Title of Invention	"APPARATUS AND METHOD FOR FRONT- TO -BACK ALIGNMENT OF PHOTOSENSITIZED SUBSTRATES"
Abstract	The present invention relates to front to back alignment apparatus for photosensitized substrates. The present invention also relates to a method for positioning a photosensitized substrate for printing a pattern from the mask symmetrically onto the front to back photosensitized substrate in an exposure apparatus used in a photoUthographic process for fabricating semiconductor devices, image pick-up devices such as charge-coupled devices, hquid crystal displays, thin film magnetic heads, sensors, actuators, MEMS or the like.

Title of Invention

"APPARATUS AND METHOD FOR FRONT- TO -BACK ALIGNMENT OF PHOTOSENSITIZED SUBSTRATES"

Abstract

The present invention relates to front to back alignment apparatus for photosensitized substrates. The present invention also relates to a method for positioning a photosensitized substrate for printing a pattern from the mask symmetrically onto the front to back photosensitized substrate in an exposure apparatus used in a photoUthographic process for fabricating semiconductor devices, image pick-up devices such as charge-coupled devices, hquid crystal displays, thin film magnetic heads, sensors, actuators, MEMS or the like.

Full Text	FRONT TO BACK ALIGNMENT APPARATUS FOR PHOTOSENSITIZED SUBSTRATES Field of the invention The present invention relates to front to back alignment apparatus for photosensitized substrates. The present invention also relates to a method for positioning a photosensitized substrate for printing a pattern from the mask symmetrically onto the front to back photosensitized substrate in an exposure apparatus used in a photolithographic process for fabricating semiconductor devices, image pick-up devices such as charge-coupled devices, liquid crystal displays, thin film magnetic heads, sensors, actuators, MEMS or the like. Background of the invention Lithographic techniques for an important part in the manufacture of semiconductor circuits. Photolithographic processes require that the wafer coated with a photosensitive compound, generally known as photoresist is positioned relative to the image of a mask or reticle containing circuit patterns thereon. Because the processing requires multiple layers to be placed on the wafer on one or both sides thereof, precise positioning of these multiple layers is required. The position of the reticle image and the wafer often must be known and aligned within a range of some nanometers. In any of various exposure apparatuses used for fabrication of semiconductor devices, liquid crystal displays or the like including a projection exposure apparatus (such as a stepper) and a proximity printing exposure apparatus, in order to transfer a circuit pattern formed on a mask or reticle onto a photoresist film formed on a photosensitized substrate such as a wafer (or a glass plate, etc.) with high registration, it is required to establish alignment between the reticle and the wafer with precision. Various types of alignment sensor systems have been proposed including: "laser step alignment (LSA) type" in which a wafer has an alignment mark formed thereon which comprises a linear array of dots, and a laser beam illuminates the alignment mark to produce diffracted or scattered beams, which are used to detect the position of the alignment mark (such as disclosed in US Patent 5,243,195); "field image alignment (FIA) type" in which an image-sensing device is used to take an image of an alignment mark which is illuminated by illumination light having a continuous spectrum of a wide wavelength range obtainable from a halogen lamp, and the picture data of the image is subjected to an image processing to measure the position of the alignment mark; and "laser interferometric alignment (LIA) type" in which a wafer has an alignment mark formed thereon which comprises a diffraction grating, two laser beams having different frequencies with a small difference between them illuminate the alignment mark from different directions to produce two diffracted beams interfering with each other, and the position of the alignment mark is determined from the phase of the interference. There have been also proposed various alignment techniques which may be categorized into three types including: "through-the-lens (TTL) type" in which the position of the wafer is measured through a projection optical system; "through-the-reticle (TTR) type" in which the relative position of a wafer with respect to a reticle is measured through both a projection optical system and the reticle; and "off-axis type" in which the position of the wafer is measured directly, or not through a projection optical system. By using any of these alignment sensor systems to detect respective positions of two points on a wafer which is placed on a wafer stage, the rotational position (or rotational angle) of the wafer may be determined in addition to the position of the wafer with respect to the translational displacement. Alignment sensor systems usable for determining the rotational angle of a wafer include LIA-type system using TTL-type technique, LSA-type system using TTL-type technique, FIA-type system using off-axis-type technique, and others. The exposure apparatus is required not only to have the ability of establishing alignment with precision between a reticle and a wafer by using the detection results obtained from the alignment sensor, but also to quickly establish such alignment so as to keep high throughput (the number of wafers that can be