Abstract:
In one embodiment of the invention, there is provided a tool for annealing a magnetic stack. The tool includes a housing defining a heating chamber; a holding mechanism to hold at least one wafer in a single line within the heating chamber, a heating mechanism to heat the at least one wafer; and a magnetic field generator to generate a magnetic field whose field lines pass through the single line of wafers during a magnetic annealing process; wherein the holding mechanism comprises a wafer support of holding the single line of wafers between the heating mechanism and the magnetic field generator. The tool may be a rapid thermal processor retrofitted with the magnetic field generator.

Description:
FIELD 
     Embodiments of the invention relate to the annealing of magnetic stacks. 
     BACKGROUND 
     Magnetic film stacks are used in a variety of devices including Magnetic Random Access Memory (MRAM) devices. During manufacture, the magnetic film stacks are annealed in a thermal processor in order to achieve desired material properties. 
       FIG. 1  shows a prior art system  100  used for magnetic annealing of magnetic film stacks. The system  100  includes an annealing chamber  102  which is surrounded by a magnetic field generation component  104 . The magnetic field generation component  104  may comprise a giant cylinder of ferromagnetic materials with coils to pass current therethrough in order to generate a magnetic field. 
     Heating elements  106  are located on the periphery of the heating chamber  102  to supply heat to heating chamber  102  during the thermal annealing process. 
     In use a plurality of wafers/substrates  108  with magnetic stacks for annealing are carried by a wafer carrier  110 . The wafer carrier  110  is then placed in the annealing chamber  102  as indicated. The process of magnetic annealing includes heating the annealing chamber  102  to a temperature of between 200 to 600 degrees Celsius for anywhere between 30 minutes to 2 hours. The process may be carried out in a vacuum. Alternatively, the annealing chamber may be filled with a gas such as hydrogen, helium, argon, etc. 
     Current flowing through the magnetic field generation component  104  in the direction of the arrows  112  cause a magnetic field of between 1 to 5 Teslas to be induced within the annealing chamber  102 . 
     The aforementioned magnetic annealing process suffer from the following disadvantages:
         Conflicting requirements: For the best film crystallographic qualities large amounts of thermal energies are required to be supplied for a relatively long time (30 minutes to 2 hours. As a result some of the magnetic materials can start diffusing or moving thereby compromising device performance due to junction deterioration.   Long annealing times and higher temperatures affect the performance of the prefabricated silicon integrated circuits to which the magnetic stacks are integrated in a backend process.   The number of wafers that can be annealed per hour is small due to long annealing times.   Device performance may be compromised by reducing the annealing temperature and time to accommodate for the above problems.   The magnetic field is supplied by huge electromagnets that are very expensive and difficult to maintain. Complicated cooling systems are needed to keep the electromagnets cold. This increases the cost of manufacturing the magnetic film stacks.   The wafers are annealed as a batch of 25 or 50 and local thermal variations on a wafer are difficult to control; thermal radiation between each wafer further increases the temperature variations and hence the magnetic device performance.   Thermal variations are further aggravated by the gas flow dynamics if the wafers are annealed in gas ambience as the gas flow between the batch of wafers is not predictable and are subject to variations.       

     SUMMARY 
     In accordance with a first aspect of the invention, there is provided a tool for annealing a magnetic stack. The tool includes a housing defining a heating chamber; a holding mechanism to hold at least one wafer in a single line within the heating chamber, a heating mechanism to heat the at least one wafer; and a magnetic field generator to generate a magnetic field whose field lines pass through the single line of wafers during a magnetic annealing process; wherein the holding mechanism comprises a wafer support of holding the single line of wafers between the heating mechanism and the magnetic field generator. 
     In accordance a first aspect of the invention, there is provided a method for retrofitting a rapid thermal processor with a grid of current carrying conductors to generate a magnetic field for annealing a single line of wafers. 
     Other aspects of the invention will become apparent from the written description below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic drawing of thermal processor, in accordance with the prior art. 
         FIG. 2  shows a temperature profile for a rapid thermal annealing process, in accordance with one embodiment of the invention. 
         FIG. 3  shows a cross-sectional view of a magnetic stack that is annealed in accordance with embodiments of the invention. 
         FIGS. 4A and 4B  show cross-sectional view of tools of annealing magnetic stacks in accordance with embodiments of the invention. 
