JPH0670897B2 - Device for removing heat from thin and flexible articles - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a heat transfer device including an object and a heat storage device, and more particularly to cooling a semiconductor wafer processed by ion implantation, sputtering or the like.

BACKGROUND OF THE INVENTION In process steps where a work piece undergoes high radiant flux, the heat generated within the work piece may be the limiting factor for the process. In particular, for ion implantation of semiconductor materials, it has been recognized for several reasons that the workpiece temperature has an upper limit. Where the wafer is coated with a resist layer as part of the lingraph process, the layer will degrade or deteriorate at elevated temperatures well above 100 ° C. Areas of a previously formed semiconductor wafer that have distinctly changed properties are subject to unwanted diffusion when the wafer is exposed to long radiation, or areas that have been previously implanted, etc., anneal faster. You will receive it.

Therefore, it is important to remove from the semiconductor wafer the heat generated therein as a result of the ion implantation process and the like.

It is known in the prior art to actively cool a semiconductor wafer by clamping it onto a convexly curved platen during ion implantation. The platen has a flexible thermally conductive material coating on its surface. A clamping ring that cooperates with the platen provides a compliment of the convexly curved platen.
t) is arranged so that the semiconductor wafer is tightly pressed against the surface, and the clamping further transfers thermal energy from the wafer to the active cooling means provided in the platen. Such a device is described in U.S. Pat. No. 4,282,924, assigned with the present invention.

The above-mentioned technology is based on a conduction mechanism to transfer heat. Thermal energy is generated near the outer surface of the wafer due to the kinetic energy of the incident beam that is absorbed by the wafer. Therefore, there is a potentially thermally conductive first component in the heat transfer properties of the wafer material because heat transfer must occur through the wafer material. (For simplicity, it is assumed that heat will pass through the thickness of the wafer.) Similarly, between the wafer and the platen surface in contact with the wafer, a platen with a cooling channel for the circulating cooling fluid is then placed. The platen is made of a material that is characterized by heat transfer characteristics to effect heat transfer through the interior region. There is a definite contribution to the heat transfer properties in the intermediate contact area between the wafer and the platen. Heat transfer in this region is approximately proportional to the actual contact area between the wafer and platen and inversely proportional to the average thermal conductivity of the two materials. On a microscopic scale, their surfaces are not perfectly flat and have irregular orientations. Based on material hardness and assumptions in the surface topographical distribution of each contact surface, the contact area can be calculated by microscopic measurements, but in reality it is only a fraction of the macroscopic area. . The theory of conduction heat transfer between solids in a vacuum has been developed by Cooper, Mickick and Yabanowick (Int. J, Heat and Mass Transfer. Volume 12, 279-30).
Page 0 (1969)). It is shown therein that the contact heat conduction depends on the conductivity and the actual contact area. The actual contact area depends on the surface density of the uneven surfaces of the mating surfaces and the elastic or plastic compatibility of the material.
liance) in turn. Due to the pressing force exerted on it, the irregularities will first be deformed into contact with each other, or in close conformity, creating more contact area. The desired effect of more contact area is limited by the maximum stress that can be supported by the wafer. In U.S. Pat. No. 4,282,924, the platen is composed of a high heat capacity metal body bonded to an outer layer of heat conductive compliant material in contact with the wafer, which is curved, though not otherwise specified, in a convex shape. Therefore, it is provided with a surface layer that is deformed to accommodate the small irregularities of the wafer. In this example, the match (co
There is a more geometrical attribute in the length of the heat path through the material. Its length decreases proportionally with increasing contact pressure. This effect first appears, but is quite small with the slight deformations that normally occur.

Various effects that affect the thermal impedance of the interface have been observed by providing a selectively controllable thermal impedance where it is desirable to maintain a given wafer temperature with respect to the heat sink. An example of this attribute is discussed in commonly assigned and filed US Pat. No. 4,453,080.

OBJECTS OF THE INVENTION It is therefore an object of the present invention to provide a modified apparatus for effectively cooling a semiconductor wafer during ion implantation, and in particular an apparatus for providing such cooling in a uniform manner.

SUMMARY OF THE INVENTION An improved heat transfer device for a semiconductor wafer that is subject to radiant flux includes a platen that preferably clamps the wafer about its circumference. The platen has a profile that optimizes the transfer of heat through the wafer to the platen independent of location on the surface of the wafer. The contact pressure distribution between the wafer and the platen corresponds to a surface that is uniform over the rest of the wafer for boundary conditions limited by the clamping means that clamp the wafer to the platen. Thus, the contours that match the wafer are calculated. The magnitude of the contact pressure is maximized to an acceptable magnitude without the wafer subsequently cracking.

