Semiconductor substrate holder for chemical-mechanical polishing comprising a movable plate

Abstract: The substrate holder (20) comprises a movable plate (23) elastically mounted inside a main body (22). With the substrate holder (20) the polishing operation can be performed in two basic operation modes corresponding to two different vertical end positions of the movable plate (23). In a first (downward) mode the movable plate (23) stays in mechanical contact with the substrate (12) whereas in a second (upward) mode an air cushion is generated in a chamber (29) between the movable plate (23) and the substrate (12) for pressurizing the substrate (12) onto the polishing pad.

Description: The present invention relates to a semiconductor substrate holder according to the preamble of claim 1 for holding a substrate to be polished whereby the substrate is held and the held substrate is pressed against a polishing pad. An example of such a holder is disclosed by EP 881 039 A. In the processing of integrated semiconductor wafers and integrated circuits many process steps require a subsequent flattening or planarizing of the semiconductor topographical structure. Therefore it is highly important to provide a method and an apparatus for polishing and flattening the surface of the semiconductor substrate to a high flatness degree. In order to achieve the extent of planarity and thickness homogeneity necessary to produce ultra high density integrated circuits, chemical-mechanical planarization processes are employed. These chemical-mechanical planarization or polishing (CMP) processes involve in general pressing a semiconductor wafer against a moving polishing surface that contains an abrasive material or is wetted with a chemically reactive, abrasive slurry. The slurries are either basic or acidic and may contain alumina, silica or other abrasive particles. Typically, the polishing surface is a planar pad made of soft, porous material, such as polyurethane foam or non-woven fabric, and the pad is generally mounted on a planar platen. A major obstacle for the achievement of a high planarity and a high thickness homogeneity of the surface of a layer to be polished lies in the fact that either the semiconductor substrate below the layer to be polished or the polishing pad may contain thickness or surface variations due to warping or waviness of the wafer or the polishing pad. These variations would normally lead to corresponding local variations in the pressure applied to the semiconductor substrate during polishing and thus to local variations of polishing rates. The construction of a semiconductor substrate holder should therefore provide facilities which would allow to compensate for these inhomogeneities. A simple design of a substrate holder includes a rigid metal plate for pressing the semiconductor substrate against the polishing pad. This standard construction, however, does not allow for any compensation measures for inhomogeneities of substrate thickness or polishing pad thickness. In the US 6,012,964 a semiconductor substrate holder (carrier) is described which is constituted by a housing, a carrier base, a retainer ring, a sheet supporter, a hard sheet and a soft backing sheet. The sheet supporter is formed by a supporter body portion having an air opening communicating with an air outlet/inlet of the carrier base, a flexible diaphragm and an outer ring. A wafer is uniformly pressed by the air pressure in the pressure chamber and fluctuation in the force pressing against the outer peripheral rim of the wafer caused by the wear of the retainer ring is countered by the diaphragm. Also presented in this document are embodiments in which holes are formed in the hard sheet and the soft backing sheet in the area of the wafer centre by which it becomes possible to apply an additional back pressure for locally enhancing the polishing rate. These embodiments are, however, applicable only in case of specific known thickness variations of the semiconductor substrate and/or the polishing pad. In the US 5,791,973 and the US 6,074,289 a substrate holding apparatus is described which comprises a rotary shaft, a substrate holding head in the form of a disc which is provided integrally with the lower edge of the rotary shaft, a sealing member in the form of a ring which is made of an elastic material and fastened to the peripheral portion of the lower face of the substrate holding head, and a guiding member in the form of a ring which is fastened to the back face of the substrate holding head to be located outside the sealing member. A fluid under pressure, preferably air, is introduced into a fluid flow path formed in the rotary shaft from one end thereof and supplied to a space from the other end of the fluid flow path so as to form an air cushion on one side of the substrate and to press the substrate against the polishing pad. Due to the fact that the semiconductor substrate can be deformed in accordance with the surface of the polishing pad and/or the semiconductor substrate the semiconductor substrate can be pressed onto the polishing pad with a locally constant contact pressing force so that also the polishing rate is locally constant over the entire wafer. However, with this design it is not possible to introduce a specific local polishing profile by a local variation of the pressing force and hence the polishing rate. The only way to achieve this would be the incorporation of a plurality of chambers to be supplied with fluids of varying pressure which