WO2008048985A2

WO2008048985A2 - Method of manufacturing integrated deep and shallow trench isolation structures

Info

Publication number: WO2008048985A2
Application number: PCT/US2007/081592
Authority: WO
Inventors: Joerg Haussman; Christoph Dirnecker; Rupert Wagner
Original assignee: Texas Instruments Incorporated
Priority date: 2006-10-17
Filing date: 2007-10-17
Publication date: 2008-04-24
Also published as: WO2008048985A3

Abstract

A method of forming an integrated deep and shallow trench isolation structure comprises depositing a hard mask on a film stack (10) having a plurality of layers formed on a substrate (11) such that the hard mask (16) is deposited on a furthermost layer from the substrate, imprinting a first pattern into the hard mask to define an open end of a first trench, imprinting a second pattern into the hard mask to define an open end of a second trench, and etching into the film stack the first trench to a first depth and the second trench to a second depth such that the first trench and the second trench each define a blind aperture in the surface of the film stack.

Description

METHOD OF MANUFACTURING INTEGRATED DEEP AND SHALLOW TRENCH ISOLATION STRUCTURES

The invention relates to a method of manufacturing integrated deep and shallow trench isolation structures. More particularly, the invention relates to a method of manufacturing simultaneously filled integrated deep and shallow trench isolation structures using an imprinted hard mask. BACKGROUND

When several semiconductor devices are provided on a chip, it is required to isolate them from each other in order to suppress parasitic capacitances. Shallow trench isolation is known, whereby trenches are etched into the surface of the semiconductor layer structure in which the devices are formed. However, for high performance bipolar applications, shallow trench isolation is not sufficient to suppress parasitic capacitances. To improve the isolation characteristics, the integration of deep trenches in addition to shallow trench isolation is required in structures formed on silicon on insulator substrates. This is especially beneficial, but not limited to cases where the structures are formed on silicon on insulator substrates. The same applies whore the structures are formed from bulk silicon.

Integrated sequences for combined deep and shallow trench isolation have been disclosed in which the deep trench is formed first and then filled with a resist plug, upon which shallow trenches are subsequently patterned. However, the known methods of forming integrated deep and shallow trench isolation structure are not suitable for very deep trenches. This is because either the number of deep trenches per unit area or the depths of the deep trenches need to be limited, in order to avoid insufficient filling with resist before the shallow trench pattern is formed. SUMMARY The invention has been devised with the foregoing in mind. The invention provides a method of forming an integrated deep and shallow trench isolation structure, comprising depositing a hard mask on a multilayer structure (designated herein as a film stack) having a plurality of layers grown on a substrate such that the hard mask is formed on an uppermost layer relative to the substrate, imprinting a first pattern into the hard mask to define an open end of a first trench and imprinting a second pattern into the hard mask to define an open end of a second trench, and etching into the film stack the first trench to a first depth and the second trench to a second depth.

Preferably, the film stack is formed by growing a silicon-oxide layer on a substrate and depositing a silicon-nitride layer on silicon on insulator (SOl) or bulk silicon substrate wafers. Ideally, a blind end of the first trench should be provided in the buried oxide layer and a blind end of the second trench should be provided in the silicon layer, in the case where a silicon on insulator substrate is used. In this case, the first trench is then a deep trench and the second trench is a shallow trench. In the case where bulk silicon substrate applications are employed, the open end of the second trench is much deeper relative to the open end of the first trench. Before the trenches are formed, the hard mask can then be deposited on the nitride layer, which is the top layer in the film stack. In this method, the deep trench can be formed before the shallow trench or the shallow trench can be formed first.

Alternatively, the hard mask can also comprise a first portion and a second portion. The first pattern can then be created on the first portion of the hard mask to define the open end of the first trench and then the second pattern can be created on the second portion of the hard mask to define an open end of the second trench.

The first trench and the second trench can be lined with a third oxide layer. The first trench and the second trench can then be filled with an insulating material, for example silicon oxide or undoped ploy silicon. This method eliminates the need for extra resist filling to enable shallow trench patterning. It also avoids insufficient filling with resist.

Furthermore, this method only requires a single sequence to simultaneously fill and planarize the deep and shallow trenches. The nitride layer can be removed after a final planarization step.