In one embodiment, the platen comprises a rigid metal member and a compliant layer bonded thereon. Due to the properties of the conforming layer and the inner contour of the part at the joining surface, the outer contour which meets the requirements mentioned above is controlled.

In either approach, the platen preferably includes active cooling means to effectively remove heat from it.

Preferred Embodiment An overview of the present invention is best shown in FIG. 1, which illustrates a typical ion implanter. High voltage terminal 2
It is typically kept at a selectable voltage from +10 kev to +20 kev with respect to ground. Within the terminal 2 an ion source 8 and its associated power supply 10 are arranged, which are provided with extractions, probes and focusing voltages which need not be detailed for the purposes of the invention. Typically, the ion source operates in a gas handling device that requires a gaseous feed, which device selects between several gas cylinders and by controlled leakage of the selected gas. May be used to supply A high flux ion beam 18 diverging from the ion source 8 is momentum analyzed at an analysis magnet 20, exits the magnet, is shaped by a hole 22, and is further defined by a variable slit system 24. The beam is then accelerated to earth potential in the accelerating tube 26.
An optical element 28, such as a quadrupole double lens, acts on the beam to produce spatial focusing at the target surface 56 or 58. The typical device of FIG. 1 utilizes an electrostatic deflection device. The electrostatic deflector comprises a y-deflector 40 and an x-deflector 40 for directing the beam 18 over the selected target plane.
Contains 42. The corrugation applied to plates 40 and 42 to form the desired scan pattern is formed in the scanning device.
A dual channel targeting chamber 46 is provided to cover the product being processed. Beam forming slits 48 and 49 are included for each processing channel, and Faraday cages 50 and 51 for charge collection and assembly are included in the target chamber. When the automated wafer handling equipment, including supply channels 52 and 54, is activated, the two processing channels will have one semiconductor wafer through each of the two vacuum locks for staggered introduction into the target chamber. Will be introduced. The wafer handling equipment properly positions, aligns, and cools the wafer during processing and removes the processed wafer from the chamber after processing is complete. The wafer handling equipment is described in the series of applications filed therewith.

It will be appreciated that the entire area traversed by the ion beam is kept in a high vacuum, eg at a pressure typically on the order of 10 ⁻⁶ mm.hg.

As shown in FIG. 2A, it will be seen that the clamping force L applied around the wafer 62 results in a non-uniform distribution of contact forces across the entire wafer surface. This causes uneven loading on the wafer. The uneven load there is due to the platen 64 having an arbitrary convex outline 65.
Is formed by. The local stress is symbolically indicated by the vector component 66. Waha 62 and Platen 64
The thermal conductivity of the boundary between and is a function of the contact force exerted on the wafer towards the platen and the thermal and mechanical properties of the material. As noted above, this is in part a feature of both the wafer and platen microscopic materials, and the microscopic surface roughness and compressibility as well as each material that can vary in proportion to the applied force. This is due to the contact area at.
In a convex spherical platen that typically cools a wafer clamped at its periphery, such as in FIG. 2A, the relatively elevated temperature regions cause a local decrease in contact pressure during processing. We know that it will be widespread in the central region of Waha.

The optimum platen contour can be unequivocally described by a differential equation describing a uniformly loaded thin disk supported around it. This issue is discussed by Marguerre and Woernle ('Elastic Plate', Chapter 11, Blythedell Publishing Company, Waltham, Massachusetts, 1969). With reference to this,
The deflection y (r) for a thin flexible disk supported in a uniformly loaded environment is Is. Where r = radial position D = 2r ₀ (wafer diameter) t = wafer thickness p = force applied to the wafer E = elastic coefficient ν = Poisson's ratio The deviation described by the above equation (1) is , Meet the criteria for optimal contact cooling sensed during processing, eg, uniform contact pressure distribution applied to the wafer. Thus, when the wafer is clamped to the platen along the periphery of the wafer, the contact pressure between the wafer and the platen becomes uniform as long as the surface of the platen has the shape represented by the formula (1). . It is further desirable to maximize the contact force between the wafer and the platen in order to maximize heat transfer from the wafer to the platen. Maximum contact force is limited by the properties of the wafer material. Since the wafer is deformed due to the convex surface, the tangential component of stress (tangential component with respect to the normal contact force) is
Bending is forced, thereby compressing near the inner surface of the wafer and tensioning near its outer (convex) surface. The tangential component of the stress on each surface represents the magnitude that gives a range to the design parameter (in particular, the tensile stress on the outer surface).

