Abstract:
A heating chuck assembly for wafer processing is provided, including heating modalities for same. The assembly generally includes hermetically sealed opposingly paired discs, and housed therebetween, a ceramic element interposed between first and second heating elements. The first heating element is adjacent a first disc of the opposingly paired discs so as to be paired therewith, the second heating element adjacent a second disc of the opposingly paired discs so as to be paired therewith. The assembly further contemplates the inclusion of temperature sensing/measuring and controlling devices, in the context of a heating chuck system.

Description:
[0001]     This is a regular application filed under 35 U.S.C. §111(a) claiming priority under 35 U.S.C. §119(e) (1), of provisional application Ser. No. 60/692,114, having a filing date of Jun. 20, 2005. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention generally relates to thermal processing of semiconductor wafers, more particularly, to a temperature controlled semiconductor wafer chuck assembly equipped and/or configured so as to promote thermal uniformity during high thermal wafer processing, i.e., up to about 600° C.  
       BACKGROUND OF THE INVENTION  
       [0003]     Semiconductor device manufacturing, more particularly, integrated circuit fabrication, is dependent upon a requisite supply of semiconductor wafers. The market is large, with capital equipment spending totaling $29.9 billion in 2003, a 7.9 percent increase from 2002 (see http://www.future-fab.com/welcome.asp).  
         [0004]     A typical wafer is made of extremely pure silicon that is grown into mono-crystalline ingots up to about twelve inches in diameter, using, e.g., the Czochralski process. The resulting ingots are thereafter sliced into wafers of select thickness, e.g., 0.75 mm, lapped, etched, and polished. Once prepared, numerous wafer processing steps, e.g., front end processing, back end processing, testing, and, packaging, are necessary to produced the desired semiconductor integrated circuit.  
         [0005]     Typical front end processing includes preparation of the wafer surface, silicon dioxide growth, patterning and subsequent implantation or dopant diffusion to obtain sought after electrical properties, and growth/deposition of a gate dielectric or isolating insulation. Having “created” the devices, circuit forming interconnections are required. This back end process generally involves depositing layers of metal and insulating material, and etching the deposition into select patterns. Upon completion of the back end processing, the semiconductor devices are subject to a variety of electrical tests to ascertain, i.e., verify, functionality. The proportion of devices on the wafer found to satisfactorily perform is referred to as “yield.” 
         [0006]     In furtherance of executing one or more of the subject wafer processing steps, the work piece, i.e., a wafer, is commonly held by a wafer chuck, i.e., a chuck or shafted pedestal assembly. Oftentimes it is necessary to control the temperature of the wafer during processing, and for this purpose, the semiconductor wafer chuck can be a temperature controlled chuck. Heated chucks and shafted pedestal heaters are ideally used in critical in-situ wafer processing applications where proximity to the wafer requires precise thermal, electrical, metallurgic and mechanical specifications.  
         [0007]     A variety of known teachings are alleged to generally improve semiconductor wafer surface temperature uniformity during select wafer processing operations. For example, U.S. Pat. No. 5,467,220 (Xu) incorporates a yoke having a parabolic or elliptical surface which acts as a reflector in a wafer pedestal assembly. Reflector positioning and spacing relative to the wafer surface encourage reflection of heat radiated from the edge portion on the wafer surface and wafer chuck back to the wafer edge to mitigate thermal loss at the wafer edge, and thereby improve temperature uniformity across the surface of the wafer.  
         [0008]     U.S. Pat. No. 6,278,600 (Shamouilian et al.) provides an electrostatic chuck having a flex circuit laminated to a contoured support pedestal. The top surface of the chuck has a contoured topography achieved by machining the upper surface of the pedestal prior to lamination of the flex circuit to the pedestal. It is believed that the contoured topography improves the flow of backside cooling gas resulting in a uniform wafer temperature profile.  
         [0009]     U.S. Pat. No. 6,583,638 (Costello et al.) provides a chuck assembly comprising a primary heater interposed between a chuck top plate and a multi-layer heat sink, with a secondary heater fitted to the bottom or the underside of heat sink. The secondary heater is intended to work against the cooling affect of the heat sink and thereby eliminate extreme variations in the temperature of the bottom of the chuck due to action of the primary heater, as well as the cooler of the chuck.  
         [0010]     Finally, U.S. Pat. No. 6,967,177 (May et al.) discloses an apparatus for controlling substrate temperature of a substrate during processing thereof at a process energy. Upon sensing chuck temperature outside a target temperature range, a controller is used to adjust a flow rate of a thermal transfer media flow, the temperature of the thermal transfer media, and the process energy to bring the sense chuck temperature within the target temperature range.  