         FIG. 5A-5D  shows the heating grid and the magnetic field generator of the tool, In greater detail. 
         FIG. 6  shows the heating grid with two wafers in a single line. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one skilled in the art that the invention can be practiced without these specific details. 
     Reference in this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Moreover, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but not other embodiments. 
     Referring now to  FIG. 3  of the drawings, there is shown a cross-section through a magnetic stack  300  that may be annealed in accordance with the systems and methods disclosed herein. As will be seen, the stack  300  comprises a bottom electrode layer  302  The bottom electrode layer may comprise materials such as tantalum, rubidium, copper, copper-nitride. A pinning layer  304  overlies the bottom electrode layer  302  and comprises materials such as platinum-manganese, or iridium manganese. Layer  306  resides atop layer  304  and comprises a ferromagnetic material such as iron-cobalt, or nickel-iron. The magnetic stack  300  comprises a tunnel dielectric layer  308  that resides on the layer  304 . The tunnel dielectric layer  308  may comprise a material such as magnesium oxide. The stack  300  is completed by a ferromagnetic layer  310  of, e.g. iron cobalt, and a capping layer  312  of e.g. ruthenium or tantalum. 
     The magnetic stack  300  may be used to define magnetic bits in a MRAM memory device. For device performance, the magnetic domains in the pinning layer  304  must be aligned. This is achieved by a magnetic annealing process. Moreover, the tunnel dielectric layer  308  must be thermally annealed to change its material propertied from amorphous to crystalline. 
     Advantageously, in one embodiment, there is provided a method for annealing magnetic stacks in which processes of thermal annealing and magnetic annealing are carried out in the same equipment and in the same process. 
     Referring now to  FIGS. 4A and 4B  of the drawings, there is shown a cross-sectional view of a magnetic annealing tool  400  and a magnetic annealing tool  400 ′, according to embodiments of the present invention. Chamber walls  402  form an outer port  404  through which a semiconductor substrate, such as wafer  406  comprising a magnetic stack, may be introduced into the tool  400 . A conventional load lock mechanism may be used for inserting and removing wafer  406  through outer port  404 . After the wafer has been loaded into tool  400 , a plate  407  is used to cover outer port  404 . In addition, a viewing window  405  may be provided for, among other things, end point detection, in situ process monitoring and wafer top surface temperature measurement. If a window is used for these purposes, a small hole or clear section must be provided through any internal chamber surfaces, such as insulating walls, to allow the wafer to be viewed. When a window is not used, a non-transmissive plate may be used to cover the viewing port for improved insulation. 
     Chamber walls  402  are relatively cold, preferably being maintained at an average temperature less than one hundred degrees Celsius (100° C.). Chamber walls  402  are aluminum and are cooled by cooling channels  408 . Water or another cooling media may be pumped through cooling channels  408  to cool the aluminum chamber walls  402 . 
     After the wafer is introduced into the chamber, it is placed upon narrow pins  410  which may comprise silicon carbide or ceramic. The pins are mounted on a pin support plate  412  that may be raised and lowered by an elevational mechanism  414 , such as a pneumatic or electromechanical lift with a vacuum-sealed bellows. After the wafer is loaded into the chamber and placed on pins  410 , the elevational mechanism  414  is lowered to place wafer  406  close to or onto a heated block  416  for thermal processing. 
     The heated block preferably has a large thermal mass that provides a stable and repeatable heat source for heating wafer  406 . Heated block  416  may provide a heating surface  418  within the chamber that is substantially parallel to the wafer to allow heat transfer across the entire backside surface area of the wafer. Heated block  416  comprises a material that will not contaminate wafer  405  even when the wafer is placed in contact with the heated block  416  at high temperatures (greater than 500° C.) and low pressures (less than 100 Torr). In one embodiment, heated block  416  comprises silicon carbide coated graphite, although other materials that will not react with the wafer at processing temperatures such as silicon carbide or quartz may be used as well. A material with high thermal conductivity is preferred to allow heat to uniformly dissipate through the block. Insulating techniques described below are used to prevent sharp temperature gradients from forming in the heated block  416  due to heat losses at the edges of the heated block  416 . 
