Vacuum type wafer holder

Abstract: A wafer holding device of vacuum attraction type, includes a structural member (1) having a protrusion (2) for supporting a wafer; and elastic members (3') made of a material having an elasticity modulus smaller than that of the wafer and that of the structural member, the elastic members being distributed on a wafer attraction plane of the structural member.

Description: This invention relates to a vacuum chuck and, more particularly, to a vacuum attraction type wafer holding device usable in a semiconductor device manufacturing exposure apparatus, for holding a wafer by attracting a bottom face of the wafer through vacuum attraction. In the field of semiconductor device manufacturing apparatuses, for transfer of a semiconductor device pattern of a mask onto a semiconductor wafer, the wafer is held by a wafer holding device. Generally, when a wafer is held by a wafer holding device and if there is a large contact heat resistance at the contact surface between the wafer and the holding device, the heat generated in the wafer by application of exposure energy, for the mask pattern transfer to the wafer, is not easily transmitted from the wafer to the holding device. This causes temperature rise in the wafer and thermal deformation of it and, thus, leads to degradation of the pattern transfer precision. Particularly, in vacuum attraction type wafer holding devices, since substantially no gas remains in the space between the wafer and the holding device except the contact surface therebetween, there occurs a considerable temperature rise in the wafer. Thus, the degradation of the pattern transfer precision is notable. When a vacuum attraction type wafer holding device is used in a proximity type exposure apparatus wherein a mask and a wafer are placed with a small clearance of about several ten microns, if there is a large contact heat resistance, the temperature rise in the wafer causes a temperature rise in the mask. If this occurs, thermal distortion is produced also in the mask and, therefore, the pattern transfer precision is degraded considerably. Conventionally, a vacuum attraction type wafer holding device comprises a main structural member (table) with a wafer contact surface finished to provide a good flatness for ensuring flatness of a wafer as the same is attracted to the attracting surface. The main structural member is made of a rigid material with a large elasticity modulus for flatness correction, for correcting any warp of the attracted wafer. Thus, the contact heat resistance of such a main structural member may be small. In this type of wafer holding device, however, even the finished wafer contact surface has small surface irregularity or waviness. Also, the bottom face of each wafer has small surface irregularity or waviness. Therefore, when the bottom face of the wafer is attracted to the wafer contact surface, the actual area of the wafer contact surface is smaller than the apparent area of the same. Thus, the contact heat resistance is not so small and it is not easy to completely prevent degradation of the pattern transfer precision. EP-A-0298564 discloses a wafer holding device for holding a wafer through vacuum attraction, said device comprising: a body portion having a plurality of protrusions for supporting the wafer; elastic members made of a material having a modulus of elasticity smaller than that of said protrusions and arranged such that when the wafer is not attracted to said body portion, each elastic member is in a state in which it projects beyond the top ends of said protrusions, and when the wafer is held through vacuum attraction and is supported by said protrusions, said elastic members are deformed. It is a primary object of the present invention to provide a wafer holding device, particularly of vacuum attraction type, with which the contact heat resistance can be made small. It is another object of the present invention to provide a wafer holding device, particularly of vacuum attraction type, with which enhanced pattern transfer precision is assured. According to the invention there is provided a wafer holding device having the features recited in claim 1. In a wafer holding device of the present invention, a wafer attracted thereto through vacuum attraction is held while being in contact with the elastic members. Since these elastic members have a modulus of elasticity smaller than that of the wafer, even if the bottom face of the wafer has small surface irregularity or waviness, the contact surface of each elastic member can deform in compliance with the surface irregularity or waviness. As a result, a small contact heat resistance at the contact surface is ensured. Further according to the invention there is provided a method of manufacturing micro-devices having the steps recited in claim 2. In the accompanying drawings: - Figure 1 is a top plan view of a wafer holding device of vacuum attraction type according to a first embodiment of the present invention; Figure 2 is a sectional view taken on a line A - A' in Figure 1; Figure 3 is an enlarged view showing a protrusion and an associated elastic member before a wafer is attracted thereto; and Figure 4 is an enlarged view showing the protrusion and the elastic