         [0011]     High temperature heating chucks, e.g., sandwiched pedestal assemblies (see  FIG. 1 ), generally comprise two hermetically sealed metal or ceramic discs  11 ,  13  which house a combination of elements  15  such as a heater (e.g., a mica heater, i.e., etched Inconel® foil between two layers of mica sheeting), ceramic paper, Kapton®, silicon rubber, etc. Such assemblies are further characterized by combinations of sensors, controllers, cabling and other electrical and or mechanical components, as well as tight dimensional tolerances, surface flatness, perpendicularity, and a select surface finish.  
         [0012]     As is to be expected, temperature specifications and tolerances of heating chucks are a function of the wafer process, e.g., chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), lithography, baking, plasma etching, cleaning, etc. Generally, thin flexible heaters or heating elements (e.g., thermofoil heaters) are advantageously utilized in heating chuck assemblies. Characteristic heaters available for the semiconductor industry are twofold, namely, low temperature, i.e., up to about 260° C., all polyamide heaters laminated to heat sinks (e.g., AP heaters by Minco, Minneapolis, Minn.) and high temperature, i.e., up to about 600° C., mica heaters (e.g., models HM—by Minco, Minneapolis, Minn.).  
         [0013]     Wafers whose diameters are 200 mm and 300 mm are most pervasive, and, inasmuch as the 300 mm wafers offer 125% more area than the 200 mm wafers, they are increasingly used in the industry. As larger wafers enable lower production costs, the most commonly used heating chucks are also 300 mm in diameter, however, as heating chucks become larger, it becomes more difficult to control the thermal tolerances during wafer processing. As a result, problematic thermal warping of the wafer is becoming increasingly common (see generally, “The Benefit of Using Double Heaters to Reduce Thermal Deformations of Heating Chuck Assemblies for Semiconductor Applications,” Mohamed, Zakaria, [publication date/bibliographic data], incorporated herein by reference).  
         [0014]     The thermal process control of both the wafer and the heating chucks are critical to wafer processing as operating temperatures are generally controlling, e.g., operating temperatures dictate, among other things, reaction kinetics of the chemical reactions of the wafer process. As previously alluded to, during such processes, layers of gases or thin films are deposited to form a solid insulating or conducting layer on the surface of a wafer. The gases react with material on the substrate thereby creating a thin film that has desirable electrical properties. High-quality films are those with a uniform chemical composition and thickness across the entire substrate area. The thermal process controls the density of the thin film deposited, which is also crucial to the overall wafer quality.  
         [0015]     In CVD processing, a gas containing metal or an insulating chemical is sprayed onto the surface of the wafer. These gases react on the heated wafer surface, forming a thin film of solid material. Energy sources such as heat or radio frequency (rf) power are used alone, or in combination, to facilitate this reaction. These CVD films range in thickness from a small fraction of a micron to a few microns, and must be deposited with extreme uniformity across the wafer surface. Thereafter, the wafer is cut to small chips that are used to create integrated circuits and electronic devices.  
         [0016]     The wafers are generally processed inside clean vacuum chambers in order to be free of impurities and out-gassing. Ideally, the chambers are maintained at one atmosphere vacuum pressure. The temperatures and pressures of the vacuum chamber remain constant throughout the process without any disturbances or variations.  
         [0017]     In addition to the strict environmental controls employed in the vacuum chamber, minimization of thermal disturbances is also critical to the creation of high-quality wafers. Reductions of temperature gradients across a heater lead to less variability in the temperature-dependent chemical reactions, which, in return, lead to higher production yield. In addition, the maintenance of dimensional tolerances of the heating chucks is crucial to the attainment of uniform heating. Any thermal deformation or warping of the chuck surface creates nonuniform temperatures across the wafer.  
         [0018]     The general industrial requirements for temperature tolerances generally are ±1%, or less, of the operating temperatures. The usual tolerances for flatness are 0.001-0.005 inches. The criteria of temperature and flatness must co-exist in order to achieve high yields of the process. In addition, other issues such as the lifetime, fatigue, and cycling that occurs daily (i.e., “on” and “off” depending on the operating and process time) are some factors which effect the performances of these chucks.  
         [0019]     Finally, the quality of the heating chuck surface finish can effect the temperature uniformity as well. Heat loss through radiation, which depends on the reflectivity and the color of the surface finish, also affect the temperature uniformity of the surfaces. Therefore, the operational control of temperature variation, surface finish, and flatness during thermal processing are essential to productive wafer production processes. Thus, there remains an unmet need in the art for high temperature heating chuck assemblies exhibiting improved thermal uniformity and mechanical stability, e.g., flatness, more particularly, for both novel structures, and attendant heating modalities for same.  