appears too complicated. In the introductory portion of the above mentioned US 5,791,973 there is further described with respect to Fig.16 another design of a semiconductor substrate holder wherein an elastic polishing pad is adhered to the top surface of a table. The bottom portion of a substrate holding head is formed with a recessed portion. The substrate is solidly supported by a plate-like elastic member which can elastically deformed in the recessed portion of the substrate. The substrate holding head, elastic member and the substrate define a hermetically sealed space into which a gas under controlled pressure is introduced through a gas supply path. The gas under pressure introduced into the hermetically sealed space presses the substrate solidly supported by the elastic member against the polishing pad, so that the pressure on the upper face of the substrate achieves equal polishing. A disadvantage of this embodiment is the rather complicated mechanism of mounting and dismounting the substrate to the elastic member. The document EP 0 881 039 A2 discloses a wafer polishing apparatus with a retainer ring wherein a rubber sheet is arranged between a head body and a retainer ring of a wafer holding head. The wafer polishing apparatus is supposed to hold a semiconductor substrate and to polish it by chemical-mechanical polishing (CMP). The head body of the apparatus holds the semiconductor substrate in a predetermined position relative to the head body and the head body comprises a base plate and a ring like elevation provided thereon and a movable plate provided inside the ring like elevation for pressurizing the semiconductors substrate from inside the ring like elevation towards an underlying polishing pad. The movable plate is mounted to the main body such that it is movable in a direction towards and away from the semiconductor substrate and a support member is provided on a portion of the inner wall of the ring like elevation and the support member comprises a support surface for supporting the semiconductor substrate. Two O-rings air-tightly close a space between the periphery of a rubber sheet, which is located above the retainer ring and the head body. When a pump supplies the compressed air to the space, the periphery of the rubber sheet is elastically deformed to press the retainer ring under uniform pressure. It is therefore an object of the present invention to provide a semiconductor substrate holder for an apparatus for polishing semiconductor substrates wherein the semiconductor substrate holder allows polishing of a semiconductor surface with excellent uniformity over the entire surface area and which also allows the introduction of a specific wanted polishing profile. This object is achieved by a holder having the features of claim 1. Exemplary and advantageous embodiments are indicated in the dependent claims. With the semiconductor substrate holder according to the present invention the polishing operation can be performed in two basic operation modes corresponding to two different vertical positions of the movable plate. In a first mode of operation the movable plate is in a lower position where it is in direct mechanical contact with the semiconductor substrate, preferably with a soft backing film in-between. The first mode of operation corresponds therefore to the standard carrier design. In the first mode of operation it is possible to vary the polishing profile in a predetermined manner, e.g. by applying a predetermined pressure to predetermined areas of the semiconductor substrate. This can be accomplished by means of a first fluid supply path formed through the movable plate and outlet openings formed in the lower surface of the movable plate and the backing film which outlet openings are connected with the first fluid supply path. Since the movable plate is in direct mechanical contact with the semiconductor substrate a pressure is exerted only on those substrate portions which are opposite the outlet openings of the movable plate when a fluid is supplied to the outlet openings. In a second mode of operation the movable plate is in an upper position where it is not in direct mechanical contact with the semiconductor substrate. In this position a chamber is formed between the movable plate and the semiconductor substrate. By means of the first fluid supply path and the outlet openings formed in the movable plate a fluid, preferably air, can be supplied to the chamber so as to form an air cushion on one side of the substrate and to press the substrate against the polishing pad. This mode of operation allows a homogeneous pressurization of the movable plate and corresponds to the "cushion mode" as known from the above-mentioned prior art documents. In a preferential embodiment the first and second modes of operation are characterized by predetermined end positions of the movable plate wherein in a first end position corresponding to the first mode of operation a lower surface of the movable plate is in contact with the backside of said semiconductor substrate and in a second end position corresponding to the second mode of operation the lower surface of the movable plate is not in contact with the backside of the semiconductor substrate. The end positions of the movable plate can be defined by an abutment member which can be provided on an inner portion of the ringlike elevation. The abutment member may comprise two abutment surfaces corresponding to said two end positions and the movable plate may comprise an extension