Method and apparatus for substrate-mask alignment

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.

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.

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. 1is an overview of a shadow mask deposition apparatus and process, which may involve high temperatures, vacuum, and corrosive and/or oxidizing gases. A deposition source110provides the material114to be deposited, and a shutter120can be placed over the deposition source110to block the flow of material. The material114is directed towards a mask124that in turn is positioned near a wafer or substrate130. The mask124generally 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 mask124includes openings505(as shown inFIG. 5A) that reflect the pattern of material to be deposited onto the substrate130. Specifically, openings (such as slots) have been formed in the mask124, so that the material114passing through these openings is deposited onto the substrate130to form a desired pattern on the wafer. The various elements shown inFIG. 1and the other figures are advantageously contained within one or more chambers (not shown), so that vacuum can be applied during the deposition process.

FIG. 2shows a receiver assembly210that includes two receivers220a,220bfor receiving a mask carrier224(that holds the mask124) and a substrate carrier230(for holding the substrate130), respectively. (Both the mask carrier224and the substrate carrier230can 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 toFIGS. 4 and 5, the mask carrier224and the substrate carrier230are constructed so that, when they are inserted into the receivers220a,220b, 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 carrier224and the substrate carrier230can be independently exchanged in and out of the receivers220a,220busing a robot (as described below in connection withFIG. 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 substrate130, ten different masks may be used in turn.

The receiver assembly210may advantageously include a heater assembly250(as seen in the cutaway portion ofFIG. 2), thereby permitting heat to be directed onto the substrate130. 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 inFIG. 2, the receiver assembly210may be rotated, thereby facilitating more uniform thickness and composition of the materials deposited onto the substrate130. To this end, a rotation motor260working with various gears and ball bearings (not shown) permit the motor to rotate the receiver assembly.

FIG. 3shows an array310of receiver assemblies210a,210b,210cthat can be used in a deposition chamber320(shown in cutaway). The array310can be rotated with a rotation motor working in conjunction with various gears and ball bearings (not shown, but similar to the rotation motor260ofFIG. 2), so that a particular receiver assembly210a,210b, or210cis positioned directly over a desired deposition source. Twelve such deposition sources110a,110b,110c, etc. are shown inFIG. 3, and each deposition source may be dedicated to a different material. Also, the mask carrier224may be exchanged out of the chamber320through a port (not shown). Thus, the configuration shown inFIG. 3facilitates the rapid deposition of layers onto the substrate130, in which each layer is patterned differently and is made of a different material.

As now described with respect toFIGS. 4 and 5, the mask124(which include a pattern of openings505) and the substrate130are 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 substrate130relative to the substrate carrier230, ii) fixing the position of the substrate carrier230relative to the mask carrier224, and iii) fixing the position of the mask124relative to the substrate carrier230. Each of these will now be considered in turn.

First, the positions of the substrate130and the substrate carrier230are fixed relative to one another as follows. (This is typically done beforehand in atmosphere, outside of any deposition chamber.) As shown inFIG. 4, the substrate carrier230includes three tabs410(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 substrate130into the substrate carrier230are possible when the flats of the tabs410are tangential to the circumference of the substrate. When the tabs410are rotated 90 degrees from this position, as shown inFIG. 4B, the tabs contact the substrate130at three places along its circumference. Three spring fingers420(e.g., made of Inconel 625®) on the side of the substrate130opposite the tabs410(see the inset inFIG. 4C) maintain a small force against the backside of the substrate to keep it in contact with the tabs. This pinching action prevents the substrate130from moving out of position relative to the substrate carrier230. The thickness of each tab410at its end411is 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 substrate130from the mask124may be kept substantially constant (e.g., about 50 microns) across the face of the substrate, in accordance with the procedures discussed below in connection withFIGS. 4 and 5. (Note that additional components related to thermal management may be incorporated into the substrate carrier230, such as a disk435(e.g., made of SiC) for heating the substrate130, heat shields437(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 carrier230and the mask carrier224are fixed relative to one another as follows. As suggested byFIG. 2, the mask carrier224and the substrate carrier230are separately inserted into the U-shaped receivers220a,220b. (A robotic mechanism, such as the one described below in connection withFIG. 6, may be used for this purpose.) The receiver220ahas edges that engage longitudinal slots550in the mask carrier224; likewise, the receiver220bhas edges that engage longitudinal slots450in the substrate carrier230. The mask carrier224and the substrate carrier230have respective integral leaf springs560and460, which provide a small restraining force to keep the carrier in place once it has been inserted into its corresponding receiver. Accordingly, the mask carrier224and the substrate carrier230are brought into coarse alignment when they are inserted into the receivers220a,220b, with a positioning tolerance of ±0.4 mm in each of the XYZ directions and ±1 degree about the vertical axis of the receiver assembly210. This by itself is not good enough for the desired shadow masking operations, but when additional adjustments are made, as described below, the mask124and the substrate130can 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 mask124and the substrate carrier230are fixed relative to one another as follows. The mask124has been previously (and precisely, as described below with respect toFIG. 5D) spot welded to a mask plate510, which is most clearly evident from the cross-sectional view ofFIG. 5C. As a result of the welding process, the mask124and the mask plate510form an integral unit. The mask plate510fits within a retainer520of the mask carrier224. The resulting structure ensures that the mask124remains substantially fixed with respect to the mask carrier224, allowing for only limited lateral, vertical, and rotational motion of the mask with respect to the mask carrier. During deposition operations, a coiled wire spring530tends to urge the mask plate510upwards (seeFIG. 5C), until the top portion of the mask plate contacts the three tabs410. (On the other hand, when the mask carrier224is not in the receiver assembly210, upward motion of the mask carrier224is constrained by the retainer520, as suggested byFIG. 5C.) Preferred materials for the mask plate510, the retainer520, and the spring530include HAYNES® 230® alloy, Stainless Steel Type 304, and Inconel X750®, respectively.

