Abstract:
A shadow masking device for use in the semiconductor industry includes self-aligning mechanical components that permit shadow masks to be exchanged while maintaining precise alignment with the target substrate. The misregistration between any two of the various layers in the formed structure can be kept to less than 40 microns.

Description:
TECHNICAL FIELD 
     The invention relates to a device for depositing material onto a substrate, and more particularly, to a device that permits the self-alignment of a shadow mask with respect to a substrate, thereby permitting different shadow masks to be used in turn. 
     BACKGROUND 
     Semiconductor devices are thin structures that are fabricated over the surface of a substrate by adding and removing material in several steps. Various layers are typically built up, with each layer having a composition and a form selected in view of the device design. The process may include steps to alter the properties of certain deposited materials, e.g., through ion implantation or annealing. In addition, chemical-mechanical planarization may be used to smoothen out the layers as they are built up. 
     The patterning of deposited material usually involves lithography. First, a uniform layer of material is deposited on the entire surface of the substrate. Unwanted material is then removed by an etching process, such as wet etching or dry etching (e.g., plasma etching or ion milling). Lithography permits certain portions of the layer to be removed while others remain. A typical lithographic process involves coating a layer with a photo-sensitive resist, selectively exposing the resist to a specific wavelength of light using focusing optics and a mask, chemically washing away the exposed (or alternatively, unexposed) portions of the resist, etching away parts of one or more layers, and then removing the remaining resist before proceeding to the next step. These steps involve the use of expensive equipment and can be time consuming. 
     Shadow masking is an alternative patterning method that does not rely on a lithographic process. Instead, a mask of a desired pattern is placed between the deposition source and the substrate, so that only material that passes through openings in the mask is deposited on the substrate. The mask is ideally positioned as close to the substrate as possible, so that the deposited pattern does not become “defocused” on the substrate; rather, the resulting deposited pattern is very nearly an exact copy of the mask. With shadow masking, the achievable feature sizes are limited by mask manufacturing capabilities, and precision mechanical alignment is required between the mask and the substrate. Nevertheless, shadow masking permits the rapid prototyping of devices, is significantly cheaper (since expensive processing equipment, such as steppers and etchers are not needed), generally involves fewer process steps than lithography, and does not require the use of harsh chemicals. 
     SUMMARY 
     Shadow mask apparatuses and methods are disclosed in which the mask is accurately aligned to the substrate. Unlike a lithographic process, in which a mask is aligned optically in a separate system (such as a stepper), the alignment of the shadow mask in this invention occurs automatically in the deposition process chamber. Mechanical alignment methods (rather than expensive optical ones) are used, which rely on self-aligning mechanical components. 
     In one preferred implementation of the invention, a first pattern of material is deposited over a substrate held in a substrate carrier by employing a first shadow mask held in a first mask carrier. A second pattern of material is deposited over the first pattern of material by employing a second shadow mask held in a second mask carrier, with the mask carriers being exchanged by a robot. Each of the first and second masks self-align passively with the substrate carrier, so that the deposited, second pattern is aligned to within 40 microns of the deposited first pattern. In a preferred implementation, at least 3 (or even 5, 10, or more) different shadow masks are used to construct a multi-layered structure onto the substrate. The depositions may take place in vacuum, and the masks are exchanged by a robot. The substrate may be heated to an elevated temperature, e.g., greater than 200° C. or even 400° C. The substrate and the first shadow mask may advantageously rotate together while material is being deposited. 
     In a preferred implementation of the invention, a first pattern of material is deposited over a substrate held in a substrate carrier by employing a first shadow mask in combination with mechanical components that self-align with the substrate carrier. A second pattern of material is deposited over the first pattern of material by employing a second shadow mask in combination with mechanical components that self-align with the substrate carrier. The masks are exchanged by a robot, and any misregistration between the first pattern and the second pattern is less than 40 microns. 
     The following exemplary method can be used in conjunction with the embodiments described herein, in which:
         (a) the first shadow mask is installed in a receiver;   (b) the first layer of material is deposited over the substrate;   (c) the first shadow mask is removed from the receiver;   (d) the second shadow mask is installed in the receiver;   (e) the second layer of material is deposited over the substrate; and   (f) the second shadow mask is removed from the receiver,
 
with steps (a), (b), (c), (d), (e), and (f) being carried out in turn.