     In one embodiment, the heated block  416  is approximately one (1) inch thick in the first embodiment and provides a thermal mass substantially larger than the wafer which is only about thirty five thousandths (0.035) of an inch thick. It is preferred that heated block  416  be at least ten times thicker than the wafer that is being processed. This provides a stable temperature heat source for thermally processing wafer  406 . 
     In one embodiment, a single heated block  416  provides a generally rectangular heating surface large enough to process two wafers  406  at a time. Using a single heated block  416  provides a simplified and cost effective design. However, the heated block  416  cannot be easily rotated to rotate the wafers during processing. Wafer rotation is often desired to enhance uniformity in semiconductor processing. In alternative embodiments, a separate rotating heating plate may be provided for each wafer  406  to further enhance uniformity. The wafers  406  could also be held slightly above the heated plate  416  and rotated on pins  410 . However, the chamber according to the first embodiment provides excellent uniformity without rotation, so a simplified and cost effective design is preferred. 
     The heated block  416  is heated by a resistive heater  420  positioned below the heated block  416 . The resistive heater  420  preferably comprises silicon carbide coated graphite although other materials may also be used. Heater support pins  422  space the resistive heater  420  from heated block  416  by a short distance (approximately 0.125 of an inch). A power source (not shown) is connected to the resistive heater  420  by a heater mounting mechanism  424  in a separate heater mounting chamber  442 . Current is driven through resistive heater  420  to heat the heated block  416  which in turn acts as a stable heat source for wafer  406 . The power applied to the resistive heater may be adjusted to control the temperature of the heated block. A thermocouple  426  or other temperature sensor may be used to measure the temperature of the heated block  416 . An optical pyrometer or thermocouple (not shown) may also be used to measure the wafer temperature directly. The temperature sensors send signals indicative of the temperature of, the heated block  416  and/or wafer  406  to a conventional temperature control system (not shown). The temperature control system then causes a transformer or other power source to apply an appropriate amount of power to the resistive heater  420  to achieve the desired processing temperature. Typically temperatures between five hundred degrees Celsius (500° C.) and one thousand degrees Celsius (1000° C.) are used for thermal processing in the chamber according to the first embodiment. 
     Other mechanisms may be used to provide a stable heat source. For instance, an RF coil could be used to induce a current in a conductive plate within the chamber or lamps could be used to heat the backside of a conductive block. 
     In one embodiment, the stable heat source may be provided by a grid of heating elements (resistive wires). The grid may be used to heat the block  416  or may be a replacement for the block  416 . The latter case is illustrated in the embodiment  400 ′ of the tool shown in  FIG. 4B  where the grid of heating elements is indicated by reference number  450 . In use the wafer  406  will make direct contact with the grid  450 . 
     In order to reduce heat loss and enhance uniformity, heated block  416  and wafer  406  are enclosed within an insulated heating chamber  428 . Heating chamber  428  is formed by insulating walls  430   a - d  spaced apart from heater  420 , heated block  416  and wafer  406 . The insulating walls  430   a - d  may comprise a material that has a low thermal conductivity. In addition, insulating walls  430   a - d  are preferably highly reflective and substantially non-transmissive to thermal radiation (particularly in the visible and infrared regions). Thus, insulating walls  430   a - d  substantially prevent heat transfer by direct radiation from within heating chamber  428  to cold chamber walls  402 . In the first embodiment, insulating walls  430   a - d  comprise opaque quartz with a thermal conductivity of approximately three and one half Watts per centimeter Kelvin (3.5 W/cmK). Opaque quartz is highly preferred in the first embodiment because it is durable and inert in virtually all processes, has a high reflectivity and low conductivity, and may be used to form an insulating wall using a single intrinsic piece of material. Opaque Silica Glass OP-1 from Nippon Silica Glass U.S.A., Inc. is an exemplary opaque quartz that may be used in one embodiment. In contrast to transparent quartz, opaque quartz is white with a nearly ideal opaque appearance. This is due to the special structure of the material which has a well-controlled distribution of micropores in the otherwise dense matrix, scattering light and thermal radiation in a very efficient and homogeneous way. Thus, the direct transmission of radiation is nearly completely suppressed (less than 1% transmission across wavelengths from 200 to 5000 nm for 3 mm path length). The surface of the opaque quartz is preferably treated to inhibit flaking and the release of particulates. This is accomplished in the first embodiment by exposing the surfaces of the opaque quartz to high temperatures which glaze the surfaces. This forms a shallow layer (approximately 1 mm deep) of clear durable quartz on the exterior surfaces of the insulating walls which acts as a protective coating. 
