DE10259322A1 - Making alignment mark in opaque layer on substrate, forms trenches of differing depth, fills selectively, then adds and polishes further layers to leave defined opaque mark - Google Patents

Abstract

Substrate comprises surface (91), first and second trenches (10, 20) of differing depth and width. First layer (50) is applied. Trench sizes and quantity applied ensure that smaller trench (20) is filled, but larger (10) is only partly filled. Second layer (30) is added which has greater etch selectivity compared with the first. Second layer thickness is adjusted during application, such that the first trench (10) is filled completely. The substrate is then chemically-mechanically polished, to lay bare the surface (91) of the substrate, and to form a planar surface over the first and second trenches. The second material is selectively etched to the first material, for partial opening (81) of the first trench, without using a resist- or resin mask. The opaque layer (100) is applied on the plane surface and into the opening (81) of the first trench (10) so that the alignment mark is formed as a depression (201) of the opaque layer, above the opening (81).

Description

The invention relates to a method to form an alignment mark in an opaque layer on a substrate, in particular on a semiconductor wafer.

To ensure the highest possible positional accuracy of structures to be formed in a layer relative to a pre-plane on a substrate such as a semiconductor wafer are so-called alignment marks in lithographic projection (English: alignment marks) used to align the substrate. These, along with the structures in the relevant layer levels formed and serve the exact position determination for an adjustment optics of the exposure device used in each case when carrying out a subsequent exposure.

The subsequent exposure takes place in a photosensitive resist, which is used for this purpose is applied to the layer currently to be structured. The The layer to be structured is in turn above the to belonging to the previous level, already structured layer within which the named Alignment marks e.g. than compared to the structural elements of the Circuit deep, wide trenches are arranged.

The layers to be structured often have the property translucent, i.e. transparent at the wavelength of the light used by the adjustment optics. The same generally also applies to the resist, so that by means of the adjustment optics through the resist and the layer to be structured through the alignment marks in the underlying layer recognized and their positions can be measured.

Different materials, in particular those that are electrically conductive, such as metals for formation of metal conductor tracks or polysilicon are used in the Manufacturing processes that are opaque, i.e. opaque to that light used by the adjustment optics. In this case, the alignment marks already arranged in layers not covered be recognized. However, it is possible that at The presence of an appropriate surface topography before application the layer to be structured is precisely this surface topography immediately in the new interface of the now applied layer to be structured is reflected. In this case, the position of the alignment mark is based on the current one surface topography identified.

In the manufacture of structures with particularly narrow structural widths, however, there is an effort to the exposure in a possible flat surface perform on a substrate. There are namely essentially identical focus conditions across the surface, so that a high image contrast is achieved.

For this purpose, for example a chemical-mechanical polishing step on the one already existing surface topography applied layer to be structured. By however, this polishing step becomes an option at the same time the detection of alignment marks significantly reduced if opaque layers be applied.

As a result, people are increasingly turning to the underlying layers with alignment marks as trenches particularly large Form widths and / or depths. The widths of these trenches are compared to those that serve the structural elements of the circuit, so big enough that these trenches of the Adjustment marks through the deposition process of the currently to be structured Layer does not become a complete backfilling of the alignment mark. In the polishing process thus a flat surface created at the positions of the alignment marks by indentations because of the unfilled trenches is marked.

However, further problems arise in this connection. As in the 1 and 2 The example of the formation of a shallow trench isolation (STI, 21) with subsequent planarization shows the surface in an environment of the trenches that are not completely filled 10 the alignment marks in the chemical-mechanical polishing process due to erosion 101 impaired.

1 shows in monocrystalline silicon 1 trenches formed of a substrate 10 , which represent alignment marks, and trenches 20 which with an HDP (high density plasma) oxide 50 to form the shallow trench isolation 21 are completely filled. Outside the trenches 10 . 20 is monocrystalline silicon 1 of the substrate is a pad nitride which forms the surface of the substrate provided 40 arranged. The trench is in the drawing, which is not to scale 10 of the oxide 50 only incompletely filled.

