STEPPER ALIGNMENT MARK FORMATION WITH DUAL FIELD OXIDE PROCESS

Abstract: The present invention is a semiconductor photomask set for producing wafer alignment accuracy in a semiconductor fabrication process. Alignment marks are used to arrange various mask layers in nearly identical orientation with the underlying semiconductor mask pattern. Their chief function is to prevent mask misalignment. Traditional microchip fabrication methods result in the alignment marks being covered with layers of oxide material making accurate alignment a troublesome process. The photomask set (M1, M3) of the present invention produces an alignement mark (17M) that is accurate for subsequent fabrication after undergoing a dual field oxide fabrication process. The mask set (M1, M3) can be used in conjunction with stepper wafer alignment tools and is especially useful in forming semiconductor memory products capable of permorming block data erasure operations. The exposed alignment marker (17M) facilitates checking and testing mask misalignment during the fabrication process.

Description: 1 STEPPER ALIGNMENT MARK FORMATION WITH DUAL FIELD OXIDE PROCESS TECHNICAL FIELD The present invention relates to integrated circuits and fabrication techniques for forming field oxide (FOX) regions on the integrated circuit substrate. More particularly, the present invention relates to fabrication techniques for improving the visibility of alignment marks used in forming dual field oxide regions on the integrated circuit substrate. BACKGROUND ART Silicon dioxide (oxide) is a dielectric material widely used in the fabrication of integrated semiconductor circuits. The oxide thickness determines whether the oxide prevents shorting (insulator), or induction of electrical charges on the wafer surface. When used to prevent electrical charge induction from the metal layers, the oxide is referred to as a field oxide (FOX) layer. The magnitude of the voltages in the integrated circuit impacts the thickness of the FOX regions. By example, in fabricating a memory product, the core of the die is used to fabricate memory circuit elements, while the periphery is used for logic circuitry. Memory circuits operate at, or below, the 5.0 Vdc range, while other circuitry, such as logic circuitry, operates in the 10 Vdc to 20 Vdc range. The higher voltages utilized in the periphery requires a thicker FOX than the FOX used in the core of the die (4000A compared to 2000A). In order to fabricate the die with the two thicknesses of FOX in the core and periphery areas of the die, a dual FOX layering process must be employed. As is known in the prior art, masks are provided with an alignment mark for use in aligning the various patterns on the wafer. A first mask creates a target on the wafer at a first patterning step. Subsequent masks contain masks portions which align to the previously formed mask. In dual field oxide fabrication processes, the second masking operation has traditionally caused a second layer of oxide material to be fabricated over the previously fabricated alignment marker. The subsequent alignment after the dual field oxide process has caused misalignment problems and device failures. Any attempts to test for mask misalignment is frustrated because of the second layer of oxide material that has been fabricated over the previously fabricated alignment marker. Thus a need is seen to exist for a fabrication process involving dual field oxide fabrication where the alignment marker is not diminished by the second field oxide layer and that facilitates checking for mask misalignment during subsequent masking operations. Accordingly, a primary object of the present invention is to provide a photomask set that produces an alignment mark that is accurate for subsequent fabrication process after undergoing a dual field oxide (FOX) fabrication process. DISCLOSURE OF INVENTION The present invention accomplishes the objects set forth above by providing a semiconductor photomask set for producing wafer alignment accuracy in a semiconductor fabrication process. The photomask set produces an alignment mark that is accurate for subsequent fabrication after undergoing a dual field oxide (FOX) fabrication process. Prior arts methods have traditionally covered the alignment marks with layers of oxide material. The method includes the steps of: (a) providing a first photomask member having mask portions for forming a plurality of first field oxide regions on a first region of a semiconductor substrate and also having a mask portion for forming an alignment marker; (b) providing a second photomask member having mask portions for forming a plurality of second field oxide regions on a second region of the semiconductor substrate and also having mask portions delineated for covering any first field oxide regions and alignment marker formed by using the first photomask member; (c) forming the first field oxide regions and the alignment marker utilizing the first photomask member; (d) covering the formed first field oxide regions and the alignment marker with a photoresist material by utilizing the second photomask member; (e) forming the second field oxide regions after utilizing the second photomask member; (f) facilitating wafer alignment accuracy by removing the photoresist material and exposing the alignment marker; and (g) aligning a semiconductor wafer by utilizing the exposed alignment marker. The mask set can be used in conjunction with stepper wafer alignment tools and is