When imprinting the patterns for the trench openings in the hard mask, the first pattern can either be imprinted relative to the second pattern such that the open end of the first trench is isolated from the open end of the second trench; or such that the open end of the first trench is coincident with the open end of the second trench; or such that the open end of the first trench is bounded by the open end of the second trench. Therefore, this method allows full flexibility in the layout of the trenches. Deep and shallow trenches can either be integrated such that the deep trenches are isolated, semi-isolated or shallow trench bounded within the same device layouts. The method also allows design of pure shallow trench areas and areas with a high density of deep trenches (deep trench arrays).

The invention also provides an integrated deep and shallow trench isolation structure comprising a film stack having a plurality of layers, a first trench defining a blind aperture in the surface of the film stack and having a first depth, and a second trench defining a blind aperture in the surface of the film stack and having a second depth. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side view of a cross-section of a film stack according to first and second embodiments of the invention;

FIG. 2 is a schematic side view of a cross-section of a film stack according to a third embodiment of the invention;

FIG. 3 is a schematic side view of a cross-section of a first stage of formation of the film stack according to a first embodiment of the invention;

FIG. 4 is a schematic side view of a cross-section of a first stage of formation according to a second embodiment of the invention; FIG. 5 is a schematic side view of a cross-section of a first stage of formation according to a third embodiment of the invention;

FIG. 6 is a further stage of formation according to a first embodiment of the invention; FIG. 7 is a further stage of formation according to a second embodiment of the invention; FIG. 9 is a further stage of formation according to a first embodiment of the invention;

FIG. 10 is a further stage of formation according to a second embodiment of the invention;

FIG. 11 is a further stage of formation according to a third embodiment of the invention; FIG. 12 is a further stage of formation according to a third embodiment of the invention;

FIG. 13 is a further stage of formation according to a first embodiment of the invention;

FIG. 14 is a further stage of formation according to a second embodiment of the invention;

FIG. 15 is a further stage of formation according to a third embodiment of the invention;

FIG. 16 is a further stage of formation according to a first embodiment of the invention;

FIG. 17 is a further stage of formation according to a second embodiment of the invention;

FIG. 18 is a further stage of formation according to a third embodiment of the invention;

FIG. 19 is a further stage of formation according to first and second embodiments of the invention; FIG. 20 is a further stage of formation according to a third embodiment of the invention;

FIG. 21 is a further stage of formation according to first and second embodiments of the invention;

FIG. 22 is a further stage of formation according to a third embodiment of the invention;

FIG. 24 is a further stage of formation according to a third embodiment of the invention;

FIG. 25 is a further stage of formation according to first and second embodiments of the invention; FIG. 26 is a further stage of formation according to a third embodiment of the invention;

FIG. 27 is a further stage of formation according to first and second embodiments of the invention;

FIG. 28 is a further stage of formation according to a third embodiment of the invention;

FIG. 29 is a filling stage according to first and second embodiments of the invention; FIG. 30 is a filling stage according to a third embodiment of the invention; FIG. 31 is a schematic side view of a cross-section of a structure with filling material etched from all areas without trenches according to first and second embodiments of the invention;

FIG. 32 is a schematic side view of a cross-section of a structure with filling material etched from all areas without trenches according to a third embodiment of the invention;

FIG. 33 is a schematic side view of a cross-section of a planarizing stage according to first and second embodiments of the invention;

FIG. 34 is a schematic side view of a cross-section of a planarizing stage according to a third embodiment of the invention;

FIG. 35 is a schematic side view of a cross-section of a finished isolation structure according to first and second embodiments of the invention and;

FIG. 36 is a schematic side view of a cross-section of a finished isolation structure according to a third embodiment of the invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

In FIG. 1 a film stack 10 is provided on a substrate 11 and comprises a first buried oxide layer 12 underneath a silicon layer 13. An initial oxide layer 14 is provided on the silicon layer 13 and a nitride layer 15 is provided on the initial oxide layer 14. A hard mask 16 is deposited on the top surface of film stack 10. The same film stack 10 shown in FIG. 1 is used for the first and second embodiments of the invention.