In 2 is the planarized surface resulting from the polishing process 91 shown. Due to the incomplete backfilling of the trench 10 with the oxide 50 the top trench edges, especially the pad nitride 40 in the vicinity of the trenches 10 badly attacked and eroded. Microscope examinations, for example of semiconductor wafers, on which memory modules are formed, show that after the planarization process, the memory structures located near the saw frame area of an exposure field (kerf) are severely affected by such erosion. The yield in semiconductor production can thus be reduced considerably.

This problem can be circumvented by the incomplete backfilling of the trench 10 emerging deepening 81 in the surface of the oxide 50 with another layer, a so-called cap layer 39 on the oxide 50 is backfilled. In an additional lithographic exposure step with subsequent etching back and removal of the resist applied to it, the trench, which has now been completely filled, can be used using a specially designed block mask 10 after an unproblematic polishing step can be opened again so that the topography of the alignment marks in the trenches for the alignment optics 10 becomes visible again. Avoiding the particles and problems of yield caused by erosion must be bought through an expensive and time-consuming manufacturing process.

It is the task of the present Invention, the effort and costs for the process of opening alignment marks forming trenches below layers to be structured, opaque layers. It is also an object of the present invention, the quality of the structuring process an opaque layer to increase.

The task is solved by a method of forming an alignment mark in a substantially opaque Layer on a substrate, comprising the steps: providing of the substrate comprising a surface and a first and a second trench, the first trench having a first width and a first depth and the second trench a second width and a second Depth, and wherein the first width is greater than the second width and / or the first depth is greater than that second depth is applying a first layer comprising a first material with a thickness such that the second trench due to its smaller width and / or depth completely and due to the first trench its greater width and / or depth is only partially filled, applying one second layer comprising a second material which has a selectivity in an etching process compared to the first Material, adjusting the thickness of the second layer when Apply such that the first digging completely is filled in by the second layer chemical-mechanical polishing of the substrate to expose the surface of the substrate under to form a flat surface over the first and second trenches, etching the second material selective to the first material to partially open the first trench, applying the opaque layer on the plane surface and into the opening of the first trench, so that the Adjustment mark formed as a recess in the opaque layer above the opening becomes.

Before performing the chemical mechanical Polishing process are not complete with the first layer, in particular an STI oxide, filled first trenches, which Represent positions of alignment marks in a substrate filled in by a second layer. This second layer has the special property of being selective in an etching process across from the trenches backfilled etched first layer to be able to. Selective here means that it there is an etching process at which the etch rate the material of the second layer is many times higher, than the etch rate the first or further layers, which are superficially exposed are.

In the following polishing process the the first trench of the surface surrounding the alignment mark Substrate is not eroded, as the wider or deeper trench there is essentially no opening or depression. The polishing process leads to a Back planarization such that the original surface of the substrate is exposed. For example, under substrate that for a wafer serving as the base monocrystalline silicon with a or more layers arranged thereon, such as one thin Oxide and / or pad nitride layer Roger that.

The trenches are in the monocrystalline Silicon and the layers of the provided Substrate formed. After the back planarization is the first trench forming the alignment mark with parts of the first and backfilled the second layer. Since the second, the formation of structural elements such as the flat Trench isolation trenches have already been completely filled with the first layer, since they have a smaller second layer Width and / or depth than the first width and / or depth of the first trenches of the alignment marks, these trenches have no share of the second Material of the second layer.

Whether the depth, the width or a combination of both, the criterion for backfilling the first and not backfilling the second Trench fulfilled, depends in particular of the currently set process parameters for the first separation process Shift or earlier deposited layers. In the majority of cases the different current depth of the trench of the alignment mark relative to the trench of a structural element to be filled for the Operation of the method according to the invention his.

With an etching process, which the material of the second layer selectively etched back to that of the first layer now the trenches of the alignment marks at least in those occupied by the second layer Parts of the space etched free. Depending on the etching selectivity set other areas of the substrate surface are also slightly etched back, however the etching depths are comparatively low.

After the etching process, the flat, planarized surface only has depressions in the Trenches of the alignment marks. If an opaque layer is now applied, the depression is transferred to the opaque layer, provided the deposition process is sufficiently conformant and the thickness of the deposited layer is not significantly greater than the width and / or depth of the first trench. On the basis of this recess, the position of the alignment mark can be recognized with an alignment lens. To carry out the method according to the invention, particles are neither generated by eroding trench edges during a planarization process, nor are additional masking steps required to avoid erosion.