processed per unit of time). Thus, it is needed to realize highly effective operations in all the steps performed in relation to the exposure apparatus, from a transfer operation of a wafer onto the wafer stage to an exposure operation. In the manufacture of semiconductors, circuitry patterns are commonly transferred by lithography to multiple chips on a single semiconductor wafer. The lithography process generally comprises coating the wafer with a photoresist, exposing the photoresist in a pattern corresponding to the circuitry pattern desired, and developing the wafer to remove the photoresist in the exposed areas. Further processing, such as etching steps, may then follow using the patterned photoresist as a mask. In commonly used integrated circuits, display devices, microprocessors, memory chips, etc., the entire sequence of photolithographic processes is carried out only on one side of the silicon wafer or other substrate. Thus the alignment of mask or reticle is with respect to the wafer or substrate 'flat' for the first mask (Ml) alignment. For subsequent stages of alignment (Mask # M2, M3, M4, ... etc.), the reference is taken from the alignment marks formed in the previous lithography or etching process. In the case of some semiconductor devices such as power diodes, thyristors, sensors, actuators, MEMS and the like, the photolithography and other processing steps such as etching, diffusion, etc., are required on both sides of the silicon wafer or other substrate. In these devices it is functionally required to register patterns by photolithographic/etching process on one side of the wafer with reference to the pattern already formed on the other side of the wafer. Since silicon wafers are opaque to visible Hght, such alignment of wafer cannot be accomphshed using conventional mask to wafer aligners, which use visible light for such purposes. The method used normally comprises double side mask aligners which are very expensive and based on infra red (IR) imaging, since IR in a specific wavelength range can pass through silicon. IR radiation is used to image the patterns formed on one side of the wafer or substrate. Using this image, the pattern is registered on the other side of the wafer/substrate. In a second method of front to back alignment, the image on one side of the wafer is captured and stored and this is then used to register the pattern on the other side of the wafer/substrate. The prior art methods do not address the problem of alignment of a wafer where front to back symmetric printing of a pattern is required. Objects of the invention. The main object of the invention is to provide an apparatus for ensuring front to back alignment of a photosensitized substrate in a photolithographic process. It is another object of the invention to provide an apparatus for ensuring symmetric printing on the two sides of a photosensitzed substrate in a photolithographic process. It is yet another object of the invention to provide a method for the front to back alignment of a photosensitzed substrate in a photolithographic process. A further object of the invention is to provide a method for the front to back alignment of a photosensitized substrate to ensure symmetric printing thereon in a photoUthographic process. Summary of the invention Accordingly the present invention provides an apparatus for the front to back alignment of a photosensitized substrate comprising a jig, said jig being provided with at least three orthogonal sides and a set of slots provided on the periphery thereof, a mask plate with a set of indicators provided thereon and with a set of slots provided on the periphery thereof corresponding to the respective slots provided on the jig and of a dimension larger than the slots on the jig, enabling the mask plate to be fixed firmly on the jig. In one embodiment of the invention, the mask plate has at least three orthogonal sides corresponding to the orthogonal sides of the jig enabling the mask plate to rest firmly by means thereof against the jig. In another embodiment of the invention, the connection means comprises of screw means passing through said respective sets of slots provided on the mask plate and on said jig. In another embodiment of the invention, the set of indicators comprise a plurality of visual indicators. In another embodiment of the invention, the visual indicators comprise of a set of cross hairs. In another embodiment of the invention, the mask plate is operatively connected to the jig by means of connection means through respective sets of slots. The present invention also relates to a method for the manufacture of a photosensitized substrate with means for front to back alignment provided thereon to enable front to back symmetric etching thereof, using an apparatus comprising a jig, said jig being provided with at least three orthogonal sides and a set of slots provided on the periphery thereof, a mask plate with a set of indicators provided thereon and with a set of slots provided on the periphery thereof corresponding to the respective slots provided on the jig and of a dimension larger than the slots on the jig, enabling the mask plate to be fixed firmly on the jig, said method comprising positioning a photosensitized substrate with at least one flat side on the alignment jig such that the flat side corresponds with at lest one flat side of the said jig, a second side of the substrate being plated against a second flat side of the jig, securing the position of the substrate by means of a vacuum, placing a mask plate with sets of indicators