especially useful in forming a memory semiconductor product capable of performing block data erasure operations. Additionally, the exposed alignment marker resulting after the second field oxide facilitates testing for mask misalignment during subsequent masking operations. Other features of the present invention are disclosed or apparent in the section entitled: "MODES FOR CARRYING OUT THE INVENTION" . BRIEF DESCRIPTION OF DRAWINGS For fuller understanding of the present invention, reference is made to the accompanying drawing in the following Detailed Description of the Invention. In the drawings: Fig. 1 is a top view of a semiconductor wafer illustrating two ways of placement of an alignment mark. Fig. 2 is a partial top view of one of the alignment marks depicted in Fig. 1. Fig. 3 is a partial top view of the other alignment mark depicted in Fig. 1, illustrating the scribe line marks used to delineate the individual integrated circuit chips. Fig. 4 is a top view of a mask containing an alignment mark for patterning a device region on a core region of the semiconductor chip. Fig. 5 is a top view of a mask containing the same alignment mark for patterning a device region on a peripheral region of the semiconductor chip. Fig. 6 is a cross-section of an integrated circuit substrate showing a nitride layer deposited on the core region and peripheral region after utilizing the mask of Fig. 4. Fig. 7 is a cross-section of the semiconductor substrate illustrated in Fig. 6 after etching the nitride layer and growing field oxide pads and a first alignment mark. Fig. 8 is a cross-section view of the semiconductor substrate illustrated in Fig. 7 showing a second nitride layer grown in the peripheral region utilizing the mask depicted in Fig. 5. Fig. 9 is a cross-section view of the semiconductor substrate illustrated in Fig. 8 shown after growing the second field oxide pads in the peripheral region and etching the nitride layers over the first field oxide pads, and particularly showing the second field oxide covering the first alignment mark. Fig. 10 is a mask, in accordance with the present invention for growing a nitride layer over the first alignment mark. Fig. 11 is a partial cross-section view of the first alignment mark being protected by the nitride layer. Figs. 12 and 13 are identical to Figs. 6 and 7 and are used in accordance with the present invention. Fig. 14 is a cross-section view of the semiconductor substrate illustrated in Fig. 13 showing a second nitride layer grown in the peripheral region utilizing the mask depicted in Fig. 10, and particularly showing the first alignment mark being protected by a nitride layer. Fig. 15 is a cross-section view of the semiconductor substrate illustrated in Fig. 14 shown after growing the second field oxide pads in the peripheral region and etching the nitride layer over the field oxide pads in the core region and the first alignment mark. Reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing. MODES FOR CARRYING OUT THE INVENTION Referring now to the drawings where Figs. 1-9 basically illustrate the prior art apparatus and fabrication steps for forming isolation regions in a dual FOX process which result in producing inferior alignment markers. Referring now to Fig. 1 which shows a top view of a semiconductor wafer illustrating two ways of placement of an alignment marks A M1 and A,^ in a wafer region R. The wafer comprises a plurality of dies 10 and are delineated on the wafer at scribe lines 11. Fig. 2 is a partial top view of alignment mark A M j depicted in Fig. 1 and Fig. 3 is a partial enlarged top view of wafer region R and the other alignment mark A M2 disposed in the space delineating the scribe line 11, also depicted in Fig. 1. Fig. 3 illustrates the actual mark in dark regions that would be fabricated on the dies, and in particular, illustrates the scribe line 11 used to separate the individual integrated circuit chips 10. Fig. 4 is a top view of an exemplary mask M, containing an alignment mark portion A M for patterning an alignment marker 17 M and mask portions 14m, 15m, and 16m for patterning oxide and active regions 14, 15, and 16 on the core device region 10c of the semiconductor chip 10, see generally Figs. 6 and 7. Fig. 5 similarly shows a top view of an exemplary mask M 2 containing mask portions 18m, 19m, and 20m for patterning oxide and active regions 18, 19 and 20 on the peripheral device region lOp of the semiconductor chip 10, see generally Figs. 8 and 9. Prior art mask M 2 allows a layer of oxide 21 to cover alignment marker 17 M to be covered as seen from Fig. 9. In a stepper alignment the stepper itself has target region to align to a previously formed alignment marker formed by the first masking process. Fig. 6 shows in cross-section integrated circuit substrate 10 having a layer of barrier oxide 12, core region 10c and peripheral region lOp fabricated after utilizing the mask Mj of Fig. 4. The patterning step preceding Fig. 6 essentially mask regions 14, 16 and marker region 17 (reference mask portion A,^, using photoresist commonly used in the industry, while facilitating the growing of silicon nitride portions 13 nl , where oxide is not to be grown, by example region 15 which will be an active region on the substrate. Fig. 7 is a cross- section of the semiconductor substrate illustrated in Fig. 6 after growing first field oxide pads 14 n , 16 n and a first alignment mark 17 M after etching the nitride layer 13 n , and exposing active region 