       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a high level illustration of the patterning of a substrate using a mask. 
         FIG. 2  shows a device that receives a substrate carrier and a mask carrier. 
         FIG. 3  shows an array of the devices shown in  FIG. 2 , in which each of the devices can be used with various deposition sources. 
         FIG. 4 , which includes  FIGS. 4A ,  4 B, and  4 C, shows views of the substrate carrier from the top, the bottom, and in cross section (taken along the cut shown), respectively. 
         FIG. 5  includes  FIGS. 5A ,  5 B,  5 C,  5 D, and  5 E, in which: 
         FIGS. 5A ,  5 B, and  5 C show views of the mask carrier from the top, the bottom, and in cross section (taken along the cut shown), respectively; 
         FIG. 5D  shows an exploded view of a mask and its underlying support structure; and 
         FIG. 5E  shows an alternative mask designed for use at high temperatures. 
         FIG. 6 , which includes  FIGS. 6A ,  6 B, and  6 C, shows a robotic end effector that can be used to transport the mask carrier and the substrate carrier. 
         FIG. 7  is a picture of materials deposited over a substrate using the devices and methods described herein. 
     
    
    
     DETAILED DESCRIPTION 
     The invention described herein allows for multiple shadow masks to be used in the sequential deposition of layers of different materials onto a single substrate. In particular, the layers within the structures so formed are precisely aligned to each other, so that well-defined structures are formed. Furthermore, the disclosed shadow masking methods lend themselves to rapid prototyping. 
     Preferred embodiments of the invention are now described with respect to the figures.  FIG. 1  is an overview of a shadow mask deposition apparatus and process, which may involve high temperatures, vacuum, and corrosive and/or oxidizing gases. A deposition source  110  provides the material  114  to be deposited, and a shutter  120  can be placed over the deposition source  110  to block the flow of material. The material  114  is directed towards a mask  124  that in turn is positioned near a wafer or substrate  130 . The mask  124  generally has a thickness between 50 microns and 100 microns and preferably is made of a high-temperature material such as molybdenum or nickel-plated BeCu. The mask  124  includes openings  505  (as shown in  FIG. 5A ) that reflect the pattern of material to be deposited onto the substrate  130 . Specifically, openings (such as slots) have been formed in the mask  124 , so that the material  114  passing through these openings is deposited onto the substrate  130  to form a desired pattern on the wafer. The various elements shown in  FIG. 1  and the other figures are advantageously contained within one or more chambers (not shown), so that vacuum can be applied during the deposition process. 
       FIG. 2  shows a receiver assembly  210  that includes two receivers  220   a ,  220   b  for receiving a mask carrier  224  (that holds the mask  124 ) and a substrate carrier  230  (for holding the substrate  130 ), respectively. (Both the mask carrier  224  and the substrate carrier  230  can be made of a Ni—Cr—W—Mo alloy such as HAYNES® 230® alloy, which is resistant to both high temperature and oxidation.) As described in more detail below with respect to  FIGS. 4 and 5 , the mask carrier  224  and the substrate carrier  230  are constructed so that, when they are inserted into the receivers  220   a ,  220   b , their positioning tolerance relative to each other is less than 0.4 mm (which is sufficient to permit the necessary finer adjustments to be made). The mask carrier  224  and the substrate carrier  230  can be independently exchanged in and out of the receivers  220   a ,  220   b  using a robot (as described below in connection with  FIG. 6 ). In this manner, a different mask may be used for each deposited layer. For example, if ten different layers are to be deposited onto the substrate  130 , ten different masks may be used in turn. 
     The receiver assembly  210  may advantageously include a heater assembly  250  (as seen in the cutaway portion of  FIG. 2 ), thereby permitting heat to be directed onto the substrate  130 . In this manner, temperatures up to 400° C., or even exceeding 1000° C., may be obtained during a deposition operation. As suggested by the arrow in  FIG. 2 , the receiver assembly  210  may be rotated, thereby facilitating more uniform thickness and composition of the materials deposited onto the substrate  130 . To this end, a rotation motor  260  working with various gears and ball bearings (not shown) permit the motor to rotate the receiver assembly. 