provided thereon on top of the substrate such that the bottom of the mask plate is in contact with the front surface of the substrate, subjecting the assembly to UV radiation in order to print a set of indicators on the front surface of the substrate which coincide exactly with the respective indicators of the mask plate, removing the wafer after formation of the indicators on the front surface thereon, and positioning it again on the jig such that the marked side is in contact with the base plate of the jig and the reverse unmarked side of the substrate is exposed, the flat side of the surface being in contact with the same flat side of the jig and at least one other side of the substrate in contact with a third flat side of the jig, securing the substrate by applying vacuum, reversing the mask plate such that the bottom side thereof is in contact with the exposed surface of the substrate, exposing the assembly to UV source such that the same set of indicators is formed on the reverse surface of the substrate and corresponding exactiy to the set of indicators formed on the front side thereof In one embodiment of the invention, the set of indicators comprise of visual indicators. In another embodiment of the invention, the visual indicators comprise of a set of cross hairs. In another embodiment of the invention, the mask plate is operatively connected to the jig by means of connection means through respective sets of slots. Brief description of the accompanying drawings Figure 1 is a schematic representation of one part of the method of the invention wherein front etching is achieved by the system of the invention. Figure 2 is a schematic representation of the second part of the method of the invention wherein back etching is achieved in a manner symmetric to the front etching by the system of the invention. Detailed description of the invention The present invention provides a simple and easy to operate system for ensuring that front to back alignment is achieved for photosensitized substrates which need to be etched or coated in a photohthographic system. The system of the invention is mechanical and does not require expensive electronic monitoring systems. The invention will now be described with reference to the accompanying drawings. Figure 1 is a schematic representation of one part of the method of the invention wherein front etching is achieved by the system of the invention. A wafer is first coated with a positive photoresist on the front side and partially prebaked according to conventional methods. The partially prebaked wafer is then coated with a positive photoresist on the reverse side thereof and the prebaking is completed. The wafer (5) is then positioned on the alignment jig (6) as shown in Figure 1. The wafer generally has at least one partially flat side (51) which is aligned with side 1-2 of the jig (6). The right side (52) of the wafer (5) is then pressed against the right side (2 - 3) of the jig (6). A vacuum is then operated to hold the wafer in position. An stainless steel mask plate (7) with a set of indicators (8) machined out thereon is placed on top of the wafer (5) and pressed in the direction of the arrow shown in Figure 1. The indicators (8) can comprise for example cross hairs or any other standard indicators. The stainless steel mask plate (7) is also provided with slots (9) on the periphery thereof which correspond with respective slots (10) provided on the jig. The slots on the mask plate are preferably larger than the respective slots in the jig, thereby enabling the mask plate to be fixed to the jig firmly through any fixing mechanism such as screws and washers. This ensures that the wafer that is sandwiched between the mask plate and the jig is also fixed firmly thereby completing the front side alignment. After firm positioning, the front side of the wafer is exposed to a UV source in order register a set of indicators thereon on the photoresist layer corresponding exactly to the respective indicators on the mask plate. Figure 2 shows a schematic representation of the second part of the method of the invention wherein back etching is achieved in a manner symmetric to the front etching as described above. The wafer (21) is removed from the jig (22) and its side reversed such that the front side now faces the top of the jig plate and the reverse side faces the mask plate. The flat side (51) of the wafer is aligned with side 1-2 of the jig and the left side (53) of the wafer is aligned with side 1-4 of the jig. The wafer is then kept in position by any conventional means such as application of vacuum. This completes the back alignment of the wafer enabling the next step of aligning the stainless steel mask plate to be placed thereon. The mask plate (23) is also reversed such that its bottom surface is now on the top and the top surface thereof is in contact with the wafer. The mask plate is pushed along the direction of the arrow shown in Figure 2 till it finally comes to a rest against side 1-2 of the jig. The mask plate is secured to the jig through the respective slots provided on both the jig and the mask plate. The wafer is now exposed to a UV source in order to ensure that a set of indicators ate formed thereon on the reverse side which correspond exactly to the respective indicators on the mask plate. The photoresist is now developed. The indications on the front and reverse side thereof correspond exactly. This is achieved primarily due to the orthogonal arrangement of the sides 1-2,1-4 and 2-3 of the jig. This ensures that the wafer and the jig