FIG. 2 shows a film stack 10 used for a third embodiment of the invention. The structure of the substrate 11 buried oxide layer 12, silicon layer 13, initial oxide layer 14 and initial nitride layer 15 of the film stack 10 is the same as that used in the first embodiment. However, instead of the hard mask 16, the third embodiment has a first hard mask 17 deposited on the surface of the nitride layer 15.

In all embodiments, the growth of the thin initial oxide layer 14 acts as stress relief for the subsequent deposition of the initial nitride layer 15. The function of the buried oxide layer 12 under the silicon layer 13 is to isolate the device from the substrate 11. The thickness of the silicon layer 13 can vary from 1 micrometer to more than 8 micrometers depending on the application. The thickness of the buried oxide layer 12 is typically in the range between 0.1 micrometers and 0.5 micrometers. In the case where a silicon on insulator substrate is used, the depth of the deep trenches is limited by the buried oxide layer 12. When bulk silicon is used, the depth of the trenches can vary depending on application requirements. The thickness of the hard mask layers 16 and 17 depends on the required depth of the trenches that are to be etched into the film stack 10. The hard mask 16 used in the first and second embodiments is relatively thicker than the first hard mask 17 used in the third embodiment. All mask layers should preferably be formed from an oxide

In FIGS. 3, 4 and 5, the first stage of formation is shown for the first, second and third embodiments, respectively. A first pattern is created in a resist 19. The pattern in the resist 19 defines the first trench openings 20 and 21. In the first embodiment the openings 20 are for deep trenches so that the deep trench openings are formed first. In the second and third embodiments shallow trench openings 21 are formed first. Deep trenches can either be designed to be isolated (not guarded by a shallow trench), semi- isolated (asymmetrically guarded by a shallow trench only on one side) or shallow trench bounded (guarded by a shallow trench on both sides). Any combinations of these layout types are allowed within the same device. FIG. 6 shows a second etching stage according to the first embodiment. The pattern in the resist 19, which defines the openings 20 of the deep trenches is transferred into the underlying film stack 10 by means of plasma etching. In the first embodiment, the complete film stack consisting of an initial oxide 14, initial nitride 15 and hard mask 16 is removed selectively from the top silicon layer 13 to form deep trench openings 20. The deep trenches 22 are formed in the film stack 10 as blind apertures such that the blind end of each deep trench 22 is located in one of the layers of the film stack 10 and the open end of a trench is located, in the initial stages of etching, in the resist 19 provided on the hard mask layer 16. At this stage of etching, the blind ends of the deep trenches 22 are formed in the silicon layer 13 in the first embodiment. In a second etching stage according to the second embodiment, as shown in FIG. 7, only a partial shallow trench 23 is etched. The shallow trench 23 is etched partially into the hard mask layer 16 such that the blind end of the shallow trench 23 is located in the hard mask layer 16 at this stage and the open end of the shallow trench 23 is located in the resist 19 provided on the hard mask layer 16. The hard mask loss in the structure of the second embodiment is designed such that later etching of deep trenches will safely remove the remaining hard mask film in the shallow trench region with limited loss of the initial nitride layer 15 below. In a second stage of etching according to the third embodiment shown in FIG. 8, the complete film stack consisting of an initial oxide 14, initial nitride 15 and hard mask 16 is removed selectively from the top silicon layer 13 to form a shallow trench 23. At this stage of etching, the blind end of the shallow trench 23 is located in the silicon layer 13 and the open end of the shallow trench 23 is located in the resist 19 provided on the hard mask layer 16.

In FIGS. 9, 10 and 11, a further stage of etching is shown according to the first, second and third embodiments, respectively. The remaining photoresist 19 from the initial imprinted pattern of the trench openings is removed using conventional ash and wet clean techniques in all three embodiments.