     Other heat resistant insulating materials, such as alumina and silicon carbide, could be used for the insulating walls. In addition, the insulating walls may be formed from a transmissive material such as clear quartz coated with a reflective material such as alumina, silicon carbide, or silicon nitride. However, these alternatives tend to be less durable than glazed opaque quartz, often flake and spall, and may interfere with the chemistry of some processes. 
     The material used for the insulating walls may have a thermal conductivity less than five Watts per centimeter Kelvin (5 W/cmK) in the first embodiment although it will be readily understood that the thickness, thermal conductivity, and transmission of the material may be varied to achieve a desired level of insulation. Additional insulating walls may also be positioned between the heating chamber  428  and chamber walls  402  to improve insulation. In particular, the heating chamber  428  may be enclosed within multiple insulating housings with vacuum regions formed between the housings. 
     As shown in  FIGS. 4A and 4B , four opaque quartz insulating walls—a top  430   a , side  430   b , base  430   c , and bottom  430   d —are used to form heating chamber  428  in the first embodiment. The top  430   a  and side  430   b  insulating walls may be formed from a single piece of opaque quartz which provides an insulating hood that may be placed over each wafer processing station. As shown in  FIG. 4A , the base  430   c  and bottom  430   d  insulating walls are closely spaced to the heated block  416  and resistive heater  420 . In the first embodiment, the base  430   c  and bottom  430   d  insulating walls substantially encapsulate the heat source except for exposed circular regions of the heating surface which are shaped to receive the wafers. This helps channel the heat flux from the heat source through a circular region normal to the wafer surface and reduces lateral thermal gradients. In addition, the heating surface extends radially from the circular region underneath the base insulating wall  430   c . This helps isolate the wafer  406  from any temperature drop off at the edge of the heated block  416 . As shown in  FIG. 4A , the heated block  416  forms shallow pockets for receiving the wafers  406  in the circular regions that are left exposed by the base insulating wall  430   c . The pockets are between one sixteenth (0.0625) and one eighth (0.125) of an inch deep in the first embodiment and may be flat, slightly concave with the center of the pocket being slightly lower (approximately 0.003 inches) than the perimeter of the pocket, or stepped with the center recessed relative to a ledge formed about the outer radius of the pocket. Recessed pockets help retain heat at the edges of the wafer and the pocket shape may affect temperature uniformity across the wafer surface. Nevertheless, outstanding process uniformities have been achieved at six hundred degrees Celsius (600° C.) and eight hundred degrees Celsius (800° C.) using both flat and recessed pocket designs. 
     The insulating walls  430   a - d  substantially enclose the heating chamber  428  and form an outer insulating chamber  434  between the insulating walls  430   a - d  and the cold chamber walls  402 . The insulating walls  430   a - d  form an inner port  436  between the heating chamber  428  and insulating chamber  434  to allow a wafer to be inserted into the heating chamber  428 . A piece of insulating material may be used to cover the inner port  436  during processing to provide additional insulation. Generally, heat is transferred in the first embodiment from the heated plate  416 , across the heating chamber  428  to the insulating walls  430   a - d , through the insulating walls  430   a - d  and across the insulating chamber  434  to the cold chamber walls  402 . Of course, some heat may be transferred through support leg  432  by conduction and through inner port  436  by direct radiation (unless an opaque cover is used). However, a substantial majority (more than 90%) of the radiation from the heated block  416  is intercepted by the insulating walls  430   a - d  and only a small portion of the bottom insulating wall (less than 10%) is in contact with the support leg  432 . Thus, the rate of heat transfer in the first embodiment is substantially dependent upon the thermal resistance across the heating chamber  428 , through the insulating walls  430   a - d , and across the insulating chamber  434 . 
     The thermal resistance across the heating chamber and insulating chamber can be adjusted by adjusting the processing pressure. Tube  438  provides a gas inlet and gas exhaust port  440  provides a gas outlet. The tube  438  is connected to a conventional gas source (not shown) and gas exhaust port  440  is connected to a conventional vacuum pump (not shown) which allows the pressure in the chamber to be controlled. In the first embodiment, pressures from seven hundred sixty (760) Torr (atmospheric) down to less than one tenth (0.1) of a Torr may be achieved. As will be described further below, pressures less than one hundred (100) Torr, and in particular pressures between two (2) Torr and fifty (50) Torr, are preferred in the first embodiment. 