are in the same relative positions during both exposures to UV source. Another advantage is that since the same mask plate is used, the indicators on both the reverse and the front side of fhe photoresist correspond exactly. The wafer with the respective sets of indications can now be subjected to etching on both sides using a suitable etching solution. The etching can be done in tandem or simultaneously. AH that is required is that each set of masks be provided with the same sets of indicators thereon corresponding to the respective indicators on the wafer. A simple mechanical alignment of the indicators with visual confirmation of alignment of the wafer and the respective mask can be done. For example a first mask is placed on the jig such that the set of indicators provided thereon correspond with the indicators provided on the front side of the wafer. A second mask with another set of corresponding indicators is then placed such it is in contact with the reverse side of the wafer such the second mask indicators. If desired, the oxide can first be partially etched in wet chemical such as buffered HF to leave a set of reverse side alignment marks in the oxide which can then be used for the second mask alignment. The system of the invention considerably simplifies the task of front to back side alignment for any photosensitized substrate to be etched. The system has its application is several fields such as in fabricating semiconductor power devices, sensors, actuators and MEMS such as pressure sensors, accelerometers, and the like. We claim: 1. An apparatus for the front to back alignment of a photosensitized substrate comprising a jig, said jig being provided with at least three orthogonal sides and a set of slots provided on the periphery thereof, a mask plate with a set of indicators provided thereon and with a set of slots provided on the periphery thereof corresponding to the respective slots provided on the jig and of a dimension larger than die slots on the jig, enabling the mask plate to be fixed firmly on the jig. 2. An apparatus as claimed in claim 1 wherein the mask plate has at least three orthogonal sides corresponding to the orthogonal sides of the jig enabling the mask plate to rest firmly by means thereof against the jig. 3. An apparatus as claimed in claim 1 wherein the connection means comprises of screw passing through said respective sets of slots provided on the mask plate and on said jig. 4. An apparatus as claimed in claim 1 wherein the set of indicators comprise a plvirality of visual indicators. 5. An apparatus as claimed in claim 4 wherein the visual indicators comprise of a set of cross hairs. 6. An apparatus as claimed in claim 1 wherein the mask plate is operatively connected to the jig by means of connection means through respective sets of slots. 7. A method for the manufacture of a photosensitized substrate with means for front to back alignment provided thereon to enable front to back symmetric etching thereof, using an apparatus comprising a jig, said jig being provided with at least three orthogonal sides and a set of slots provided on the periphery thereof, a mask plate with a set of indicators provided thereon and with a set of slots provided on the periphery thereof corresponding to the respective slots provided on the jig and of a dimension larger than the slots on the jig, enabling the mask plate to be fixed firmly on the jig, said method comprising positioning a photosensitized substrate with at least one flat side on the alignment jig such that the flat side corresponds with at lest one flat side of the said jig, a second side of the substrate being plated against a second flat side of the jig, securing the position of the substrate by means of a vacuum, placing a mask plate with sets of indicators provided thereon on top of the substrate such that the bottom of the mask plate is in contact with the front surface of the substrate, subjecting the assembly to UV radiation in order to print a set of indicators on the front surface of the substrate which coincide exactly with the respective indicators of the mask plate, removing the wafer after formation of the indicators on the front svirface thereon, and positioning it again on the jig such that the marked side is in contact with the base plate of the jig and the reverse unmarked side of the substrate is exposed, the flat side of the surface being in contact with the same flat side of the jig and at least one other side of the substrate in contact with a third flat side of the jig, securing the substrate by applying vacuum, reversing the mask plate such that the bottom side thereof is in contact with the exposed surface of the substrate, exposing the assembly to UV source such that the same set of indicators is formed on the reverse surface of the substrate and corresponding exactly to the set of indicators formed on the front side thereof 8. A method as claimed in claim 7 wherein the set of indicators comprise of visual indicators. 9. A method as claimed in claim 8 wherein the visual indicators comprise of a set of cross hairs. 10. A method as claimed in claim 7 wherein the mask plate is operatively connected to the jig by means of connection means through respective sets of slots. 11. An apparatus for the front to back alignment of a photosensitized substrate substantially as described hereinbefore and with reference to the accompanying drawings. 12. A method for the manufacture of a photosensitized substrate substantially as described hereinbefore and with reference to the accompanying drawings.