FIG. 12 shows the insertion of a second hard mask 18, which is only employed in the third embodiment. The second hard mask 18 is deposited on the surface of the first hard mask 17 and also lines the trench over the initial nitride layer 15, the initial oxide layer 14 and the silicon layer 13. The first hard mask 17 and the second hard mask 18 adopt approximately the same total thickness as the hard mask 16 used on the first and second embodiments. The thickness of the additional hard mask 18 depends on the amount of consumption of the hard mask during deep trench formation. The first hard mask 17 and the second hard mask 18 are preferably made of the same material but they may also be made from different materials. In FIGS. 13, 14 and 15, which show a further stage of formation according to the first, second and third embodiments, respectively, a second resist pattern is created in the resist. The second resist pattern defines openings for the other type of trenches (shallow trench openings 21 in the first embodiment and deep trench openings 20 in the second and third embodiments). For shallow trench bounded deep trenches 29, the shallow trench opening 21 overlaps the deep trench opening 20 on both sides. For isolated deep trenches 30 a minimum spacing to the next shallow trench opening is maintained. For semi-isolated deep trenches 28, the shallow trench opening 21 overlaps the deep trench opening 20 on one side while the deep trench opening 20 overlaps the shallow trench opening 21 on the other side. All three configurations — shallow trench bounded deep trenches 29, isolated deep trenches 30 and semi-isolated deep trenches 28 are provided in a single structure in all three embodiments. In FIGS. 16 to 18 the second resist pattern is now transferred into the upper film stack 10 by means of plasma etching for all three embodiments. In the first embodiment, the hard mask 16 is partially etched to a certain extent with high selectivity to the exposed silicon layer 13 in the existing deep trench openings 20. In the second embodiment and the third embodiment the remaining film stack is completely etched through with high selectivity to silicon in the silicon layer 13. High selectivity to silicon is desirable but not necessarily required.

In FIGS. 19 and 20, the structure for the first, second and third embodiments, respectively, is shown after the second resist pattern has been removed by conventional ash and wet clean techniques. In the second and third embodiments it is possible to keep the resist pattern during the subsequent deep trench etch step.

Deep trenches 22 are etched into the silicon layer 13 with high selectivity to the hard mask 16, 17 and 18. This takes place for all three embodiments, the first and second embodiments being shown in FIG. 21 and third embodiment being shown in FIG. 22. In the case where a silicon on insulator substrate is used for the substrate 11, the buried oxide layer 12 acts as a stopping layer and defines the depth of the deep trenches 23 such that the blind ends of the deep trenches are now located in the buried oxide layer 12. Ideally, the initial nitride layer 15 can be used as an etch stop in the shallow trench regions such that the hard mask film 16 or 17 and 18 is safely removed completely while still leaving enough of the initial nitride 15 to safely stop in the trenches (as shown in FIG. 19 for the first and second embodiments).

The process can also be designed to stop within the hard mask film instead, if required. Because the initial nitride has already been removed in shallow trench areas in the third embodiment, the deep trench etch has to stop within the hard mask in this case. Resist 19 can still be present during the deep trench etch process. The remaining resist 19 can be removed by ashing at an intermediate state of etching before exposing the hard mask or it can be fully consumed by the etching process itself. This can occur in the second and third embodiments without limiting the flexibility of the trench layout. In the first embodiment the deep trench etching operation allows the presence of the resist 19 only if isolated and semi- isolated deep trenches are not included in the layout. In the third embodiment, shown in FIG. 22, the formation of a spacer 24 at the edge of a shallow trench 23 starts to take place when etching the deep trenches 22. This particular spacer 24 will remain part of the isolation structure until the completion of the isolation process sequence.

Once the deep trenches 22 are etched to the target depth such that the blind end of each deep trench 22 is provided in the buried oxide layer 12 and all photoresist has been removed from shallow trench areas 23, another etch process with inverse selectivity requirement is applied to remove the remaining upper film stack layers (nitride 15 and initial oxide 14) from the shallow trench regions 23, while not significantly removing any of the silicon layer 13. The thicker hard mask provided on top of the active device areas prevents unwanted loss of the nitride layer 15 and remains thick enough to withstand the subsequent shallow trench etch operation. This is shown in FIG. 23 for the first and second embodiments and in FIG. 24 for the third embodiment.

In FIGS. 25 and 26 etching of the silicon layer 13 to the targeted shallow trench depth is shown for the first and second embodiment in FIG. 25 and for the third embodiment in FIG. 26. The shallow trench depth is selectively etched to the hard mask 16 and 17 and this completes the shape formation of the integrated deep and shallow trench structures. For compatibility with existing shallow trench planarization schemes it is desirable to have the initial nitride layer 15 still protected by leftover of a minimum of the hard mask 16 or 17 from all etching processes at this stage.