For uniformly loaded thin flexible disks, the maximum surface pressure resulting from the deviation thus formed is: And its maximum pressure is limited by the maximum tension stress at which the wafer is safe, as in the equation below.

Where p = contact force ν = Poisson's ratio σ = stress stress on the outer surface σ _max = r ₀ , t, the maximum tension pressure that can be tolerated when given a uniformly thin flexible disk according to equation (2) , The maximum deviation is given by

Using σ _max , Becomes

The equation without dimension is given by dividing equation (1) by equation (3) which describes the desired contour for the platen.

Therefore, the equation (4) expresses the joint condition that the uniform heat transfer is carried out over the entire surface of the wafer and the maximum heat transfer from the wafer is carried out. Thus, when the wafer is clamped to the platen along the periphery of the wafer, that is, by the clamping means for pressing the periphery of the wafer, in order to make the contact pressure between the wafer and the platen uniform, the contact surface of the platen is It must have the shape represented by (4), that is, its contact surface is uniquely determined. It should be noted that equation (4) does not depend on Young's modulus, and Poisson's ratio hardly changes in various materials. Expression (4) represents a contour that can be uniformly applied to the proposed purpose. In actual use, the value of y _max imposes a constraint on the magnitude or scale of the curve, but it is also possible that different wafer materials can be accommodated without substantially sacrificing heat transfer properties. You should be careful.

Thus, in FIG. 2B, the platen 70 of the present invention has the surface contour described by equation (4). The wafer 72 is pressed against the platen 70 with a clamping force L by a peripheral clamping ring 74. Cooling channel 76
Are provided on the platen to further remove heat. The stress distribution symbolically represented by the vector component 78 is the boundary condition of the surrounding tightening and the wafer 72 and the contour 71.
It can be seen that it is uniform across the wafer because of the uniform constant force load imposed by matching and. It is assumed that the contact pressure is a value that does not change with a further increase in L, with respect to the magnitude of the constant force L ₀ for making contact over the entire wafer surface. The further increase in contact force ΔL is simply transferred to the substrate through the wafer without affecting the load distribution.

In another embodiment (FIG. 2C), the platen assembly includes a rigid platen 80 and a suitable heat transfer coating 82.
It may be formed from Platen 80 Middle contour 90
Is determined by taking into consideration the load that the wafer 86 receives and the characteristics of the compatible material 82. The load is the boundary profile 84 between the wafer 86 and the platen 80 under design load conditions,
It is imposed to meet the boundary conditions determined by clamping the wafer to the platen to match the desired contour that creates a uniform contact pressure distribution across the entire surface of the platen-wafer interface. The simplest approach is to contact a layer of conformal material of uniform thickness to the intermediate contour 90 so that the boundary contour 84 and the intermediate contour 90 are substantially identical. In the general case, the compressed conforming material will exhibit a radial thickness distribution due to increasing ambient loads,
Complex design procedures will be required. Such a design is easily accomplished by an iterative procedure in a computer model. In that computer model, the function y ₁ (r, θ), which represents the intermediate contour 90 of the bare platen, is varied to account for deviations in the compliant material layer 82 to form the desired outer surface contour. For the boundary conditions defined by the continuous tightening of the circumference, the θ dependence disappears and these functions become radially symmetrical.

562.46 Kg / cm ² for a typical silicon wafer (100 mm diameter, 525 micron thickness) with the size of a processed product
(8000psi) tension bonding stress is acceptable. A maximum deviation of 0.134 cm (0.0529 inch) is taken as the size of the contour given by equation (4). 0.471Kg / cm
A constant pressure of ² (0.67 psi) is maintained towards the aluminum (6061-T6) heat sink so contoured. It is known that its thermal contact resistance is about 65 ° C / watt / cm ² in vacuum. In one embodiment, the aluminum platen body, conformal substrate and typical wafer are:
It becomes 0.31, 2.11 and 0.33-0.074 ° C / watt / cm ² , respectively. The major contributor to the total thermal impedance is expected to be the contact impedance, but the actual value of the thermal contact resistance depends entirely on both the heat sink and wafer surface properties. Soft aluminum or indium are believed to have excellent properties for this purpose. Flexible materials offer the good property of having the advantage of being able to tolerate surface particulates.

A related aspect of the invention is that gas is introduced into the wafer-platen boundary region to a pressure approximately equal to the contact pressure to enhance heat transfer. The material in question is described in this and in a series of applications.