Full Text

FRONT TO BACK ALIGNMENT APPARATUS FOR PHOTOSENSITIZED SUBSTRATES
Field of the invention
The present invention relates to front to back alignment apparatus for photosensitized substrates. The present invention also relates to a method for positioning a photosensitized substrate for printing a pattern from the mask symmetrically onto the front to back photosensitized substrate in an exposure apparatus used in a photolithographic process for fabricating semiconductor devices, image pick-up devices such as charge-coupled devices, liquid crystal displays, thin film magnetic heads, sensors, actuators, MEMS or the like. Background of the invention
Lithographic techniques for an important part in the manufacture of semiconductor circuits. Photolithographic processes require that the wafer coated with a photosensitive compound, generally known as photoresist is positioned relative to the image of a mask or reticle containing circuit patterns thereon. Because the processing requires multiple layers to be placed on the wafer on one or both sides thereof, precise positioning of these multiple layers is required. The position of the reticle image and the wafer often must be known and aligned within a range of some nanometers.
In any of various exposure apparatuses used for fabrication of semiconductor devices, liquid crystal displays or the like including a projection exposure apparatus (such as a stepper) and a proximity printing exposure apparatus, in order to transfer a circuit pattern formed on a mask or reticle onto a photoresist film formed on a photosensitized substrate such as a wafer (or a glass plate, etc.)
with high registration, it is required to establish alignment between the reticle and the wafer with precision.
Various types of alignment sensor systems have been proposed including: "laser step alignment (LSA) type" in which a wafer has an alignment mark formed thereon which comprises a linear array of dots, and a laser beam illuminates the alignment mark to produce diffracted or scattered beams, which are used to detect the position of the alignment mark (such as disclosed in US Patent 5,243,195); "field image alignment (FIA) type" in which an image-sensing device is used to take an image of an alignment mark which is illuminated by illumination light having a continuous spectrum of a wide wavelength range obtainable from a halogen lamp, and the picture data of the image is subjected to an image processing to measure the position of the alignment mark; and "laser interferometric alignment (LIA) type" in which a wafer has an alignment mark formed thereon which comprises a diffraction grating, two laser beams having different frequencies with a small difference between them illuminate the alignment mark from different directions to produce two diffracted beams interfering with each other, and the position of the alignment mark is determined from the phase of the interference. There have been also proposed various alignment techniques which may be categorized into three types including: "through-the-lens (TTL) type" in which the position of the wafer is measured through a projection optical system; "through-the-reticle (TTR) type" in which the relative position of a wafer with respect to a reticle is measured through both a projection optical system and the reticle; and "off-axis type" in which the position of the wafer is measured directly, or not through a projection optical system.
By using any of these alignment sensor systems to detect respective positions of two points on a wafer which is placed on a wafer stage, the rotational position (or rotational angle) of the wafer may be determined in addition to the position of the wafer with respect to the translational displacement. Alignment sensor systems usable for determining the rotational angle of a wafer include LIA-type system using TTL-type technique, LSA-type system using TTL-type technique, FIA-type system using off-axis-type technique, and others.
The exposure apparatus is required not only to have the ability of establishing alignment with precision between a reticle and a wafer by using the detection results obtained from the alignment sensor, but also to quickly establish such alignment so as to keep high throughput (the number of wafers that can be processed per unit of time). Thus, it is needed to realize highly effective operations in all the steps performed in relation to the exposure apparatus, from a transfer operation of a wafer onto the wafer stage to an exposure operation.
In the manufacture of semiconductors, circuitry patterns are commonly transferred by lithography to multiple chips on a single semiconductor wafer. The lithography process generally comprises coating the wafer with a photoresist, exposing the photoresist in a pattern corresponding to the circuitry pattern desired, and developing the wafer to remove the photoresist in the exposed areas. Further processing, such as etching steps, may then follow using the patterned photoresist as a mask.
In commonly used integrated circuits, display devices, microprocessors, memory chips, etc., the entire sequence of photolithographic processes is carried out only on one side of the silicon wafer or other substrate. Thus the alignment of
mask or reticle is with respect to the wafer or substrate 'flat' for the first mask (Ml) alignment. For subsequent stages of alignment (Mask # M2, M3, M4, ... etc.), the reference is taken from the alignment marks formed in the previous lithography or etching process.