     The low pressure, insulating walls, and other thermal properties (described further below) allow a very compact chamber design to be used with a high level of uniformity. In the first embodiment, the heated block  416  is approximately ten (10) inches wide which is only slightly wider than the wafer  406  and its edges may be within one (1) inch of the cold chamber walls  402 . The base insulating wall  430   c  is approximately one hundred twenty five thousandths (0.125) of an inch from the edge of heated block  416  and the distance from the base insulating wall to the chamber wall (across the insulating chamber  434 ) is less than half an inch. Thus, the width of the heated block  416  is more than eighty percent (80%) of the interior width of the processing chamber. In addition, the heated block  416  occupies more than ten percent (10%) of the interior volume of the processing chamber. 
     The tool also includes a magnetic field generator in form of a plurality of elongate current carrying conductors  452  disposed within the chamber  428 . 
     Referring now to  FIG. 5A  of the drawings, the heating grid  450  of  FIG. 4B  is shown in greater detail. As will be seen, the grid  450  comprises resistive wires Y 0  to Y n  disposed in a first direction and intersecting resistive wires X 0  to X n  disposed in a second direction. The wafer  406  with the magnetic stack to be annealed is placed on the grid  450  as is illustrated in  FIG. 5B .  FIG. 5C  shows the elongate current carrying conductors Z 0  to Z n  that are used to create the magnetic field during the magnetic annealing process. The conductors Z 0  to Z n  overlie the wafer  406  so that the wafer  406  is sandwiched between the heating grid  450  and the conductors Z 0  to Z n . 
     Thus, it is to be appreciated that embodiments of the invention have disclosed a three tiered approach to annealing in which at a first tier heat is supplied to a wafer  406  held at a second tier on a substrate, and a magnetic field is applied to the wafer  406  through energizing of current carrying conductors located at a third tier. 
       FIG. 2  shows a temperature profile  200  for an annealing process performed with the annealing tools disclosed herein. Also shown in  FIG. 2  and indicated by reference numeral  202 , is the temperature profile for a conventional process for annealing of a magnetic stack. It will be appreciated that the profile  200  indicated a much higher temperature peak and a much shorter duration when compared with the profile  202 . The thermal energy delivered by the profile  202  is by the shaded portion indicated by reference numeral  204 . The equivalent of this thermal energy may be delivered as a single pulse  202   a , or a series of additional pulses (only one additional pulse  202   b  is shown) in the profile  200 . Beneficially, the shorter times and higher peak temperature of the profile  200  mitigate the problems of material diffusion and junction breakdown associated with conventional annealing processes for magnetic stacks. Further, with the tools disclosed herein thermal and magnetic annealing can occur at the same time, thus improving the number wafers that can be processed over time. 
     The tools and method disclosed herein may be used to: 
     1. Achieve perfect to near-perfect crystalline structures of materials such as PtMn (Platinum Manganese), IrMn (Iridium Manganese), MgO (Mangesium Oxide). 
     2. Achieve the best device performance characteristics such as MR (Magneto Resistance) ratio, RA (Resistance Area product), tunnel junction dielectric breakdown. 
     3. Increase the throughout (number of wafers annealed per hour) by cutting down the annealing time by annealing at higher temperature to supply adequate thermal energy in a shorter time. 
     4. Reduce the cost of the equipment by adopting single wafer processing that requires less complex less expensive annealing equipment. 
     5. To simplify annealing equipment design and size by applying a heat cycle followed by the application of a magnetic field cycle. 
     6. To achieve film and crystallographic uniformity by localized temperature control and adjustment. 
     In one embodiment, preexisting semiconductor equipment may be retrofitted with a magnetic field generator to realize the tool disclosed herein. 
     Although the present invention has been described with reference to specific exemplary embodiments, it will be evident that the various modification and changes can be made to these embodiments without departing from the broader spirit of the invention. Accordingly, the specification and drawings are to be regarded in an illustrative sense rather than in a restrictive sense.