Other tightening devices are also conceivable. Another such clamping device has an equal clamping force L at symmetrically discrete points, ie at three points equally spaced around the circumference of the wafer, as shown in the example of FIG. In addition to the wafer. In such cases, complex saddle-shaped contours result. That is, the wafer will be transformed into a surface y (r, θ) where θ is the azimuth angle around its perimeter. Such a surface can be calculated by referring to the above-mentioned Margaree and Wohrunle. The platen 108 is provided with such a saddle-shaped profile which is further compelled by the conditions of maximum deviation. As described above, the shape of the contact surface of the platen differs depending on the tightening device, but is uniquely determined by the equation determined by the tightening condition. In this sense, the shape of the contact surface of the platen cooperates with the tightening device. Producing the necessary and complex contours can be readily obtained by using multi-axis machining techniques. As another example, the platen 108 may be formed with a layer of compliant material as in the previous embodiments, and the contours may be desired with the more complex calculations required. The advantage obtained with this discrete clamping embodiment is that the additional wafer area is accessible to the fabrication of devices on the wafer. Taking the fraction of the tightening part around the continuous width ω and the entire surface, Which is on the order of a few percent of the typical values of ω (2 mm) and r ₀ . This example is the purpose of this and a series of other applications.

Although the description is directed to obtaining a uniform contact pressure between the wafer and the heat sink member, it will be clearly understood that other desired distributions can be obtained as well. For example, temperature gradients may be needed for other special processing steps than the discussed example, and a corresponding contact force distribution could be obtained by a suitable design of the contact contour.

The description has consisted of removing heat from the semiconductor wafer during ion implantation. It will be readily appreciated that ion implantation is just one example of radiation treatment for which the present invention is suitable. Furthermore, heat transfer from thin flexible disks is closely related to heat transfer to such materials. Therefore, treatments such as heating thin flexible workpieces would also receive similar advantages according to the present invention.

The apparatus for removing heat from a flat article of the present invention is a device for processing a flat article (wafer), in which the wafer is heated by a clamping means with a specific contact surface represented by a specific equation. Being clamped on the sink means, the distribution of contact pressure between them can be made uniform, the heat transfer can be made uniform, and its particular equation does not depend on Young's modulus, Since the Poisson's ratio hardly changes with different materials, the contact surface can be uniquely determined, and the wafer can be uniquely clamped to its elastic limit regardless of different wafer materials. Furthermore, since the wafer is clamped in the heat sink means close to the elastic limit of the wafer by the clamping means, its heat transfer can be maximized and the wafer is not cracked.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a typical ion implanter including the present invention. FIG. 2A is a schematic illustration of a non-optimal profile platen and wafer clamping thereon. FIG. 2B is an explanatory diagram of the effect of the present invention. FIG. 2C is an explanatory diagram of another embodiment of the present invention. FIG. 3 is an explanatory diagram of another embodiment of tightening. [Explanation of main symbols] 62 …… wafer, 64 …… platen 65 …… contour, 66 …… vector component 70 …… platen, 71 …… contour 72 …… wafer, 74 …… tightening ring 76 …… cooling channel, 80 …… Platen 82 …… Compatible material layer, 84 …… Wheel 86 …… Wafer, 88 …… Tightening ring 90 …… Contour, 108 …… Platen

Claims (3)

[Claims] 1. An apparatus for removing heat from a flat article under radiant flow velocity, comprising: a) a heat sink means for removing transfer heat from the article, which is non-planar; A heat sink means having a spherical convex contact surface; and b) a fastening means for clamping the peripheral portion of the flat article to the contact surface, wherein the article has a radius of r ₀ and a thickness of r _0. Is a disk of t, the peripheral portion is a narrow continuous area around the disk, and the contact surface is of a general formula Where α = (1 + ν) / (5 + ν) and γ
= 2 (3 + ν) / (1 + ν), ν is the Poisson's ratio, Y _max is the maximum deviation of the disc, and y is the height of the contact surface from the radial coordinate plane at the radial coordinate r. An apparatus for removing heat from a flat article, characterized in that the clamping means cooperate with the contact surface to create a tension stress at the outer convex surface of the disc near its elastic limit. . 2. An apparatus for removing heat from a flat article according to claim 1 wherein said disk is a semiconductor wafer. 3. An ion source, and means for directing an ion beam from the ion source to the semiconductor wafer, and a vacuum chamber surrounding the ion source, means for directing the ions, the heat sink means and the front clamping means. An apparatus for removing heat from a flat article according to claim 2, wherein the heat sink means removes heat from the semiconductor wafer while being irradiated with the ion beam.