member of Figure 3, after the wafer is attracted thereto. An embodiment of the present invention will now be explained in conjunction with the attached drawings. Wafer holding device of vacuum attraction type according to this embodiment includes a main structural member 1 which is made of a rigid material such as stainless steel or ceramics, for example, having a large elasticity modulus, for correction of warp of a semiconductor wafer to be attracted thereto (see Figure 3). The main structural member 1 has an outer circumferential wall 1a formed as a unit therewith. On the upper surface of the main structural member as encircled by the outer circumferential wall 1a, which surface is to be opposed a wafer to be attracted, there are provided plural protrusions 2, of a number of twelve in this example, each having a pin-like shape. These protrusions 2 have the same height (level) and are formed as a unit with the main structural member 1. Also, these protrusions are disposed with good symmetry. Each protrusion 2 has an elastic member 3 which surrounds the associated protrusion The elastic members 3 have the same height (level). The main structural member 1 has a vacuum port 4 formed therein for communicating the central portion of the upper surface of the main structural member with a vacuum source, not shown. Each elastic member 3 is made of a material having an elasticity modulus smaller than that of the main structural member 1 and that of a wafer 5 to be attracted. Examples of usable material are: synthetic resin such as polypropylene, polyester, nylon, silicon rubber; synthetic rubber and the like. Further, a metal such as indium or the like which easily cause plastic deformation may be used, if within the range of elastic deformation. As seen in Figure 3, the elastic member 3 has a height D + d (mm) which is higher than the height D (mm) of the protrusion 2, by an amount d (mm). In other words, the elastic member 3 projects beyond the top of the protrusion 2 by d (mm). Each of the protrusion 2 and the elastic member 3 has a top surface (contact surface) having been finished to obtain a predetermined surface flatness, for the contact with an attracted wafer 5. The wafer attracting operation will now be explained by reference to Figures 3 and 4. Before start of wafer attracting operation, a semiconductor wafer 5 to be attracted is conveyed by means of a conveying hand (not shown) to a position such as shown in Figure 3 whereat the bottom face of the wafer 5 contacts the contact surface of the elastic member 3. After this, the vacuum port 4 is opened to the vacuum source (not shown) and the vacuum attraction to the bottom face of the wafer 5 starts. Here, if the ambience pressure (e.g. atmospheric pressure) at the upper face side of the wafer 5 is T (Torr), then the gas in the space enclosed by the main structural member 1, the outer circumferential wall 1a and the wafer 5 is evacuated by the vacuum source until the pressure in that space (i.e. attraction pressure) becomes equal to t (Torr) which is zero (0), for example. Thus, a pressure difference of T - t (Torr) is produced between the top surface and the bottom surface of the wafer 5. As a result, if the wafer attraction area of the wafer holding device is Sw, then an attraction force of (T - t)/760 x 10 5 x Sw (Pa) is applied to the wafer 5. On the other hand, as the wafer is attracted toward the upper surface of the main structural member 1 by means of this attraction force, the elastic member 3 is contracted by an amount corresponding to the quantity of displacement of the wafer 5. As a result, a repulsive force of the elastic member 3 is applied to the bottom surface of the wafer 5 in proportion to the displacement of the wafer. Therefore, when the wafer 5 is displaced by an amount corresponding to the difference d (mm) (where d > 0) between the height (D + d (mm)) of the elastic member 3 and the height (D (mm)) of the protrusion 2 and if the Young's modulus of the elastic member 3 is E (Pa) and the contact surface area between the wafer 5 and the elastic members 3 (total) is Se, then the repulsive force F to be applied to the wafer 5 is expressed as: F = E x d/(D + d) x Se (Pa) It is seen therefrom that, in order to assure engagement of the bottom surface of the wafer 5 with the contact surface of the protrusion 2 against the repulsive force F, the level difference d (mm) should satisfy a relation (3) such as follows: E x d/(D + d) x Se < (T - t)/760 x 10 5 x Sw On the other hand, in order to retain a space for allowing contraction of the elastic member 3, between the wafer attraction area Sw and the contact surface area Se, there should be a relation: 0 < d ≤ (Sw - Se)/Se x D It follows that, if 760·E·Se > (T - t)·10 5 ·Sw, the relation (3) is rewritten into a relation (5), below, and therefore if the level difference d is so set as to satisfy the relation (4) as well as the relation (5), the attracting operation can be performed without problem: 0 < d < [(T-t)x10 5 ]/[760·E·Se/Sw-(T-t)x10 5 ]·D If on the other hand 760·E·Se ≤ (T - t)·10 5 · Sw, the attracting operation can be performed without problem provided that the level difference d is so set as to satisfy the relation (4). After the attracting operation is completed in the manner