In the case of some semiconductor devices such as power diodes, thyristors, sensors, actuators, MEMS and the like, the photolithography and other processing steps such as etching, diffusion, etc., are required on both sides of the silicon wafer or other substrate. In these devices it is functionally required to register patterns by photolithographic/etching process on one side of the wafer with reference to the pattern already formed on the other side of the wafer. Since silicon wafers are opaque to visible Hght, such alignment of wafer cannot be accomphshed using conventional mask to wafer aligners, which use visible light for such purposes. The method used normally comprises double side mask aligners which are very expensive and based on infra red (IR) imaging, since IR in a specific wavelength range can pass through silicon. IR radiation is used to image the patterns formed on one side of the wafer or substrate. Using this image, the pattern is registered on the other side of the wafer/substrate. In a second method of front to back alignment, the image on one side of the wafer is captured and stored and this is then used to register the pattern on the other side of the wafer/substrate.
The prior art methods do not address the problem of alignment of a wafer where front to back symmetric printing of a pattern is required. Objects of the invention.
The main object of the invention is to provide an apparatus for ensuring front to back alignment of a photosensitized substrate in a photolithographic process.
It is another object of the invention to provide an apparatus for ensuring symmetric printing on the two sides of a photosensitzed substrate in a photolithographic process.
It is yet another object of the invention to provide a method for the front to back alignment of a photosensitzed substrate in a photolithographic process.
A further object of the invention is to provide a method for the front to back alignment of a photosensitized substrate to ensure symmetric printing thereon in a photoUthographic process. Summary of the invention
Accordingly the present invention provides an apparatus for the front to back alignment of a photosensitized substrate comprising a jig, said jig being provided with at least three orthogonal sides and a set of slots provided on the periphery thereof, a mask plate with a set of indicators provided thereon and with a set of slots provided on the periphery thereof corresponding to the respective slots provided on the jig and of a dimension larger than the slots on the jig, enabling the mask plate to be fixed firmly on the jig.
In one embodiment of the invention, the mask plate has at least three orthogonal sides corresponding to the orthogonal sides of the jig enabling the mask plate to rest firmly by means thereof against the jig.
In another embodiment of the invention, the connection means comprises of screw means passing through said respective sets of slots provided on the mask plate and on said jig.
In another embodiment of the invention, the set of indicators comprise a plurality of visual indicators.
In another embodiment of the invention, the visual indicators comprise of a set of cross hairs.
In another embodiment of the invention, the mask plate is operatively connected to the jig by means of connection means through respective sets of slots.
The present invention also relates to a method for the manufacture of a photosensitized substrate with means for front to back alignment provided thereon to enable front to back symmetric etching thereof, using an apparatus comprising a jig, said jig being provided with at least three orthogonal sides and a set of slots provided on the periphery thereof, a mask plate with a set of indicators provided thereon and with a set of slots provided on the periphery thereof corresponding to the respective slots provided on the jig and of a dimension larger than the slots on the jig, enabling the mask plate to be fixed firmly on the jig, said method comprising positioning a photosensitized substrate with at least one flat side on the alignment jig such that the flat side corresponds with at lest one flat side of the said jig, a second side of the substrate being plated against a second flat side of the jig, securing the position of the substrate by means of a vacuum, placing a mask plate with sets of indicators provided thereon on top of the substrate such that the bottom of the mask plate is in contact with the front surface of the substrate, subjecting the assembly to UV radiation in order to print a set of indicators on the
front surface of the substrate which coincide exactly with the respective indicators of the mask plate, removing the wafer after formation of the indicators on the front surface thereon, and positioning it again on the jig such that the marked side is in contact with the base plate of the jig and the reverse unmarked side of the substrate is exposed, the flat side of the surface being in contact with the same flat side of the jig and at least one other side of the substrate in contact with a third flat side of the jig, securing the substrate by applying vacuum, reversing the mask plate such that the bottom side thereof is in contact with the exposed surface of the substrate, exposing the assembly to UV source such that the same set of indicators is formed on the reverse surface of the substrate and corresponding exactiy to the set of indicators formed on the front side thereof
In one embodiment of the invention, the set of indicators comprise of visual indicators.
In another embodiment of the invention, the visual indicators comprise of a set of cross hairs.
In another embodiment of the invention, the mask plate is operatively connected to the jig by means of connection means through respective sets of slots. Brief description of the accompanying drawings