       FIG. 3  shows an array  310  of receiver assemblies  210   a ,  210   b ,  210   c  that can be used in a deposition chamber  320  (shown in cutaway). The array  310  can be rotated with a rotation motor working in conjunction with various gears and ball bearings (not shown, but similar to the rotation motor  260  of  FIG. 2 ), so that a particular receiver assembly  210   a ,  210   b , or  210   c  is positioned directly over a desired deposition source. Twelve such deposition sources  110   a ,  110   b ,  110   c , etc. are shown in  FIG. 3 , and each deposition source may be dedicated to a different material. Also, the mask carrier  224  may be exchanged out of the chamber  320  through a port (not shown). Thus, the configuration shown in  FIG. 3  facilitates the rapid deposition of layers onto the substrate  130 , in which each layer is patterned differently and is made of a different material. 
     As now described with respect to  FIGS. 4 and 5 , the mask  124  (which include a pattern of openings  505 ) and the substrate  130  are aligned with respect to each other in a fixed and reproducible manner. This permits masks to be exchanged in a way that permits layers to be deposited over each other without significant misregistration. This is accomplished by i) fixing the position of the substrate  130  relative to the substrate carrier  230 , ii) fixing the position of the substrate carrier  230  relative to the mask carrier  224 , and iii) fixing the position of the mask  124  relative to the substrate carrier  230 . Each of these will now be considered in turn. 
     First, the positions of the substrate  130  and the substrate carrier  230  are fixed relative to one another as follows. (This is typically done beforehand in atmosphere, outside of any deposition chamber.) As shown in  FIG. 4 , the substrate carrier  230  includes three tabs  410  (e.g., made of Inconel 625®, a high-Ni content alloy that is resistant to both high temperature and oxidation) that are mounted in the substrate carrier but are free to rotate with respect to it. Insertion and removal of the substrate  130  into the substrate carrier  230  are possible when the flats of the tabs  410  are tangential to the circumference of the substrate. When the tabs  410  are rotated 90 degrees from this position, as shown in  FIG. 4B , the tabs contact the substrate  130  at three places along its circumference. Three spring fingers  420  (e.g., made of Inconel 625®) on the side of the substrate  130  opposite the tabs  410  (see the inset in  FIG. 4C ) maintain a small force against the backside of the substrate to keep it in contact with the tabs. This pinching action prevents the substrate  130  from moving out of position relative to the substrate carrier  230 . The thickness of each tab  410  at its end  411  is precisely machined (to within a tolerance of ±5 microns) using an electrical discharge machine (EDM) method; by controlling the thickness of each tab in this manner, the distance separating the substrate  130  from the mask  124  may be kept substantially constant (e.g., about 50 microns) across the face of the substrate, in accordance with the procedures discussed below in connection with  FIGS. 4 and 5 . (Note that additional components related to thermal management may be incorporated into the substrate carrier  230 , such as a disk  435  (e.g., made of SiC) for heating the substrate  130 , heat shields  437  (e.g., made of Inconel 625®), and slots (not shown) in the substrate carrier to help manage thermal conduction.) 
     Next, the positions of the substrate carrier  230  and the mask carrier  224  are fixed relative to one another as follows. As suggested by  FIG. 2 , the mask carrier  224  and the substrate carrier  230  are separately inserted into the U-shaped receivers  220   a ,  220   b . (A robotic mechanism, such as the one described below in connection with  FIG. 6 , may be used for this purpose.) The receiver  220   a  has edges that engage longitudinal slots  550  in the mask carrier  224 ; likewise, the receiver  220   b  has edges that engage longitudinal slots  450  in the substrate carrier  230 . The mask carrier  224  and the substrate carrier  230  have respective integral leaf springs  560  and  460 , which provide a small restraining force to keep the carrier in place once it has been inserted into its corresponding receiver. Accordingly, the mask carrier  224  and the substrate carrier  230  are brought into coarse alignment when they are inserted into the receivers  220   a ,  220   b , with a positioning tolerance of ±0.4 mm in each of the XYZ directions and ±1 degree about the vertical axis of the receiver assembly  210 . This by itself is not good enough for the desired shadow masking operations, but when additional adjustments are made, as described below, the mask  124  and the substrate  130  can be aligned well enough to permit multiple layers to be deposited over each other, to within 40 microns of the desired alignment. 