       SUMMARY OF THE INVENTION  
       [0020]     A heating chuck assembly for wafer processing is provided, including heating modalities for same. The assembly generally includes hermetically sealed opposingly paired discs, and housed therebetween, a ceramic element interposed between first and second heating elements. The first heating element is adjacent a first disc of the opposingly paired discs so as to be paired therewith, the second heating element adjacent a second disc of the opposingly paired discs so as to be paired therewith. The assembly further contemplates the inclusion of temperature sensing/measuring and controlling devices, in the context of a heating chuck system.  
         [0021]     The discs, which advantageously are selected from the group consisting of aluminum, stainless steel, nickel, or alloys thereof, essentially house dual heating elements, more particularly, dual mica heaters having a ceramic element interposed therebetween. Preferably, the heating elements are substantially identical, characterized by substantially similar watt densities and heating profiles. Although not necessary, it is further advantageous that the heating elements have greater than one heating zone, and, generally, not more than four separately operable heating zones, with greater than four heating zones nonetheless contemplated and a function of heat sink area, thickness, uniformity, materials, etc. Operatively, the heating elements may function individually, in parallel, or simultaneously.  
         [0022]     The discs, which are generally sealed about a common periphery, are further united interior of the common periphery via a plurality of bosses. In furtherance of thermal and mechanical stability, the assembly of the subject invention advantageously includes bosses spaced on about three inch centers for the device described herein, boss spacing being less or greater to the extent that the plate is thinner or thicker. More specific features and advantages obtained in view of those features will become apparent with reference to the drawing figures and DETAILED DESCRIPTION OF THE INVENTION. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     Referring now to the drawings wherein like numerals are used to designate like parts of the invention throughout the figures:  
         [0024]      FIG. 1  illustrates a conventional high temperature heating chuck in exploded perspective plan view depicting heretofore known, typical components thereof;  
         [0025]      FIG. 2  illustrates, in perspective view, a heating chuck assembly of the subject invention;  
         [0026]      FIG. 3  is a sectional view of the heating chuck assembly of  FIG. 2 ;  
         [0027]      FIG. 3   a  is a detailed view of area “ 3   a ” of  FIG. 3  showing the heating elements of the assembly of  FIG. 2 ;  
         [0028]      FIG. 4  is a plan view of the bottom plate of the heating chuck assembly of  FIG. 3 , i.e., an overhead view of the  FIG. 2  structure;  
         [0029]      FIG. 5  is a plan view of the top (i.e., wafer receiving) plate of the heating chuck assembly of  FIG. 3 , i.e., an underside view of the  FIG. 2  structure; and,  
         [0030]      FIG. 5A  illustrates the plate of  FIG. 5  with preferred heating zones indicated. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]     The heating chuck assembly of the subject invention is generally shown in  FIG. 2 , with details thereof illustrated in  FIGS. 3-5 . As a preliminary matter, although the subject disclosure is generally directed to “high” temperature wafer processing, i.e., at temperatures up to about 600° C., the subject assembly is not intended to be so limited. The subject assembly, more particularly, a 6061 aluminum alloy chuck, demonstrated particular utility in a thermal range of about 100-300° C.  
         [0032]     With general reference to the figures, the subject assembly  10  generally includes hermetically sealed opposingly paired discs or plates  12 ,  14  supported upon a stem  16 , namely, a top or wafer receiving plate  12 , and a bottom or stem receiving plate  14 . It is to be noted that the notions or conventions of “top,” “bottom,” “up,” “down,” etc., as the case may be, are relative, and primarily provided to facilitate a discussion of feature relationships and/or interrelationships.  
         [0033]     The assembly  10  further, and advantageously, includes first  18  and second  20  heating elements, i.e., heaters, each heating element being adjacent to each plate of the opposingly paired plates in the assembly. Essentially, each plate of the set or pair has an associated or paired heater. Interposed at least between the first  18  and second  20  heating elements is one or more sheets of ceramic paper  22  or the like. As will later be discussed, the heaters are preferably mica heaters.  
         [0034]     The chuck plates are advantageously fabricated from aluminum, and alloys thereof (e.g., 6061), stainless steel, or nickel, with aluminum alloys being preferred for thermal applications below about 375° C., due to, among other things, their high thermal conductivity, light weight, ease of machinabilty and ability to be welded (i.e., hermetically united via electron beam welding). For thermal processing in excess of about 375° C., stainless steel and nickel are options, with nickel generally providing a five fold increase in thermal conductivity compared to stainless steel, and with nickel (i.e., Ni 200) offering a lower degree of thermal deformation as compared with stainless steel (i.e., 316 SS). Thermophysical properties of select elements of the chuck assembly of the subject invention are summarized in Table 1 herein.  
         [0035]     With particular reference to  FIGS. 3-5 , the top plate  12  is generally adapted to receive the bottom plate  14 , e.g., as shown, the top plate  12  includes a rim  24 , more particularly, a peripheral sidewall, within which the bottom plate  14  is received. The top plate  12  further includes opposing plate surfaces, i.e., an “exterior” or wafer receiving plate surface  26  and an “interior” or heater receiving plate surface  28 .  