15. The first field oxide pads 14 n , 16 π comprise, by example, silicon dioxide material having a thickness of 2000A. The next phase as depicted in Figs. 8 and 9 comprises growing the second and thicker field oxide pads in a dual FOX process and the use of second mask M 2 of Fig. 5. The patterning step preceding Fig. 8 essentially mask regions 18, and 20 while targeting on alignment marker 17 M . As before, the process comprises using a photoresist commonly used in the industry to mask regions, by example active region 15, 19 and first oxide pads 14 π , 16 π , but not regions 18 and 20 on chip 10, while facilitating the growing of silicon nitride portions 13 ώ , where oxide is not to be grown. As seen in Fig. 8, the alignment marker 17 M is not protected by silicon nitride layer \ >^. Fig. 9 is a cross-section view of the semiconductor substrate 10 illustrated in Fig. 8 after growing second field oxide pads 18 β , 20 β and an additional coat of oxide 21 over alignment mark 17 M . After the second oxide pads are grown nitride layer 13^ is etched to expose active regions 15 and 19 and the first oxide pads 14 π , 16 π . The second field oxide pads 18 β , 20 β comprise, by example, silicon dioxide material having a thickness of 4000A. The oxide coating 21 over the alignment marker 17 M prevents any subsequent use of the alignment marker 17 M to check for misalignment of subsequent masks used during the fabrication process after the second field oxide pads 18 β , 20 β are formed. Fig. 10 is a mask M 3 , having mask portion B MJ ^ , in accordance with the present invention, for growing a nitride layer 13,^ over the first alignment marker 17 M , as depicted in Fig. 11. Basically, mask M 3 replaces mask M 2 in the dual FOX fabrication process, and accordingly, Figs. 12 and 13 are identical to Figs. 6 and 7 and whose description is repeated for convenience with change in Fig. numeral reference. Thus, Fig. 12 shows in cross-section integrated circuit substrate 10 having a layer of barrier oxide 12, core region 10c and peripheral region lOp fabricated after utilizing the mask M, of Fig. 4. The patterning step preceding Fig. 12 essentially mask regions 14, 16 and marker region 17 (reference mask portion A^, using photoresist commonly used in the industry, while facilitating the growing of silicon nitride portions 13 nl , where oxide is not to be grown, by example region 15 which will be an active region on the substrate. Fig. 13 is a cross-section of the semiconductor substrate illustrated in Fig. 12 after growing first field oxide pads 14 π , 16 π and a first alignment mark 17 M after etching the nitride layer 13 nJ and exposing active region 15. The first field oxide pads 14 π , 16 π comprise, by example, silicon dioxide material having a thickness of 2000A. Figs.14 and 15 differ from Figs. 8 and 9 with respect to the present invention of protecting the initially formed alignment marker 17 M and the fabrication benefits associated with being able to use the alignment marker. Accordingly, the next phase of the present invention, as depicted in Figs. 14 and 15, comprises growing the second and thicker field oxide pads in a dual FOX process and the use of mask M 3 of Fig. 10. The patterning step preceding Fig. 14 now masks alignment mark 17 M which was not masked using prior art process steps as shown in Figure 8. As before, the process comprises using a photoresist commonly used in the industry to mask regions, by example active region 15, 19 and first oxide pads 14 n , 16 π , except regions 18 and 20, but differs in that during the growing of silicon nitride portions 13„ 2 , the alignment marker 17 M is now included in those other regions where oxide is not to be grown. As seen in Fig. 14, the alignment marker 17 M is now protected by silicon nitride layer 13 Mn2 . Fig. 15 is a cross-section view of the semiconductor substrate 10 illustrated in Fig. 14 after growing second field oxide pads 18 β , 20 β without an additional coat of oxide over alignment mark 17 M . After the second oxide pads 18 β , 20 β are grown, nitride layer 13^ and 13,^ are etched to expose active regions 15 and 19, the first oxide pads 14 n , 16 π , and now alignment marker 17 M . The second field oxide pads 18 β , 20 β comprise, by example, silicon dioxide material having a thickness of 4000A. Having alignment marker 17 M exposed after the formation of the second field oxide pads 18 β , 20 β allows testing and checking for misalignment of subsequent masks that are required to complete the fabrication of the semiconductor device. The present invention has been particularly shown and described with respect to a certain preferred embodiment and features thereof. However, it should be readily apparent to those of ordinary skill in the art that various changes and modifications in form, semiconductor material, material conductivity type i.e. N-type, or P-type, and detail may be made without departing from the spirit and scope of the inventions as set forth in the appended claims. The inventions illustratively disclosed herein may be practiced without any element which is not specifically disclosed herein. INDUSTRIAL APPLICABILITY The present invention applies to the semiconductor fabrication industry. The invention enables the fabrication of robust and accurate alignment marks despite subsequent FOX processing. The alignment marks are robust and do not degrade with subsequent processing thereby preventing mask misalignment errors during repeated process steps.

Claims: 6 CLAIMS What is Claimed is: 1. A semiconductor photomask set apparatus for forming dual field oxide regions on a semiconductor substrate, said apparatus comprising: a first mask member having mask portions for forming a plurality of first field oxide regions on a first region of a semiconductor substrate, said first mask member having an alignment mark portion for forming an alignment marker; and a second mask member having mask portions for forming a plurality of second field oxide regions on a second region of said semiconductor substrate, said second mask member also having mask portions delineated for covering any formed alignment marker. 2. A semiconductor apparatus, said apparatus comprising: a semiconductor substrate member; a core region of said semiconductor substrate having a plurality of first field oxide pads having a first thickness, an alignment marker formed on said semiconductor substrate, said alignment marker having said first thickness; a dielectric material deposited over said plurality of first field oxide pads and said alignment marker, said dielectric material being a protective layer to prevent formation of oxide material on said plurality of first field oxide pads and said alignment marker. 3. A semiconductor apparatus as described in claim 2, said apparatus further comprising: a second region of said semiconductor substrate formed as a peripheral region, said peripheral region having a plurality of second field oxide pads having a second thickness, said second thickness being greater than said first thickness; and said dielectric material being removed to expose said alignment marker for facilitating subsequent wafer alignment accuracy using a stepper alignment technique. 4. A method for producing wafer alignment accuracy in a semiconductor fabrication process after undergoing a dual field oxide semiconductor fabrication process, said method comprising the steps of: (a) providing a first photomask member having mask portions for forming a plurality of first field oxide regions on a first region of a semiconductor substrate, said first mask member also having a mask portion for forming an alignment marker; (b) providing a second photomask member having mask portions for forming a plurality of second field oxide regions on a second region of said semiconductor substrate, said second mask member also having mask portions delineated for covering any first field oxide regions and alignment marker formed by using said first photomask member; (c ) forming said first field oxide regions and said alignment marker utilizing said first photomask member; (d) covering said formed first field oxide regions and said alignment marker with a photoresist material by utilizing said second photomask member; (e) forming said second said field oxide regions after utilizing said second photomask member; and (f) facilitating wafer alignment accuracy by removing said photoresist material and exposing said alignment marker for use in subsequent semiconductor fabrications process steps. 5. A method for producing wafer alignment accuracy in a semiconductor fabrication process as described in claim 4, wherein: said steps (c), (d), (e), and (f) comprise utilizing a stepper wafer alignment means. 6. A method for producing wafer alignment accuracy in a semiconductor fabrication process as described in claim 4, wherein: said step (d) comprises covering said formed first field oxide regions and said alignment marker with a silicon nitride material having a thickness of 1700A. 7. A method for producing wafer alignment accuracy in a semiconductor fabrication process as described in claim 4, wherein: said step (c) comprises ) forming said first field oxide regions and said alignment marker with a silicon dioxide material having a thickness of 2000A; and said step (e) comprises ) forming said second field oxide regions with a silicon dioxide material having a thickness of 4000A. 8. A method for producing wafer alignment accuracy in a semiconductor fabrication process as described in claim 4, wherein: said step (f) facilitates a further series of fabrication steps for forming a memory semiconductor apparatus capable of performing block data erasure operations. 9. A method for producing wafer alignment accuracy in a semiconductor fabrication process as described in claim 4, wherein: said step (f) facilitates checking for alignment of subsequent masks used in subsequent fabrication steps by using said exposed alignment marker. 10. A method for producing wafer alignment accuracy in a semiconductor fabrication process as described in claim 4, wherein: said step (0 facilitates testing for mask misalignment as part of the monitoring operations of said fabrication process by using said exposed alignment marker.

Inventors: CHANG MARK S, KAJITA TATSUYA, CHANG, MARK, S., KAJITA, TATSUYA
Applicants: ADVANCED MICRO DEVICES INC, FUJITSU LTD, ADVANCED MICRO DEVICES, INC., FUJITSU LIMITED
Application year: 1999
Main classification: H01L23
IPC codes: G03F1, G03F7, H01L21, H01L23