acting in combination with said abutment member. On an inner portion of the ringlike elevation a support member is formed, which comprises a support surface for supporting the semiconductor substrate. The support surface is flush with the surface of the movable plate in its first end position. In a preferred embodiment the above mentioned abutment member is formed integral with the support member. In a preferential embodiment the movable plate is actuated by applying a fluid pressure on one side thereof. The movable plate can be mounted on the main body by an impermeable sealing member like a membrane, so that a chamber is formed by the inner walls of the movable plate and the main body and the membrane. A second fluid supply path can be provided for supplying a fluid into this chamber for pressurizing the movable base plate and thereby effecting the movement of the movable base plate. Preferably the sealing member is provided with elastic properties like a spring such that a resting position of the spring corresponds to one of the first or second end positions of the movable plate. In the following specific embodiments of the semiconductor substrate holder of the present invention and the different modes of operation are described with respect to the accompanying drawings, in which Fig.1 depicts a schematic cross-sectional view of a semiconductor substrate holder according to a first embodiment of the present invention together with a polishing pad in a state according to the first mode of operation; Fig.2 depicts a schematic cross-sectional view of a semiconductor substrate holder according to the first embodiment of the present invention in a state according to the second mode of operation; Fig.3 depicts a schematic cross-sectional view of a second embodiment of the semiconductor substrate holder of the present invention in a state according to the second mode of operation. In Fig.1 is a cross-sectional view of a semiconductor substrate holder to be polished according to a first embodiment of the present invention, in which are shown: a rotatable table 10 having a flat surface which is made of a rigid material and an elastic polishing pad 11 adhered to the top surface of the table 10. Above the table 10 is provided a substrate holder for holding a semiconductor substrate 12. The substrate holder 20 comprises a rotary shaft 21 rotated by rotary driving means (not shown) and a main body 22 in the form of a disc provided on the lower edge of the rotary shaft 21. The main body 22 is comprised of a base plate 22.1 and a ringlike elevation 22.2 thereon. A downward vertical force can be exerted on the rotary shaft 21 and transmitted to the main body 22 by an apparatus not shown in this Figure. Inside the ringlike elevation 22.2 a disc-like movable plate 23 is affixed to the main body 22, i.e. to a portion of the main body 22 corresponding to the ringlike elevation 22.2 thereof, by an elastic sealing membrane 24. Between the movable plate 23 and the main body 22 a chamber 25 is formed wherein the walls of the chamber 25 are constituted by portions of the inner walls of the movable plate 23 and the main body 22 and by the elastic membrane 24. This chamber 25 can be supplied with a fluid like air via a fluid supply path 25.1 formed in a portion of the wall of the main body 22 in order to generate a pressure P1 inside the chamber 25 which is higher than atmospheric pressure to thereby pressurize the movable plate 23 in a downward direction. It is also possible to evacuate the chamber 25 via the fluid supply path in order to generate a pressure inside the chamber 25 which is lower than atmospheric pressure to thereby suck the movable plate 23 in an upward direction. Alternatively it would be also possible to omit the chamber 25 and the fluid supply path 25.1 and to exert a force on the movable plate 23 merely by mechanical means. On an outer surface of the main body 22, i.e. on the ringlike elevation 22.2 a retaining ring 26 is provided which can be formed integral with the main body. On a portion of the inner wall of the ringlike elevation 22.2 a ringlike support member 27 is provided which comprises a corresponding ringlike support surface for receiving the back surface of the semiconductor substrate 12 thereon. The radial width of the ringlike support member 27 and thus of the support surface is preferably in the range of 2 - 10 mm. The support member 27 is provided such that the height difference between the support surface and the surface of the retaining ring 26 is less than the height of the substrate 12 by an infinitesimal amount. The lower surface of the movable plate 23 and the support surface of the support member 27 can be covered with a soft backing film 28. The support member 27 can also be formed integral with the main body 22. The support member 27 has also the function of an abutment member 27 for defining the end positions of the movable plate 23. For this purpose the abutment member 27 comprises a recess 27.1 on an inner wall thereof, wherein an extension 23.1 of the movable plate 23 engages and is movable therein between upper and lower end faces of the recess 27.1. Alternatively it would also be possible to provide an abutment member which is not formed integral with the support member 27. The elastic spring-like membrane 24 can be mounted such between the main body 22 and the movable plate 23 that it is in a resting position when the movable plate 23 is in its upper (second) end position and that it is in an elongated position when the