         [0036]     The bottom plate  14  is generally adapted for cooperative engagement with the stem  16 , as is generally well known in the art. Likewise, the stem is of conventional design and is functionally required to, among other things, support the subassembly of plates, heaters, etc.  
         [0037]     The bottom plate  14  generally includes a plurality of supporting bosses  30 , with about a three inch span between adjacent bosses believed advantageous, and thus preferred. Functionally, the supporting bosses must be capable of withstanding the shearing forces acting from the upper and the lower plates. During the heating and vacuuming processes, the welded boss joints experience continuous shearing forces. Any slightly uneven supports, or weak weld joints, may cause deformation in the plates. Thus, the number of the supporting bosses, and their locations (i.e., general configuration thereof) are a further consideration in an improved chuck assembly configuration.  
         [0038]     With reference to  FIG. 4 , a particularly advantageous boss configuration is shown in connection with a 6061 anodized aluminum plate/mica heater assembly for a 300 mm wafer, more particularly, for a 13 inch diameter chuck having about a 1.15 inch thickness (i.e., 0.5 inch top plate thickness, 0.5 inch bottom plate thickness, and 0.15 inch gap for the subassembly comprised of the two mica heaters and ceramic paper). Radially from an axial centerline  32 , four boss rings are indicated, namely, in increasing dimensional magnitude, r 1 , r 2 , r 3 , and r 4 , with fifteen (15) total bosses, the occurrence thereof in relation the radial rings being 3/3/6/3. Furthermore, bosses are distributed in 30° angular increments from the plate centerline  34 , more particularly, in a repeating occurrence of 2/1/1/1 through a 120° arc. As indicated in  FIG. 4 , and beginning at a “1 o&#39;clock” position, bosses are positioned as follows:  1 , r 1 , r 4 ;  2 , r 3 ;  3 , r 2 ;  4 , r 3 ;  5 , r 1 , r 4 ;  6 , r 3 ;  7 , r 2 ;  8 , r 3 ;  9 , r 1 , r 4 ;  10 , r 3 ;  11 , r 2 ; and,  12 , r 3 .  
         [0039]     As to the heating elements of the subject invention, dual mica heaters are critical for optimal thermal and mechanical performance of the chuck, and by extension, wafer processing. Mica heaters generally include an etched foil element sandwiched between layers of mica. An organic material binds the layers together and burns off during initial warm up. Such heaters are characterized by high thermal capability, i.e., up to about 600° C., and a power rating of up to about 110 watts per square inch.  
         [0040]     In connection to heating modalities, it is advantageous, but not necessary, that each of the heaters  18 ,  20  of the assembly  10  include greater than one heating zone  36 , more particularly, that each heater include up to about four heating zones (i.e., independently operable heating zones  36   a - 36   d , see e.g.,  FIG. 5A ). Likewise, simultaneous or substantially simultaneous operation of each of the heater of the assembly is preferred. It is to be understood that attendant controllers, sensors, indicators, etc. are contemplated although not necessarily shown and/or explicitly disclosed, such items being believed well know to those of ordinary skill in the subject art.  
         [0041]     Interposed between the “top” and “bottom” heaters is at least a single sheet of ceramic fabric paper  22 , i.e., a ceramic element, or the like. Among several critical relationships in the subject assembly or subassembly, is a twofold requirement that the etched foil element and mica sheets of the heater remain substantially integrated, and that the heater per se be substantially and uniformly contacting the heat sink, i.e., plate or disc. In furtherance thereof, incorporating at least a single ceramic fabric paper sheet, e.g., about 0.125″ thick, between the dual heating elements provides a resilient padding. A plate interposed laminate structure comprising the heating elements  18 ,  20  and ceramic paper  22  is typically compressed by about half, and aides realization of the aforementioned relationships and interrelationships.  
         [0042]     As illustrated in inventor testing, temperature disturbance phenomena were noted, namely, when four zones in each heater were controlled independently and simultaneously while trying to maintain the temperature at a select set temperature (see  FIG. 5   a ), the heat transfers quickly throughout the heating chuck and affects the neighboring zones. These temperature disturbances from the neighboring zones adversely affected the overall temperature uniformity. In contrast, when running only two heaters without zones, these disturbances were not manifest. It is believed that the time response of the temperature controllers could be modified in an effort to reduce this phenomenon.  
         [0043]     There are other variations of the subject invention, some of which will become obvious to those skilled in the art. It will be understood that this disclosure, in many respects, is only illustrative. Changes may be made in details, particularly in matters of shape, size, material, and arrangement of parts, as the case may be, without exceeding the scope of the invention. Accordingly, the scope of the subject invention is as defined in the language of the appended claims.