described, the bottom face of the wafer 5 is vacuum attracted to and held by the wafer holding device, while being in contact to the contact surfaces of the protrusions 2 and the elastic members 3. Although the contact surface of each protrusion 2 has been finished, it may have small surface irregularity or waviness. Also, the bottom face of the wafer 5 may have small surface irregularity or waviness. Therefore, usually it is not possible to attain small contact heat resistance at the contact surface of the protrusion 2. Since, however, each elastic member 3 is made of a material having an elasticity modulus smaller than that of the wafer 5, the contact surface thereof can easily deform in compliance with the surface irregularity or waviness of the bottom face of the wafer 5. As a result, the contact heat resistance at the contact surface of the elastic member can be made very small. Accordingly, when in this state an exposure energy is applied to the wafer 5 for lithographic pattern transfer to the wafer, for semiconductor device manufacture, any heat applied to or produced in the wafer can be easily transmitted to the main structural member 1 and, therefore, degradation of the pattern transfer precision due to heat generation can be prevented effectively. This has been confirmed by the inventors of the subject application. Namely, the inventors prepared a wafer holding device wherein its main structural member was made of stainless steel and its elastic members were made of silicon rubber. The height D of the main structural member 1 was 1 (mm), and the height D + d of each elastic member 3 was 1.05 (mm), wherein the level difference d = 0.05 (mm). After a wafer having a relatively good surface flatness was attracted, the contact heat resistance R1 was measured. The result is that: R1 = 1 x 10 -3 (K·m 2 /W) Also, the contact heat resistance R2 of a comparative example device without any elastic member was measured. The result is that: R2 = 1 x 10 -2 (K·m 2 /W) It is seen therefrom that, in accordance with the present embodiment, the contact heat resistance can be reduced to one-tenth (1/10) of that of the conventional wafer holding device and also that the temperature rise in the wafer can be reduced to one-tenth. As a result, the pattern transfer precision can be improved significantly. While in the present embodiment each protrusion has a columnar pin-like shape as illustrated in Figure 1, it is not limited to the disclosed form. Prismatic pin-like shape may be adopted, as an example. In the foregoing embodiment the protrusions and the elastic members are disposed with good symmetry. However, they may be disposed asymmetrically. Since however a wafer usually has a circular shape, the symmetrical structure of elastic members can ensure better flatness precision of the wafer after it is attracted. Further, in the foregoing embodiment, each elastic member has an associated protrusion. While the invention has been described with reference to the structure disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the scope of the following claims.

Claims: A low thermal resistance wafer holding device for holding a semiconductor wafer (5) through vacuum attraction, said device comprising: a body portion (1) comprising a peripheral wall (1a); a central cavity surrounded by said wall (1a); a plurality of protrusions (2) within the central cavity for supporting the wafer; elastic members (3) made of a material having a modulus of elasticity smaller than that of said semiconductor wafer and that of said protrusions (2) and arranged such that when the wafer is not attracted to said body portion, each elastic member is in a state in which it projects beyond the top ends of said protrusions, and when the wafer is held through vacuum attraction and is supported by said protrusions, said elastic members are deformed into thermally conducting contact with the wafer, the device being characterised in that said peripheral wall (1a) is a substantially rigid wall whose upper edge is at the same level as the ends of the protrusions (2) and is capable of forming a seal with a peripheral region of a wafer, and in that each of said protrusions (2) consists of a pin (2) and wherein each elastic member (3) is provided to surround a respective pin (2) so as to be guided thereby during deformation. A method of manufacturing semiconductor devices, comprising the steps of: placing a semiconductor wafer on a wafer holding device according to claim 1; applying a vacuum to the wafer holding device to deform said elastic members whereby the wafer is held through vacuum attraction to the wafer holding device and supported by said protrusions; applying an exposure energy to the wafer held by the wafer holding device to transfer a semiconductor device pattern of a mask on to the wafer, and fabricating a semiconductor device from the exposed wafer.

Inventors: EBINUMA RYUICHI, HARA SHINICHI, MARUMO MITSUJI, SAKAMOTO EIJI, EBINUMA, RYUICHI, HARA, SHINICHI, MARUMO, MITSUJI, SAKAMOTO, EIJI, EBINUMA, RYUICHI, C/O CANON KABUSHIKI KAISHA, HARA, SHINICHI, C/O CANON KABUSHIKI KAISHA, MARUMO, MITSUJI, C/O CANON KABUSHIKI KAISHA, SAKAMOTO, EIJI, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: H01L21
IPC codes: C30B25, C30B31, H01L21