafers to within the reaction core and has a first slotted portion for receiving a first group of semiconductor wafers to be cleaned alternating with a second slotted portion for receiving a second group of semiconductor wafers forming collector electrodes. The first slotted portion is connectable to a first voltage source Vb and the second slotted portion is connectable to a second voltage source Vc.

Finally, there is provided, in accordance with a preferred embodiment of the present invention, a reaction core cleaning unit including a heatable reaction core, a core collector electrode and a conductive element. . The core collector electrode is mounted to the reaction core and is connectable to a first, core voltage source Va. The conductive element is locatable within the reaction core and is connectable to at least a second voltage source having a voltage level greater than the voltage level of the core voltage source. The core collector electrode can be mounted to the inside or outside of the reaction core.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which: Fig. 1 is a schematic illustration of a prior art thermoemission system; Fig. 2 is an exemplary prior art semiconductor processing machine; Fig. 3 is a schematic illustration of a thermoemission system for batch processing, constructed and operative in accordance with a preferred embodiment of the present invention; Fig. 4 is a schematic illustration of a boat carrying a batch of semiconductor wafers; Fig. 5 is a schematic illustration of a further thermoemission system for batch processing, constructed and operative in accordance with a second preferred embodiment of the present invention; Fig. 6 is a schematic illustration of a thermoemission system for batch processing, constructed and operative in accordance with a third preferred embodiment of the present invention; and Fig. 7 is a schematic illustration of a thermoemission system for cleaning a processing oven, constructed and operative in accordance with a fourth preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Reference is now made to Fig. 2 which illustrates an exemplary prior art semiconductor processing machine 100. The machine has a reaction core 102 housed within a housing 104. The desired process, annealing, oxidation, epitaxial growth, LPCVD etc., occurs within the reaction core 102 to a batch of semiconductor wafers 106. Typically, the wafers 106 are mounted on a transfer device 108 of some kind, such as a boat, paddle, carrier, etc., with which the batch of wafers 106 are transferred. As shown in Fig. 2, the processing machine includes a conveying unit 110 which carries the transfer device 108 from the outside to the inside of the processing machine.

In accordance with a preferred embodiment of the present invention, the electric field cleaning of EP Publication EP-A-0749153 is performed within the reaction core 102 of a prior art oven or rapid thermal tool to one or more wafers 106. In each of the embodiments described hereinbelow, a single wafer 106 or a batch of contaminated wafers 106 are placed within the transfer device 108 after which the transfer device 108 is placed within the reaction core 102. Within transfer device 108 is formed a first electrode to which all of the wafers 106 are connected. The location of tne collertor^ejectrode varies with the different embodiments. In each embodiment, a vacuum is created within the reaction core 102 into which an inert gas or gas mixture is pumped. The reaction core is then heated to a temperature between 300 and 1300°C and an electric field is formed between thejw^ejectrpdes. Alternatively, there can be many electrodes among which the electric field is formed.

If desired, a plasma can be introduced into the reaction core 102 to strengthen the electric field, as described in EP Publication EP-A-0749153. Also, if desired, oxygen gas can be introduced into the reaction core 102 thereby to cause oxidation to occur during the cleaning process.

The various embodiments of the present invention provide in-situ cleaning for most semiconductor processing machines, such as those providing high temperature annealing, high temperature oxidation, photoresist removal, plasma processing, chemical vapor deposition (CVD), silicon epitaxial growth, rapid thermal processing, etc. For each of these machines, the cleaning operation can occur before, during or after the manufacturing process.

Reference is now made to Fig. 3 which illustrates a first apparatus for batch processing. In this embodiment, the collector electrodes are not metal sheets as described in EP Publication EP-A-0749153. Rather, they are other semiconductor wafers, labeled 120, connected to the collector voltage source Vc. Thus, the collector wafers 120 collect the contamination impurities from the contaminated wafers, labeled 122, to be cleaned. This is particularly advantageous since semiconductor wafers easily absorb the contaminants and thus, the collector wafers 120 typically collect more contaminant than metal sheets.