The translucency of the structure to be structured Layer does not only refer to the optical or ultraviolet Wavelength range, like conventional ones Adjustment optics come into consideration. For future exposure devices Technology generations can Such optics are also suitable, which are used in X-ray, Ion or electron beam processes work. These definitions are included in the present invention.

The etching step according to the invention can also be a sum of subsequent separate etching steps may include, for example, first after an STI process wet chemical Oxide etching process, then a wet chemical Nitride etching process and after again a wet chemical oxide etching process can be carried out. A particular advantage arises precisely when the etching step according to the invention to expose the brand opening in Combination with an etching step to remove a superficial applied further layer, for example the pad nitride or an oxide layer arranged directly on the monocrystalline silicon, carried out becomes.

Applying the first layer can also have several sub-steps each with one application of sub-layers include. With future technologies it is provided, for example, that the process parameters during a Deposition to vary in a suitable manner, so that there is a horizontal sequence of layer structures in such a first one Layer.

The method according to the invention is not based on the use of alignment marks for alignment in one exposure unit limited. Rather, this document also includes an alignment mark measuring mark to determine the relative position accuracy of two on one Understood structured layer planes arranged on the substrate, which generally only after exposure has been carried out in one Microscope measuring device examined become.

The invention will now be based on a embodiment with the help of a drawing explained become. In it show:

1 - 2 Method steps for opening a filled alignment mark according to a first example according to the prior art,

3 - 7 Method steps for opening a filled alignment mark according to a second example according to the prior art,

8th - 11 Method steps for opening a filled alignment mark according to an exemplary embodiment according to the prior art.

The method according to the invention can be applied particularly advantageously to the structuring of poly-silicon tracks which are used for gate contacting of the active regions of selection transistors, for example for trench capacitors. For this purpose, a substrate is provided, which is monocrystalline silicon 1 comprises, on which a thin oxide and then a pad nitride layer 40 is arranged, which can serve as an end mark for subsequent etching processes.

The substrate is a semiconductor wafer on which memory cell fields are to be formed.

Trench capacitors (deep trenches) serving as storage nodes and active regions for forming selection transistors are already arranged in the substrate provided. The memory cells are separated from each other by a shallow trench isolation, for what purpose in the monocrystalline silicon 1 trenches 20 were formed with shallow depth.

The width of these trenches 20 (in the following STI trenches or second trenches 20 ) have a structure width of 240 nanometers (nm). Before the flat STI trenches are formed within the memory cell fields, the first trenches become 10 of the alignment marks formed in the monocrystalline silicon 1 in the saw frame of the exposure field, which includes the memory cell field, among other things. For simplification and not to scale are in 3 the trenches 10 and 20 drawn immediately next to each other.

The profile of the trenches 10 and / or trenches 20 can be any, in particular V-shaped or U-shaped profiles, etc. are possible in addition to the rectangular profiles shown in the figures.

The first ditches 10 of the alignment marks have a much greater depth than the STI trenches 20 or further trenches that are structured within the memory cell array or in its periphery. The width of the first trenches 10 is, for example, 1.2 μm. The depth is 1.5 μm. Trenches experimentally formed in the saw frame area, which have a width of, for example, 0.5 μm and a depth of approximately 100 to 150 nm, show the properties similar to the second trenches in relation to backfilling when the method according to the invention is carried out 20 , The different depths of the trenches originally etched up to 5 μm deep into the silicon for formation of alignment marks is due to a different filling profile, here of poly-silicon for filling trenches for storage capacitors (deep trenches). In the exemplary embodiment, the trenches lie 10 depth, which is characteristic of alignment marks, for example above 0.5 μm.

The depth of the trenches 10 . 20 a special role here for backfill behavior. In the case of shallow trench isolation (STI), the depth is by definition small, in the exemplary embodiments it is approximately 280 nm.