Additional features in the top portion of the mask plate510interact with features in the substrate carrier230to ensure that, during deposition operations, the mask124remains fixed with respect to the substrate carrier (and hence the substrate130). Specifically, three radial slots440in the substrate carrier230(see especiallyFIG. 4B) mate with the three alignment balls540ofFIG. 5A. (The width of the slots440is 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 plate510and positioned within 3 microns of their target locations within the mask plate. When the top portion of the mask plate510is brought into contact with the three tabs410of the substrate carrier230, the three alignment balls540simultaneously engage the three radial slots440to 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 mask124is parallel to the surface of the substrate130and separated from it at a distance of nominally 50 microns. The three alignment balls540are preferably precision ground silicon nitride balls, such as those used in the manufacture of ball bearings. The balls540are 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 balls540also act as guiding surfaces for the mask plate510as the mask plate is brought into position. By using alignment balls540with various mask plates510(having respective masks124), different masks can be aligned accurately and precisely with respect to the same substrate130(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 mask124is relatively thin, so that the mask plate510shown inFIG. 5can help support the mask from deforming. As shown inFIG. 5D, the mask plate510need not be completely open where it underlies those portions of the mask124that do not include the openings505; rather, the mask plate510can be used to provide support to the overlying mask124. The mask124and the mask plate510can be spot welded together using the following procedure. First, holes565ain the mask124can be aligned with holes565bin the mask plate510. 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 mask124, with each mask having holes565ain the same positions, one can exchange masks during a deposition process and expect the relative alignment between the mask and the substrate130to be the same for each mask/substrate combination.

If the mask124and the mask plate510are 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 mask124′, such as the one shown inFIG. 5E, may be used. The mask124′ includes slots575(four are shown) that are advantageously curvilinear or serpentine in shape. Portions580of the mask124′ between the slots575and the circumference of the mask124′ may be spot welded to the underlying mask plate510(not shown inFIG. 5E). Those portions585between the distal end of the slot575and the circumference of the mask124′ constitute the weakest portions of the mask124′, and thus are most likely to bend as the mask124′ expands. By using this design, the mask124′ is less likely to buckle in its middle, which could lead to distortion of the mask124′, misalignment of the mask124′ with respect to the mask plate510, and/or physical contact between the mask124′ and the substrate130. By employing a symmetrical arrangement of the slots575, the mask124′ is more likely to maintain its desired alignment with respect to the mask plate510.

The mask carrier224and the substrate carrier230are preferably transferred to the receivers220a,220brobotically. When this happens, the receiver assembly210(seeFIG. 2) may be located inside the deposition chamber, where the material114is deposited onto the substrate130; alternatively, the receiver assembly210may be in a chamber adjacent to the deposition chamber when the carriers224,230are transferred. In principle, a number of different techniques and apparatuses may be employed to transfer the carriers224,230to the receivers220a,220b; one approach is now described with reference toFIG. 6. A robotic end effector610includes two prongs620a,620bthat slide underneath respective lips630a,630bof the mask carrier224(seeFIG. 6A). Once the mask carrier224is held by the end effector610(seeFIG. 6B), a compressor640moves forward and over the mask plate510(seeFIG. 6C). At this point, the compressor640moves downward and touches surfaces570on opposite sides of the mask plate510, thereby pushing the mask plate towards the mask carrier224, which compresses the spring530. With the mask plate510pushed down, the mask carrier224can be freely inserted into the receiver220a, without touching the substrate carrier230. After insertion, the compressor640is moved slightly upward, which permits the mask plate510to rise (as it is urged upwards by the spring530). The alignment is complete once the mask plate510contacts the three tabs410(which are part of the previously inserted substrate carrier230) and the alignment balls540mate with their counterparts (the radial slots440). At this point, the spring530comes to an equilibrium position. Note that the fork tines650of the compressor640are designed to engage the surfaces570, so that the tines no longer touch the mask plate510once the alignment is complete. This allows the fork tines650to be retracted without disturbing the alignment.

To remove the mask carrier224from the receiver220a, this series of movements may be performed in reverse. Furthermore, the invention allows for masks124to be exchanged while leaving the substrate130and its substrate carrier230in the receiver assembly210. The movement of the robotic end effector610and the compressor640may be controlled by a combination of mechanical components, such as gears, pulleys, levers, limit switches, and screws (not shown). Furthermore, the substrate130can be at deposition temperature when the masks124are 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 inFIG. 7as a vertically oriented dogbone-shaped region encompassed by the box labeled710. 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 labeled720. The geometric centers of the first and second materials are represented by the points730and732, respectively, and are separated by 28 microns.

In a second experiment, first and second materials were deposited over a TiO2substrate that had been previously uniformly coated with a 10 nm thick layer of VO2. Using the devices and methods described herein, 6 nm of TiO2(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 TiO2at 400° C. The two deposited layers were found to be aligned to within 38 microns.