Collector wafers 120 can be processed or unprocessed. It is believed that unpolished wafers are advantageous. A rough surface and one which is oxidized are also preferable, though not necessary.

In the embodiment of Fig. 3, the collector wafers 120 are placed between the contaminated wafers 122 in a comb-like manner. With a collector wafer 120 on either side of a contaminated wafer 122, each side of each contaminated wafer 122 will be cleaned. This is indicated by impurity ions 124 which flow from either side of contaminated wafers 122 towards collector wafers 120.

The wafer arrangement of Fig. 3 is placed in a transfer device (not shown) in which the collector wafers 120 are connected to collector voltage source Vc and the contaminated wafers 122 are connected to the wafer voltage source Vb. Wafer voltage source Vb is more electropositive than Vc. The transfer device is conveyed to within the reaction core 102 which is then heated to a sufficient temperature, such as in the range 400-1250 C, in the environment of a vacuum, or a low pressure gas or gas mixture, or a gas or gas mixture at atmosphere pressure. This encourages the movement of positive ions.

Collector wafers 120 can be processed or unprocessed. It is believed that unpolished wafers are advantageous. A rough surface, and for one which is oxidized is also preferable, though and necessary.

The transfer device can be any suitable device which separately connects the contaminated wafers 122 and the collector wafers 120 to their respective power sources and which isolates the two sets of wafers from each other.

One exemplary embodiment of a transfer device 130 is shown in Fig. 4 to which reference is now made. Transfer device 130 is formed of two outer layers 132 and 134 formed of silicon and an inner isolating layer 136 formed of quartz. Outer layers 132 and 134 form the connectors to the collector wafers 120 and the contaminated wafers 122, respectively. Thus, wires 140 and 142 respectively connect voltage sources Vb and Vc to outer layers 132 and 134.

Contaminated wafers 122 are supported by slots 137 formed in lower layer 132. Collector wafers 120 are supported by slots 138 in upper layer 134. Since contaminated wafers 122 would provide a short circuit between the two layers 132 and 134 if they touched upper layer 134, upper layer 134 has large openings 139 over the locations of slots 137.

Reference is now made to Fig. 5 which illustrates an embodiment of the present invention similar to that of Fig. 3 but in which a core collector electrode 150 is mounted to the reaction core 102. Similar elements carry similar reference numerals.

Core collector electrode 150 is typically formed of a suitably conductive and heat-resistant material, such as silicon carbide or a heat-resistant metal alloy, and is formed on the outside or inside of reaction core 102. Core collector electrode 150 is connected to a core voltage source Va whose voltage level is typically between that of the wafer voltage source Vb and the first collector voltage source Vc. Thus, an electric field from the contaminated wafers 122 (at voltage level Vb) to the core collector electrode 150 (at voltage level Va) is generated in addition to the electric field existing between contaminated wafers 122 and collector wafers 120.

As long as reaction core 102 is formed of a material, such as quartz, in which the mobility of ions at temperatures above 600°C is high enough, the ions can extend through reaction core 102 and thus, as shown, core collector electrode 150 can be mounted on the outside of reaction core 102. Otherwise, core collector electrode 150 must mounted on the inside of reaction core 102.

For the embodiment of a quartz reaction core 102, processing device 100 must heat the reaction core 102 to a temperature at or above 600°C. In the presence of the heat, ions 124 will move out of contaminated wafers 122 towards collector wafers 120, towards reaction core 102 and, through reaction core 102, towards core collector 150. If desired, a plasma can be generated within reaction core 102 to help the movement of the ions 124.

It will be appreciated that when core collector 150 is mounted outside reaction core 102, the batch cleaning apparatus of Fig. 5 not only cleans contaminated wafers 122 but also helps to clean reaction core 102, since core collector 150 also attracts ions from within reaction core 102. This cleaning is not just surface cleaning, as provided for in the prior art, but cleaning from the bulk, as the ions move through reaction core 102 to its outside surface and onto core collector 150.

Reference is now made to Fig. 6 which illustrates an alternative embodiment of the batch apparatus of Fig. 5. In this embodiment, there are only contaminated wafers 122 attached to an at least partially conductive, transfer device 160 connected to wafer voltage source Vb. As in the embodiment of Fig. 5, core collector 150 is mounted on reaction core 102 and is connected to core voltage source Va.

In the presence of heat and the electric field generated between contaminated wafers 122 and core collector 150 (and, if desired, a plasma), ions 124 moves out of both sides of the contaminated wafers 122 and are collected by core collector 150. As in the previous embodiment, this embodiment also cleans the reaction core 102 if core collector 150 is mounted on the outside of reaction core 102.

Reference is now briefly made to Fig. 7 which illustrates cleaning apparatus for just cleaning the reaction core 102 when such is formed of quartz or of other materials. The core cleaning apparatus comprises a conductive element, such as a silicon carbide rod, connected to the wafer voltage source Vb and the core collector 150 mounted outside the reaction core 102 and connected to the core voltage source Va. In the presence of temperatures above 600°C and the voltages Vb and Va, an electric field is generated which is directed towards core collector 150. This electric -field encourages positive ions 124 within reaction core 02 to collect on collector 50, thereby cleaning reaction core 102.

The core cleaning operation can be performed between semiconductor processing operations thereby to minimize contamination of reaction core 102.

It will be appreciated that the embodiments of the batch apparatus of the present invention not only clean the contaminated wafers 122 but, if the apparatus is not removed from the reaction core 102, the batch apparatus will maintain the cleanliness of the now cleaned wafers. Thus, the present invention incorporates a storage unit having the elements of any of the embodiments provided hereinabove. The storage unit is heated to the appropriate temperature and a vacuum, and, if desired, an inert gas, is provided therewithin. In addition, the appropriate voltages are provided to the various electrodes, thereby maintaining the electric field and other elements of the clean environment.

It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined only by the claims which follow: 121010/2

Claims (1)

1. CLAIMS Semiconductor cleaning apparatus comprising: i. a heatable reaction core; ii. a core collector electrode mounted to said reaction core and connectable to a first, core voltage source; and iii. a wafer transfer device for transferring at least one semiconductor wafer to be cleaned to within said reaction core, said wafer transfer device being connectable to at least a second, wafer voltage source having a voltage level greater than the voltage level of said core voltage source. Apparatus according to claim 1 and wherein said core collector electrode is mounted to the outside of said reaction core. Apparatus according to claim 1 and wherein said wafer transfer device has a first slotted portion for receiving a first group of semiconductor wafers alternating with a second slotted portion for receiving a second group of semiconductor wafers, said first slotted portion being connectable to said wafer voltage source and said second slotted portion being connectable to a third, collector voltage source. Apparatus according to claim 3 and wherein said first semiconductor wafers are to be cleaned and said second semiconductor wafers are collector electrodes. Semiconductor cleaning apparatus comprising: i. a heatable reaction core; and ii. a wafer transfer device for transferring a plurality of semiconductor wafers to within said reaction core, said wafer transfer device having a first slotted portion for receiving a first group of semiconductor wafers to be cleaned alternating with a second slotted portion for receiving a second group of semiconductor wafers forming collector electrodes, said first slotted portion being connectable to a first voltage source and said 121010/2 second slotted portion being connectable to a second voltage source with a voltage level below that of said first voltage source. Reaction core cleaning apparatus comprising: i. a heatable reaction core; ii. a core collector electrode mounted to said reaction core and connectable to a first, core voltage source; and iii. a conductive element beatable within said reaction core, said conductive element being connectable to at least a second voltage source having a voltage level greater than the voltage level of said core voltage source. Apparatus according to claim 6 and wherein said core collector electrode is mounted to the outside of said reaction core. An apparatus according to any of claims 1 - 7 substantially as described hereinabove. An apparatus according to any of claims 1 - 7 substantially as illustrated in any of the drawings. Eitan, Pearl, Latzer & Cohen-Zedek Lawyers, Patent Attorneys & Notaries P-1200-IL