     Finally, the positions of the mask  124  and the substrate carrier  230  are fixed relative to one another as follows. The mask  124  has been previously (and precisely, as described below with respect to  FIG. 5D ) spot welded to a mask plate  510 , which is most clearly evident from the cross-sectional view of  FIG. 5C . As a result of the welding process, the mask  124  and the mask plate  510  form an integral unit. The mask plate  510  fits within a retainer  520  of the mask carrier  224 . The resulting structure ensures that the mask  124  remains substantially fixed with respect to the mask carrier  224 , allowing for only limited lateral, vertical, and rotational motion of the mask with respect to the mask carrier. During deposition operations, a coiled wire spring  530  tends to urge the mask plate  510  upwards (see  FIG. 5C ), until the top portion of the mask plate contacts the three tabs  410 . (On the other hand, when the mask carrier  224  is not in the receiver assembly  210 , upward motion of the mask carrier  224  is constrained by the retainer  520 , as suggested by  FIG. 5C .) Preferred materials for the mask plate  510 , the retainer  520 , and the spring  530  include HAYNES® 230® alloy, Stainless Steel Type 304, and Inconel X750®, respectively. 
     Additional features in the top portion of the mask plate  510  interact with features in the substrate carrier  230  to ensure that, during deposition operations, the mask  124  remains fixed with respect to the substrate carrier (and hence the substrate  130 ). Specifically, three radial slots  440  in the substrate carrier  230  (see especially  FIG. 4B ) mate with the three alignment balls  540  of  FIG. 5A . (The width of the slots  440  is preferably accurately machined using an EDM to a tolerance of ±5 microns.) The balls in turn are securely fixed within respective holes in the mask plate  510  and positioned within 3 microns of their target locations within the mask plate. When the top portion of the mask plate  510  is brought into contact with the three tabs  410  of the substrate carrier  230 , the three alignment balls  540  simultaneously engage the three radial slots  440  to center the mask plate relative to the substrate carrier and to fix the angular orientation of the mask plate relative to the substrate carrier. At this point, the mask  124  is parallel to the surface of the substrate  130  and separated from it at a distance of nominally 50 microns. The three alignment balls  540  are preferably precision ground silicon nitride balls, such as those used in the manufacture of ball bearings. The balls  540  are ground to a high degree of sphericity (0.13 microns) and diametric tolerance (±1.3 microns) to ensure accurate alignment. In view of their smoothness, shape and hardness, the balls  540  also act as guiding surfaces for the mask plate  510  as the mask plate is brought into position. By using alignment balls  540  with various mask plates  510  (having respective masks  124 ), different masks can be aligned accurately and precisely with respect to the same substrate  130  (e.g., within 20 microns of the desired alignment, so that adjacent layers are aligned to within 40 microns of each other). 
     As noted above, the mask  124  is relatively thin, so that the mask plate  510  shown in  FIG. 5  can help support the mask from deforming. As shown in  FIG. 5D , the mask plate  510  need not be completely open where it underlies those portions of the mask  124  that do not include the openings  505 ; rather, the mask plate  510  can be used to provide support to the overlying mask  124 . The mask  124  and the mask plate  510  can be spot welded together using the following procedure. First, holes  565   a  in the mask  124  can be aligned with holes  565   b  in the mask plate  510 . Next, dowel pins (not shown) can be passed through the corresponding sets of holes to maintain alignment between the mask and the mask plate during the spot welding process. By adopting this procedure for each mask  124 , with each mask having holes  565   a  in the same positions, one can exchange masks during a deposition process and expect the relative alignment between the mask and the substrate  130  to be the same for each mask/substrate combination. 