A thin oxide liner 25 is then grown on the exposed surfaces of the silicon layer 13, as shown in FIG. 27 for the structure according to the first and second embodiments and in FIG. 28 for the structure according to the third embodiment.

After this step of liner oxidation, the integrated deep and shallow trenches are simultaneously filled with an oxide insulation filling material 26. This is shown in FIG. 29 for the first and second embodiments and in FIG. 30 for the third embodiment. Several deposition methods for filling the trench structures are known in the art that are suitable for this application, for example HDP or chemical vapour deposition (SACVD or APCVD). Also combined deposition and reflow procedures can be used to fill the structures. After deposition the filling material can then be stabilized during a furnace operation.

FIG. 31 and 32 illustrate the structures of the first and second embodiments, and the third embodiment, respectively, after pattern and etch operations in the active silicon areas are used to relax planarization requirements. The filling material 26 has been etched from all areas without trenches to reduce the total amount of oxide to be removed during a subsequent planarization process. This etch operation stops on the surface of the nitride layer 15 and will automatically remove any hard mask film left over, except for the spacer provided in the third embodiment. FIGS. 33 and 34 show the device with a flat surface after planarization has taken place.

In FIG. 35 and 36, which show the finished isolation structure of the first and second embodiments, and the third embodiment, respectively, the nitride layer 15 has been stripped from the surface of the device. It can be seen that formation of the spacer 24 in the third embodiment shown in FIG. 36 is independent of formation of the deep trenches 22. The spacer 24 will protect the interface between the oxide layer 14 and the silicon layer 13 from being attacked during subsequent cleaning steps with hydrofluoric acid. Thus a reduced complexity in the filling of the deep and shallow trenches and increased layout flexibility of the trenches on the device is provided.

Although the invention has been described herein above with reference to specific embodiments, it is not limited to these embodiments, and no doubt alternatives will occur to the skilled person that lie within the scope of the claimed invention.

Claims

CLAIMSWhat is claimed is:

1. A method of forming an integrated deep and shallow trench isolation structure, the method comprising: depositing a hard mask on a film stack having a plurality of layers formed on a substrate such that the hard mask is deposited on a furthermost layer from the substrate; imprinting a first pattern into the hard mask to define an open end of a first trench; imprinting a second pattern into the hard mask to define an open end of a second trench; and etching into the film stack the first trench to a first depth and the second trench to a second depth.

2. A method according to Claim 1 , wherein the film stack is formed by depositing a first oxide layer on the substrate, providing a silicon layer on the first oxide layer, depositing a second oxide layer on the silicon layer and depositing a nitride layer on the second oxide layer.

3. A method according to Claim 2, wherein the step of etching comprises providing a blind end of the first trench in the first oxide layer and providing a blind end of the second trench in the silicon layer.

4. A method according to Claim 1, wherein the film stack is formed on bulk silicon.

5. A method according to Claim 4, wherein the step of etching comprises providing a blind end of the first trench and a blind end of the second trench in the bulk silicon.

6. A method according to Claim 3, wherein the hard mask is deposited on the nitride layer.

7. A method according to Claim 6, wherein the first pattern and the second pattern are imprinted simultaneously.

8. A method according to Claim 6, wherein the hard mask comprises a first portion and a second portion.

9. A method according to Claim 8, wherein the first pattern is created on the first portion of the hard mask to define the open end of the first trench and the second pattern is created on the second portion of the hard mask to define an open end of the second trench.

10. A method according to Claim 9, further comprising removing the hard mask.

11. A method according to Claim 10, further comprising removing the nitride layer.

12. A method according to Claim 11, wherein the first pattern is imprinted relative to the second resist pattern such that the open end of the first trench is isolated from the open end of the second trench.

13. A method according to Claim 11 , wherein the first pattern is imprinted relative to the second pattern such that the open end of the first trench is coincident with the open end of the second trench.

14. A method according to Claim 11, wherein the first pattern is imprinted relative to the second pattern such that the open end of the first trench is bounded by the open end of the second trench.