Figure 1 is a schematic representation of one part of the method of the invention wherein front etching is achieved by the system of the invention.
Figure 2 is a schematic representation of the second part of the method of the invention wherein back etching is achieved in a manner symmetric to the front etching by the system of the invention.
Detailed description of the invention
The present invention provides a simple and easy to operate system for ensuring that front to back alignment is achieved for photosensitized substrates which need to be etched or coated in a photohthographic system. The system of the invention is mechanical and does not require expensive electronic monitoring systems.
The invention will now be described with reference to the accompanying drawings.
Figure 1 is a schematic representation of one part of the method of the invention wherein front etching is achieved by the system of the invention.
A wafer is first coated with a positive photoresist on the front side and partially prebaked according to conventional methods. The partially prebaked wafer is then coated with a positive photoresist on the reverse side thereof and the prebaking is completed. The wafer (5) is then positioned on the alignment jig (6) as shown in Figure 1. The wafer generally has at least one partially flat side (51) which is aligned with side 1-2 of the jig (6). The right side (52) of the wafer (5) is then pressed against the right side (2 - 3) of the jig (6). A vacuum is then operated to hold the wafer in position. An stainless steel mask plate (7) with a set of indicators (8) machined out thereon is placed on top of the wafer (5) and pressed in the direction of the arrow shown in Figure 1. The indicators (8) can comprise for example cross hairs or any other standard indicators. The stainless steel mask plate (7) is also provided with slots (9) on the periphery thereof which correspond with respective slots (10) provided on the jig. The slots on the mask plate are preferably larger than the respective slots in the jig, thereby enabling the mask plate to be fixed
to the jig firmly through any fixing mechanism such as screws and washers. This ensures that the wafer that is sandwiched between the mask plate and the jig is also fixed firmly thereby completing the front side alignment. After firm positioning, the front side of the wafer is exposed to a UV source in order register a set of indicators thereon on the photoresist layer corresponding exactly to the respective indicators on the mask plate.
Figure 2 shows a schematic representation of the second part of the method of the invention wherein back etching is achieved in a manner symmetric to the front etching as described above.
The wafer (21) is removed from the jig (22) and its side reversed such that the front side now faces the top of the jig plate and the reverse side faces the mask plate. The flat side (51) of the wafer is aligned with side 1-2 of the jig and the left side (53) of the wafer is aligned with side 1-4 of the jig. The wafer is then kept in position by any conventional means such as application of vacuum. This completes the back alignment of the wafer enabling the next step of aligning the stainless steel mask plate to be placed thereon. The mask plate (23) is also reversed such that its bottom surface is now on the top and the top surface thereof is in contact with the wafer. The mask plate is pushed along the direction of the arrow shown in Figure 2 till it finally comes to a rest against side 1-2 of the jig. The mask plate is secured to the jig through the respective slots provided on both the jig and the mask plate.
The wafer is now exposed to a UV source in order to ensure that a set of indicators ate formed thereon on the reverse side which correspond exactly to the respective indicators on the mask plate.
The photoresist is now developed. The indications on the front and reverse side thereof correspond exactly. This is achieved primarily due to the orthogonal arrangement of the sides 1-2,1-4 and 2-3 of the jig. This ensures that the wafer and the jig are in the same relative positions during both exposures to UV source. Another advantage is that since the same mask plate is used, the indicators on both the reverse and the front side of fhe photoresist correspond exactly.
The wafer with the respective sets of indications can now be subjected to etching on both sides using a suitable etching solution. The etching can be done in tandem or simultaneously. AH that is required is that each set of masks be provided with the same sets of indicators thereon corresponding to the respective indicators on the wafer. A simple mechanical alignment of the indicators with visual confirmation of alignment of the wafer and the respective mask can be done.
For example a first mask is placed on the jig such that the set of indicators provided thereon correspond with the indicators provided on the front side of the wafer. A second mask with another set of corresponding indicators is then placed such it is in contact with the reverse side of the wafer such the second mask indicators. If desired, the oxide can first be partially etched in wet chemical such as buffered HF to leave a set of reverse side alignment marks in the oxide which can then be used for the second mask alignment.
The system of the invention considerably simplifies the task of front to back side alignment for any photosensitized substrate to be etched. The system has its application is several fields such as in fabricating semiconductor power devices, sensors, actuators and MEMS such as pressure sensors, accelerometers, and the like.