     If the mask  124  and the mask plate  510  are made from different materials, they will in general expand at different rates as the temperature increases, e.g., during a high temperature deposition process. To mitigate the problem of differential expansion of materials, an alternative mask  124 ′, such as the one shown in  FIG. 5E , may be used. The mask  124 ′ includes slots  575  (four are shown) that are advantageously curvilinear or serpentine in shape. Portions  580  of the mask  124 ′ between the slots  575  and the circumference of the mask  124 ′ may be spot welded to the underlying mask plate  510  (not shown in  FIG. 5E ). Those portions  585  between the distal end of the slot  575  and the circumference of the mask  124 ′ constitute the weakest portions of the mask  124 ′, and thus are most likely to bend as the mask  124 ′ expands. By using this design, the mask  124 ′ is less likely to buckle in its middle, which could lead to distortion of the mask  124 ′, misalignment of the mask  124 ′ with respect to the mask plate  510 , and/or physical contact between the mask  124 ′ and the substrate  130 . By employing a symmetrical arrangement of the slots  575 , the mask  124 ′ is more likely to maintain its desired alignment with respect to the mask plate  510 . 
     The mask carrier  224  and the substrate carrier  230  are preferably transferred to the receivers  220   a ,  220   b  robotically. When this happens, the receiver assembly  210  (see  FIG. 2 ) may be located inside the deposition chamber, where the material  114  is deposited onto the substrate  130 ; alternatively, the receiver assembly  210  may be in a chamber adjacent to the deposition chamber when the carriers  224 ,  230  are transferred. In principle, a number of different techniques and apparatuses may be employed to transfer the carriers  224 ,  230  to the receivers  220   a ,  220   b ; one approach is now described with reference to  FIG. 6 . A robotic end effector  610  includes two prongs  620   a ,  620   b  that slide underneath respective lips  630   a ,  630   b  of the mask carrier  224  (see  FIG. 6A ). Once the mask carrier  224  is held by the end effector  610  (see  FIG. 6B ), a compressor  640  moves forward and over the mask plate  510  (see  FIG. 6C ). At this point, the compressor  640  moves downward and touches surfaces  570  on opposite sides of the mask plate  510 , thereby pushing the mask plate towards the mask carrier  224 , which compresses the spring  530 . With the mask plate  510  pushed down, the mask carrier  224  can be freely inserted into the receiver  220   a , without touching the substrate carrier  230 . After insertion, the compressor  640  is moved slightly upward, which permits the mask plate  510  to rise (as it is urged upwards by the spring  530 ). The alignment is complete once the mask plate  510  contacts the three tabs  410  (which are part of the previously inserted substrate carrier  230 ) and the alignment balls  540  mate with their counterparts (the radial slots  440 ). At this point, the spring  530  comes to an equilibrium position. Note that the fork tines  650  of the compressor  640  are designed to engage the surfaces  570 , so that the tines no longer touch the mask plate  510  once the alignment is complete. This allows the fork tines  650  to be retracted without disturbing the alignment. 
     To remove the mask carrier  224  from the receiver  220   a , this series of movements may be performed in reverse. Furthermore, the invention allows for masks  124  to be exchanged while leaving the substrate  130  and its substrate carrier  230  in the receiver assembly  210 . The movement of the robotic end effector  610  and the compressor  640  may be controlled by a combination of mechanical components, such as gears, pulleys, levers, limit switches, and screws (not shown). Furthermore, the substrate  130  can be at deposition temperature when the masks  124  are exchanged. 
     EXAMPLES 
     Using the devices and methods described herein, two layers of different materials were deposited onto a silicon wafer at room temperature (approximately 25° C.). The first material, an alloy of Mg/Ta/IrMn/CoFe having a thickness of 49 nm, is evident in  FIG. 7  as a vertically oriented dogbone-shaped region encompassed by the box labeled  710 . The second material, an alloy of CoFe/Ta/Ru having a thickness of 19 nm, was deposited on top of the first material and appears as 5 horizontally oriented dogbone-shaped regions encompassed by the box labeled  720 . The geometric centers of the first and second materials are represented by the points  730  and  732 , respectively, and are separated by 28 microns. 
     In a second experiment, first and second materials were deposited over a TiO 2  substrate that had been previously uniformly coated with a 10 nm thick layer of VO 2 . Using the devices and methods described herein, 6 nm of TiO 2  (the first material) was deposited at room temperature over the substrate. This was followed by depositing 100 nm of Au (the second material) over the deposited TiO 2  at 400° C. The two deposited layers were found to be aligned to within 38 microns. 
     The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is therefore indicated by the appended claims rather than the foregoing description. All changes within the meaning and range of equivalency of the claims are to be embraced within that scope.