movable plate 23 is in its lower (first) end position. In order to bring the movable plate 23 to the lower position air is supplied to the chamber 25 and the movable plate 23 is pressurized in a downward direction against the force of the elastic spring-like membrane 24. In Fig.1 the semiconductor substrate holder 20 is shown in the downward (first) position wherein in Fig.2 the semiconductor substrate holder 20 is shown in the upward (second) position. The first mode of operation as depicted in Fig.1 corresponds to a standard carrier design as it is known from the prior art. In this operation mode it is possible to generate a predetermined polishing profile over the wafer by applying a pressure on pre-selected portions of the semiconductor substrate. For this purpose a fluid supply path 25.2 is provided which includes a tube extending from an opening in the wall of the main body 22 into an inner chamber 23.3 of the movable plate 23. From the inner chamber 23.3 connection paths are formed to connect the inner chamber 23.3 with openings 23.2 formed in the rear surface of the movable plate 23 and the backing film 28. In the present case two openings 23.2 are formed symmetrically with respect to the center of the movable plate 23. By applying a pressure P2 to the fluid supply path 25.2 and thus to those portions of the semiconductor substrate 12 opposite to the openings 23.2 there can be adjusted a radial gradient of the pressing force and of the polishing rate. Due to the pressure P2 the substrate 12 is deformed underneath the openings 23.2. The pressure P2 which is applied to the fluid supply path 25.2 can be chosen such that it is higher than atmospheric pressure which is exerted on the backside of the semiconductor substrate 12 due to the vertical force applied to the rotary shaft 21 in order to generate a higher polishing rate in the area of the openings 23.2. Alternatively a pressure P2 can be applied, e.g. by evacuating the chamber 23.3 through the fluid supply path 25.2, which pressure P2 is lower than atmospheric pressure of the movable plate 23 on the back surface of the substrate in order to generate a lower polishing rate in the area of the openings 23.2. In the second operation mode of the movable plate 23 which is shown in Fig.2 a homogeneous pressurization over the substrate area and hence a homogeneous polishing profile can be generated. In this mode the fluid supply path 25.2 serves for establishing an air cushion in a chamber 29 surrounded by the substrate 12, the movable plate 23 and the supporting member 27. In this case the openings 23.2 serve as distribution openings for distributing the air which is supplied via the fluid supply path 25.2 within the chamber 29. The pressure P2 can be chosen such high that a clearance will be formed between the substrate 12 and the support member 27 so that a part of the pressurized air can leak out of the chamber 29 through this clearance. Alternatively a third fluid supply path 25.3 can be provided which extends from a through hole in the wall of the main body 22 and a through hole in the support member 27 into the chamber 29. In a part of the third fluid supply path 25.3 outside the main body 22 an adjustable valve (not shown) can be implemented by which a controlled leak out of air out of the chamber 29 can be achieved. In addition this third fluid supply path 25.3 can be used to generate a pressure gradient in the air cushion in the chamber 29 and a corresponding inhomogeneity of the polishing rate either by supplying air to the chamber 29 or by sucking out air therefrom. Figure 3 shows an alternate embodiment of a semiconductor substrate holder 20 in which a fourth fluid supply path 25.4 is provided which should fulfil the same function as was previously described with respect to the third fluid supply path 25.3. The fourth fluid supply path 25.4 includes a tube extending from an opening in the wall of the main body 22 into an outer area of the inner chamber 23.3 of the movable plate 23. This outer area is separated from the inner area by a concentric sealing ring 30. From the outer area an opening 23.4. extends into the chamber 29. The fourth fluid supply path 25.4 or the fluid supply path 25.3 could be used also for supplying a cleaning agent like water to the chamber 29 in order to clean the inner surfaces of the substrate holder 20 from slurry waste. The fourth fluid supply path 25.4 may be employed instead or in addition to the third fluid supply path 25.3. By supplying a sufficient pressure through the third and/or the fourth fluid supply paths and at the same time adjusting a reduced pressure P2 in the fluid supply path 25.2 a deformation of the substrate occurs such that the polishing rate at substrate edge is high and the polishing rate in the center of the substrate is low. In a polishing process a time division between the two operation modes will be employed in that in a part of the polishing time the first operation mode will be applied and in another part of the polishing time the second operation mode will be carried out. In a preferred embodiment the touch down and lift off of the wafer onto the polishing pad is performed with the movable plate in the lower position in order to prevent high polishing rates at the outer wafer edge during these phases of the polishing process. In another preferred embodiment the retaining ring can be moved relative to the support surface of support member 27, in order to influence the polishing rate at the wafer edge in both operation modes.

Claims: Semiconductor substrate holder (20) for holding a semiconductor substrate (12) to be polished by chemical-mechanical polishing (CMP), said holder comprising a main body (22) for holding a semiconductor substrate (12) in a predetermined position relative to said main body (22), said main body (22) comprising a base plate (22.1) and a ringlike elevation (22.2) provided thereon, and pressurizing means for pressurizing said semiconductor substrate (12) from inside said ringlike elevation (22.2) towards an underlying polishing pad (11), said pressurizing means includes a movable plate (23) provided inside said ringlike elevation (22.2), said movable plate (23) is mounted to said main body (22) such that it is movable in a direction toward and away from said semiconductor substrate (12) characterized in that said movable plate (23) is movable between a first end position and a second end position, wherein in said first end position a main surface of said movable plate (23) is in contact with the backside of said semiconductor substrate (12), and wherein in said second end position said main surface of said movable plate (23) is not in contact with the backside of said semiconductor substrate (12). Semiconductor substrate holder according to claim 1, characterized in that a first fluid supply path is provided for supplying a fluid into a chamber (29) between said movable plate (23) and said semiconductor substrate (12). Semiconductor substrate holder according to claim 2, characterized in that said first fluid supply path (25.2) extends through said movable plate (23). Semiconductor substrate holder according to claim 3, characterized in that said first fluid supply path (25.2) includes a chamber (23.1) formed inside said movable plate (23) and a plurality of openings (23.2) from said chamber (23.1) to said chamber (29) between said movable plate (23) and said semiconductor substrate (12). Semiconductor substrate holder according to claim 1, characterized in that an abutment member is provided on a portion of the inner wall of said ringlike elevation (22.2), said abutment member comprising two abutment surfaces corresponding to said two end positions, wherein said movable plate (23) comprises an extension (23.1) acting in combination with said abutment member. Semiconductor substrate holder according to one of the preceding claims, characterized in that a second fluid supply path (25.1) is provided for supplying a fluid into a space between said movable plate (23) and said base plate (22.1) for pressurizing said movable plate (23) and thereby effecting the movement of said movable plate (23). Semiconductor substrate holder according to one of the preceding claims, characterized in that said movable plate (23) is connected to said base plate (22.1) with a flexible connecting member (24). Semiconductor substrate holder according to claims 6 and 7, characterized in that said flexible connecting member (24) is an impermeable sealing membrane so that a chamber (25) is formed by the said base plate (22.1), said movable plate (23) and said sealing membrane. Semiconductor substrate holder according to claim 7, characterized in that said flexible connecting member (24) is a spring-like member, said spring member (24) connected such with said movable plate (23) and said base plate (22.1) that in one of said two end positions of said movable plate (23) said spring-like member is in its resting position. Semiconductor substrate holder according to one of the preceding claims, characterized in that a support member (27) is provided on a portion of the inner wall of said ringlike elevation (22.2), said support member (27) comprising a support surface for supporting the semiconductor substrate (12). Semiconductor substrate holder according to claim 5, characterized in that said support member (27) is a part of said abutment member or directly connected with said abutment member or formed integral with said abutment member. Semiconductor substrate holder according to one of the claims 2 to 12, characterized in that a third and/or fourth fluid supply path (25.3, 25.4) is provided for supplying a fluid into an outer area of said chamber (29). Apparatus for polishing a semiconductor substrate (12) by chemical-mechanical polishing, said apparatus comprising a semiconductor substrate holder (20) according to one or more of the preceding claims, and a polishing pad (11).

Inventors: HOLLATZ MARK, LAHNOR PETER, HOLLATZ, MARK, LAHNOR, PETER
Applicants: INFINEON TECHNOLOGIES AG
Application year: 2001
Main classification: B24B37
IPC codes: B24B41, B24B37, H01L21