We claim:
1. An apparatus for the front to back alignment of a photosensitized substrate comprising a jig, said jig being provided with at least three orthogonal sides and a set of slots provided on the periphery thereof, a mask plate with a set of indicators provided thereon and with a set of slots provided on the periphery thereof corresponding to the respective slots provided on the jig and of a dimension larger than die slots on the jig, enabling the mask plate to be fixed firmly on the jig.
2. An apparatus as claimed in claim 1 wherein the mask plate has at least three orthogonal sides corresponding to the orthogonal sides of the jig enabling the mask plate to rest firmly by means thereof against the jig.
3. An apparatus as claimed in claim 1 wherein the connection means comprises of screw passing through said respective sets of slots provided on the mask plate and on said jig.
4. An apparatus as claimed in claim 1 wherein the set of indicators comprise a plvirality of visual indicators.
5. An apparatus as claimed in claim 4 wherein the visual indicators comprise of a set of cross hairs.
6. An apparatus as claimed in claim 1 wherein the mask plate is operatively connected to the jig by means of connection means through respective sets of slots.
7. A method for the manufacture of a photosensitized substrate with means for front to back alignment provided thereon to enable front to back symmetric etching thereof, using an apparatus comprising a jig, said jig being provided with
at least three orthogonal sides and a set of slots provided on the periphery thereof, a mask plate with a set of indicators provided thereon and with a set of slots provided on the periphery thereof corresponding to the respective slots provided on the jig and of a dimension larger than the slots on the jig, enabling the mask plate to be fixed firmly on the jig, said method comprising positioning a photosensitized substrate with at least one flat side on the alignment jig such that the flat side corresponds with at lest one flat side of the said jig, a second side of the substrate being plated against a second flat side of the jig, securing the position of the substrate by means of a vacuum, placing a mask plate with sets of indicators provided thereon on top of the substrate such that the bottom of the mask plate is in contact with the front surface of the substrate, subjecting the assembly to UV radiation in order to print a set of indicators on the front surface of the substrate which coincide exactly with the respective indicators of the mask plate, removing the wafer after formation of the indicators on the front svirface thereon, and positioning it again on the jig such that the marked side is in contact with the base plate of the jig and the reverse unmarked side of the substrate is exposed, the flat side of the surface being in contact with the same flat side of the jig and at least one other side of the substrate in contact with a third flat side of the jig, securing the substrate by applying vacuum, reversing the mask plate such that the bottom side thereof is in contact with the exposed surface of the substrate, exposing the assembly to UV source such that the same set of indicators is formed on the reverse surface of the substrate and corresponding exactly to the set of indicators formed on the front side thereof
8. A method as claimed in claim 7 wherein the set of indicators comprise of visual indicators.
9. A method as claimed in claim 8 wherein the visual indicators comprise of a set of cross hairs.
10. A method as claimed in claim 7 wherein the mask plate is operatively connected to the jig by means of connection means through respective sets of slots.
11. An apparatus for the front to back alignment of a photosensitized substrate substantially as described hereinbefore and with reference to the accompanying drawings.
12. A method for the manufacture of a photosensitized substrate substantially as described hereinbefore and with reference to the accompanying drawings.

Documents:

http://ipindiaonline.gov.in/patentsearch/GrantedSearch/viewdoc.aspx?id=hJxArpXZEMQENwUfFmAeDw==&loc=+mN2fYxnTC4l0fUd8W4CAA==

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Patent Number

269157

Indian Patent Application Number

669/DEL/2003

PG Journal Number

41/2015

Publication Date

09-Oct-2015

Grant Date

06-Oct-2015

Date of Filing

05-May-2003

Name of Patentee

INDIAN INSTITUTE OF TECHNOLOGY

Applicant Address

HAUZ KHAS, NEW DELHI 110 016, INDIA

Inventors:

#	Inventor's Name	Inventor's Address
1	SUDHIR CHANDRA	APPLIED RESEARCH IN ELECTRONICS, INDIAN INSTITUTE OF TECHNOLOGY, DELHI, (IIT) HAUZ KHAS, NEW DELHI 110 016, INDIA

PCT International Classification Number

G03B 29/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA