Abstract:
Hybrid alignment markings with a plurality of parallel marks on an active area of a silicon layer on which a multilayer structure are formed by forming initial marks in the active area by modifying the profile of the active area producing an active area surface of the active area with initial marks and then forming on the active area surface a set of interleaved marks from a second, polysilicon layer to form a single composite alignment marking composed of the initial marks and the interleaved marks. One technique is to form shallow steps with shallow trenches with low mesas in the silicon layer followed by forming low ribs of the second, polysilicon layer on the low mesas adjacent interleaved with the shallow trenches. Alternatively, form shallow cavities with low ribs in the silicon layer forming exposed low cavity surfaces of the silicon layer, and then form additional low ribs of a polysilicon layer in the shallow cavities on the exposed low cavity surfaces of the silicon layer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to alignment systems for manufacturing and more particularly to alignment marks employed on parts to be aligned. 
     2. Description of Related Art 
     The present invention relates to structures such as semiconductor devices with combination of an active region such as a silicon layer or a silicon substrate. On the silicon layer or substrate there are contact layers and polysilicon layers. During front end processing steps, usually both the polysilicon layers and contact layers are aligned to the active layer; and in such a case, there is a possibility for misalignment between the contact layer and the polysilicon layer. Many circuit designs, demand good alignment of the contact layers with both the active layer and the polysilicon layers. A nine (9) mark Field Image Alignment (FIA) marking system is employed in a system manufactured by Nikon Corporation in which alignment markings are provided on a work piece. 
     U.S. Pat. No. 4,962,318 of Nishi for “Alignment System for Exposure Apparatus”, assigned to Nikon Corporation describes Field Image Alignment (FIA) markings at Col. 6, line 59 to Col. 7, line 61 and in FIG. 1A thereof. Light from a halogen lamp is directed onto an alignment mark. At Col. 10, line 15 to Col. 11, line 11, such an alignment marking is described as a pattern of stripes raised from the surface of a wafer covered with photoresist. 
     U.S. Pat. No. 5,808,910 of Irie et al., “Alignment Method” assigned to Nikon Corporation describes a Field Image Alignment (FIA) marking system at Col. 8, line 34 to Col. 9, line 7 and in FIG. 2 thereof. 
     U.S. Pat. No. 5,783,340 of Farino et al. “Method for Photolithographic Definition of Recessed Features on a Semiconductor Wafer Utilizing Auto-Focusing Alignment” discloses a method for forming an alignment marking formed a several marks in a trench for a stepper. 
     U.S. Pat. No. 5,401,691 of Caldwell “Method of Fabrication an Inverse Open Frame Alignment Mark” teaches a method of forming alignment marks. 
     U.S. Pat. No. 5,705,320 of Hsu et al. for “Recovery of Alignment Marks and Laser Marks after Chemical-Mechanical-Polishing” shows method steps for forming topographic alignment marks (topographical features or steps formed in a substrate) located in a window. 
     SUMMARY OF THE INVENTION 
     This invention provides a methods for combining a set of active area marks and an interleaved set of polysilicon marks to form a single composite alignment marking. 
     The invention teaches a method for making a hybrid alignment marking for optimal alignment of three (3) layers comprising: 
     (a) forming integral alignment marks in a lower layer from the material of the lower layer, 
     (b) forming alignment ribs on a surface of the lower layer interleaved with the integral alignment marks. 
     Alternatively this invention provides a method for making a hybrid alignment marking for optimal alignment of three (3) layers comprising: 
     (1) Forming CD alignment marks by as follows: 
     (a) forming shallow steps with shallow trenches with low mesas in silicon material, 
     (b) forming low polysilicon ribs on the low mesas adjacent interleaved with the shallow trenches, or 
     (2) Forming CD alignment marks as follows: 
     (a) forming shallow cavities with low ribs in silicon material exposing low cavity surfaces of the silicon material, and 
     (b) forming low polysilicon ribs in the shallow cavities on the exposed low cavity surfaces surfaces of the silicon material. 
     In accordance with this invention, a method is provided for forming hybrid alignment markings with a plurality of parallel marks on an active area of a silicon layer on which a multilayer structure are formed by forming initial marks in the active area by modifying the profile of the active area producing an active area surface of the active area with initial marks and then forming on the active area surface a set of interleaved marks from a second, polysilicon layer to form a single composite alignment marking composed of the initial marks and the interleaved marks. One technique is to form shallow steps with shallow trenches with low mesas in the silicon layer followed by forming low ribs of the second, polysilicon layer on the low mesas adjacent interleaved with the shallow trenches. Alternatively, form shallow cavities with low ribs in the silicon layer forming exposed low cavity surfaces of the silicon layer, and then form additional low ribs of a polysilicon layer in the shallow cavities on the exposed low cavity surfaces of the silicon layer. 
     In another aspect of this invention, the hybrid markings resulting from the process are provided on a workpiece. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other aspects and advantages of this invention are explained and described below with reference to the accompanying drawings, in which: 
     FIG. 1A shows a plan view of a portion of a device with an partially formed alignment marking on a silicon substrate in accordance with this invention in an early stage of manufacture. 
     FIG. 1B shows a sectional view of the device of FIG. 1A taken along line  1 B- 1 B′. 
     FIG. 2A shows the device of FIGS. 1A after formation of polysilicon ribs thereon to complete an alignment marking. 
     FIG. 2B shows a sectional view of the device of FIG. 2A taken along line  2 B- 2 B′. 
     FIG. 3A shows a plan view of a portion of a device with a partially formed alignment marking formed on a P-silicon substrate layer in accordance with embodiment of this invention. 
     FIG. 3B shows a sectional view of the device of FIG. 3A taken along line  3 B- 3 B′. 
     FIG. 4A shows the device of FIGS. 3A after formation of polysilicon ribs thereon to complete another alignment marking. 
     FIG. 4B shows a sectional view of the device of FIG. 4A taken along line  4 B- 4 B′. 
     FIG. 5 shows a sectional view of a larger portion of the device of FIG. 4B with the adjacent structure included laterally of the alignment marking after coating of the device with dielectric insulator. 
     FIG. 6 shows a workpiece in the form of a semiconductor wafer is shown with device areas thereon and with a set of hybrid alignment marks in accordance with this invention, the marks being oriented orthogonally, i.e. vertically and horizontally respectively. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Consider a method of manufacturing semiconductor devices employing alignment procedures, as follows: 
     1. In the case of multilayer semiconductor devices, contact alignment to both a polysilicon layer and an active layer are critical in different directions, say alignment is critical in the X-direction for polysilicon and alignment is critical in Y-direction for the active layer, so a contact X is aligned to the polysilicon layer and a contact Y is aligned to the active layer. 
     2. With respect to both polysilicon and active layers, use is made of multiple alignment marks, i.e. alignment to both an X marks and a Y mark, to achieve an average alignment performance. 
     Problems identified by the inventors with the above alignment practices are as follows: 
     1. Job files have to be maintained depending on the directional sensitivity. 
     2. In cases in which the alignment is critical in both the X direction and the Y direction, the above two alignment procedures are not feasible. 
     3. The use of a multiple mark alignment system involves duplicate steps which we have found will reduce throughput as contrasted to the improved method of this invention. 
     The present invention resolves the problems encountered as follows: 
     1. A hybrid mark provides an optimal alignment of the contacts to both the polysilicon layer and the active layer. 
     2. Alignment performance is better in both the X direction and the Y direction. 
     3. Unlike a process in which multiple alignment marks (alignment to both an X marks and a Y mark), throughput is not sacrificed. 
     4. It is possible to assign a weight function if the directional sensitivity of the device is known before the mark design. 
     The methods presented herein combine both a set of active area marks and an interleaved set of polysilicon marks to form a single composite alignment marking. 
     According to a nine (9) mark Field Image Alignment (FIA) marking scheme, the combined number of marks in an FIA marking still remains at nine (9). The number of marks belonging to the active layer or the polysilicon layer out of the nine (9) marks basically determines the weight of alignment with reference to the active layer or the polysilicon layer respectively. In the case of experimental lots with either an active layer or a polysilicon layer which is not masked beforehand, alignment to one of the previous layers is possible just by changing the marking type parameter setting in the stepper job file. FIG.  2 A/FIG.  2 B and FIG.  4 A/FIG. 4B depict two alternative configurations of a hybrid marking for FIA sensors in Nikon Steppers. 
     FIG. 1A shows a plan view of a portion of a device  8  with a partially formed alignment marking with marks formed on a P-silicon substrate layer  10  in accordance with this invention. The device  8  is in an early stage of manufacture with four (4) shallow, narrow trenches T 1 , T 2 , T 3 , and T 4  in the center thereof (oriented vertically on the page) which are portions of a set of alignment marks which will comprise a nine mark FIA marking. FIG. 1B shows a sectional view of the device of FIG. 1A taken along line  1 B- 1 B′. 
     A set of wide trenches T 0  and T 5  are formed on the ends for a total of six (6) shallow trenches T 0 , T 1 , T 2 , T 3 , T 4  and T 5 . Between the trench T 0  and trench T 1  a mesa M 1  is formed. Between the trench T 1  and trench T 2  a mesa M 2  is formed. Between the trenches T 3  and trench T 4  a mesa M 3  is formed. Between the trench T 4  and trench T 5  a mesa M 4  is formed. Between the trench T 4  and trench T 5  a mesa M 5  is formed. 
     Referring to FIG. 1A, the mesas M 1 -M 5  between the trenches T 0  and T 5  span one hundred eighty micrometers (180 μm). The width of the mesas M 1 -M 5  is seventy-five and two tenths micrometers (75.2 μm). The length of the narrow trenches T 1 -T 4  is seventy micrometers (70 μm); and the narrow trenches T 1 , T 2 , T 3 , and T 4  are six micrometers (6 μm) wide and trenches T 1 , T 2 , T 3 , and T 4  are spaced eighteen micrometers (18 μm) apart. It is noted that the mesas M 1 -M 5  interconnect at the ends of the trenches T 1 -T 4  which are shorter than the width of the region defined by trenches T 1 -T 4 . 
     Trenches in FIGS. 1A and 1E are made by etching the silicon (P-Sub) substrate layer  10  after forming a mask thereon, as will be well understood by those skilled in the art. 
     The mask for the substrate layer  10  comprises a coating of a photoresist layer patterned with mask features formed on the substrate layer  10 . After etching through that mask, the substrate layer  10  is ready for further processing. 
     FIGS. 2A and 2B show the device  8  of FIGS. 1A and 1B after formation of polysilicon ribs RA, RB, RC, RD and RE, about six micrometers (6 μm) wide, on the surface of mesas M 1 , M 2 , M 3 , M 4  and M 5  respectively, parallel to the trenches T 1 -T 4  and spaced about six micrometers (6 μm) from the adjacent trenches. As shown the ribs RB, RC and RD are located between two of the sets of narrow trenches T 1 -T 4  with the ribs RA and RE being located outside of the outer ones T 1  and T 4  of the four narrow trenches T 1 -T 4 . As seen in FIG. 2A, the spacings between trenches T 1 -T 4  and the ribs RA-RE are equal and the ribs resemble the trenches from that point of view even though the profile seen in FIG. 2B is quite different. 
     Referring to FIG.  2 A and FIG. 2B, the ribs RA, RB, RC, RD and RE are formed by forming a polysilicon layer on the device  8  of FIG.  1 A and FIG.  1 B and after forming a polysilicon patterning mask over the polysilicon layer to protect the future ribs RA, RB, RC, RD and RE. While that patterning mask is in place, the next step is to etch away the unprotected portions of the polysilicon layer. During polysilicon masking, alignment is made to layer  10  using a regular mark, not the hybrid mark. 
     As to how the alignment and spacings seen in FIGS. 2A and 2B are achieved, as noted above, the polysilicon patterning mask is aligned to Layer- 10  regular mark.- This assures the spacing of Ribs between the trenches T 1 -T 4  of Layer- 10 , within the tolerance of stepper capability. Typically +/−0.1 μm. 
     FIG. 3A shows a plan view of a portion of a device  8 A with a partially formed alignment marking formed on a P-silicon substrate layer  10 A in accordance with embodiment of this invention. Device  8 A is in an early stage of manufacture with five (5) shallow, interconnected cavities CA, CB, CC, CD and CE between a pair of mesas MA and MB. In FIG. 3A, several ribs R 1 , R 2 , R 3 , R 4 , and R 5  are formed in the center of the spaced defined by cavities CA-CE. FIG. 3B shows a sectional view of the device of FIG. 3A taken along line  3 B- 3 B′. 
     As described above, although the pattern is complementary to the pattern in FIGS. 1A-1B, the mask for the substrate layer  10 A comprises a coating of a photoresist layer patterned with mask features to be formed on the substrate layer  10 A. After etching through that mask, the substrate layer  10 A is ready for further processing. 
     Referring to FIGS. 3A and 3B, rib R 1  separates cavities CA and CB. Rib R 2  separates cavities CB and CC. Rib R 3  separates cavities CC and CD. Rib R 4  separates cavities CB and CC. Rib R 3  separates cavities CD and CE. 
     As shown in FIG. 3A, the ribs R 1 -R 4  are oriented vertically on the page which are portions of a set of alignment marks which will comprise a nine mark FIA marking as seen in FIG.  4 A. Between mesas MA and MB the length of the spaced spanned by the cavities CA to CB and ribs R 1 -R 4  is one hundred eighty micrometers (180 μm). The width of the cavities CA-CE is seventy-five and two tenths micrometers (75.2 μm). It is noted that the cavities CA-CE interconnect at the ends of the ribs R 1 -R 4 , which are shorter than the width of the region defined by cavities C 1 -CE. The narrow ribs R 1 -R 4  are six micrometers (6 μm) wide. The length of the narrow ribs R 1 -R 4  is seventy micrometers (70 μm). The narrow cavities CB, CC, and CD are eighteen micrometers (18 μm) wide so the silicon ribs R 1 , R 2 , R 3 , and R 4  are spaced eighteen micrometers (18 μm) apart. 
     FIGS. 4A and 4B show the device  8 A of FIGS. 3A and 3B after formation of polysilicon ribs RA′, RB′, RC′, RD′ and RE′, about six micrometers (6 μm) wide, on the surface of cavities CA, CB, CC, CD and CE respectively, parallel to the ribs R 1 -R 4  and spaced about six micrometers (6 μm) from the adjacent ribs R 1 -R 4 . As shown the ribs RB′ RC′ and RD′ are located between two of the sets of narrow trenches T 1 -T 4  with the ribs RA′ and RE′ being located outside of the outer ones R 1  and R 4  of the four narrow ribs R 1 -R 4 . As seen in FIG. 4A, the spacings between ribs R 1 -R 4  and the ribs RA′-RE′ are equal and the polysilicon ribs RA′-RE′ resemble the silicon ribs R 1 -R 4  from that point of view and the profile seen in FIG. 4B shows that the two sets of rib are quite similar. 
     Referring to FIG. 6, a workpiece M comprising a semiconductor wafer W is shown with device areas DA 1  and DA 2  thereon and with a set of hybrid alignment marks AM 1  and AM 2  in accordance with this invention with the marks oriented vertically (parallel to the Y-axis) and horizontally (parallel to the X-axis) respectively. The set of hybrid marks AM 1  and AM 2  are incorporated in the horizontal (X-mark) and vertical (Y-mark) scribe lanes to provide X and Y alignment coordinates respectively. Typically, marks such as marks AM 1  and AM 2  can be placed anywhere within a field. During stepper alignment, X and Y marks from about eight (8) fields across the wafer are scanned to calculate alignment parameters such as wafer translation, wafer rotation, wafer scaling, etc. The stepper compensates for these parameters, during the exposure. 
     FIG. 5 shows a sectional view of a larger portion of the device  8 B of FIG. 4B including the adjacent structure, laterally of the marking MK′ after coating of the device with dielectric insulator layer IN. At the contact hole CH 1 , a gate oxide layer GOX formed on mesa and is shown lying below a metallization layer ME 1  on the left. Metallization ME 1  has a width of 0.35 micrometers (0.35 μm). 
     On the right of FIG. 5, a Field Effect Transistor device FET is shown on a MFET mesa. Device FET includes a gate oxide layer GOX and gate electrode G formed of the polysilicon layer from which the ribs RA′-RE′ have been formed. Atop the gate electrode G is a metallization layer ME 2  formed of metal or silicide. Another rib R 0  is shown for another portion of the device. A mesa MA is shown between the rib RA′ and the mesa on which the contact metallization ME 1  is formed. Ribs R 0 , R 1 -R 4  and the mesas MA, MB and MFET are formed of the active region of P-silicon substrate layer  10 B. Contact hole CH 1  is formed over metallization layer ME 1 . 
     Referring to FIG. 5, the FET is formed on a mesa MFET. The contact hole CH shown in the dielectric insulator layer IN is formed after contact mask and etch steps. The dielectric insulator layer IN is nearly flat after a Chemical Mechanical Polishing (CMP) Step. A contact layer mask, is aligned to the hybrid mark formed by the ribs R 1 , R 2 , R 3 , R 4  and RA′, RB′, RC′, RD′, RE′. The device  8 B is covered by a nearly flat dielectric insulation layer IN. Contact hole CH 2  is formed over metallization layer ME 2 . 
     It will be understood by those skilled in the art that the composite alignment markings formed as described above can be used in aligning layers above the layers described above using the composite alignment markings for that purpose. 
     While this invention has been described in terms of the above specific embodiment(s), those skilled in the art will recognize that the invention can be practiced with modifications within the spirit and scope of the appended claims, i.e. that changes can be made in form and detail, without departing from the spirit and scope of the invention. Accordingly all such changes come within the purview of the present invention and the invention encompasses the subject matter of the claims which follow.