In an HDP process (HDP - High Density Plasma, highly compressed plasma), an electrically insulating oxide is applied to the substrate and into the trenches as the first layer 50 deposited. The thickness of the layer is 480 nm. In the HDP deposition process, a greater thickness of the first layer is produced on horizontal surfaces than on vertical surfaces due to parallel sputtering processes. The specified layer thickness refers to the thickness over a horizontal surface. In the exemplary embodiment of the present invention which is yet to be described, an NDP method is used, but other suitable deposition methods, in particular those with a conformal deposition profile, can also be used according to the invention.

To better illustrate the advantages of the present invention, reference is first made to the 3 to 7 a process sequence according to the prior art using an additional mask step to open the filled trench 10 shown. As in 3 can be seen on the oxide layer deposited with the HDP process 50 a cap layer 39 deposited such that the trench 10 the alignment mark to above that through the pad nitride layer 40 marked trench edge is filled.

A chemical-mechanical polishing step (CMP step) is then carried out until the surface 91 of the originally provided substrate, ie the pad nitride layer 40 is exposed ( 4 ). In the further course, a photosensitive layer 70 applied and exposed with an uncritical block mask, the exposed structures on the filled trenches 10 come to rest in the frame area of the exposure fields. After developing and removing the exposed resist parts ( 5 ) in the first trenches 10 located oxide with the cap layer 39 etched back in a time-controlled etching process. The cap layer is also an oxide with the chemical and physical properties similar to those of the STI oxide 50 ( 6 ).

After removing the photosensitive layer 70 and the implementation of further process steps, for example wet chemical etching steps, oxidation, implantation, etc., is doped polysilicon as an opaque layer 100 applied. Over the etched back openings 180 the first trenches 10 any profile forms a deepening 200 in the substantially conformally deposited opaque layer 100 , After applying a further photosensitive layer which is translucent for the adjustment optics, this depression can be made 200 can be recognized and used in an alignment process for determining the position of the alignment mark.

The method according to the invention is now applied to a comparison substrate. The starting point is the described, provided wafer with first trenches 10 (Alignment marks) and second trenches 20 (STI trenches), on and in which a 480 nm thick oxide layer 50 was deposited in an HDP process. In the opening 81 of the not completely filled first trench 10 the alignment mark and on the oxide layer 50 becomes a flow-fill oxide 30 deposited. In contrast to the STI oxide 50 is this flow fill oxide 30 not been exposed to a high temperature process. As a result, its internal structure differs significantly from that of the STI oxide 50 , as well as the oxide of the cap layer 39 according to the state of the art. The flow fill oxide can consequently be selected with high selectivity in an etching process compared to the STI oxide 50 remove. 8th shows the state after application of this second layer with high etching selectivity.

After performing the CMP step ( 9 ), in which an essentially flat surface in the memory cell array and also in the frame 91 a wet etching process is used. The original conformal layer thickness of the flow fill oxide 30 was 160 nm. After a 24-second etching with BHF, the flow-fill oxide is 30 to a depth of 200 nm from the previous opening 81 in the STI oxide 50 away ( 10 ). At the same time, the STI has oxide 50 compared to the neighboring pad nitride 40 suffered a loss of approximately 40 nm in height due to etch losses (not shown). However, this loss is tolerable. The etching selectivity thus achieved is approximately one factor 5 ,

For repeating the opening 81 in the first ditch 10 no additional mask step is required for the alignment mark. The pad nitride 40 and the STI oxide 50 remain almost unaffected by the selective etching process. The pad nitride 40 is to be deducted in the following example. The etching step required for this can advantageously be performed using the etching step according to the invention to expose the opening 81 be combined. To this end, the selectivity of the etching step according to the invention must be set accordingly. The same applies to the combination with an etching step to adjust the height of the STI oxide 50 ,

It is also possible to deposit a liner made of polysilicon, for example, instead of just the flow-fill oxide, for example one Oxide plugs. By under-etching using a poly-silicon etching, the oxide plug can be removed from the opening 81 be removed.

It now becomes the opaque layer 100 comprising poly-silicon, in which the shape of the opening is due to an essentially conformal deposition 81 as a deepening 201 transfers. As described at the beginning, you can use this specialization 201 that are only slightly smaller than the depression 200 according to the state of the art, can be used for alignment of the substrate or wafer by means of alignment optics in an exposure device.

It is clear that the in the embodiment described materials do not limit the present invention represent. The one in the field of etching techniques knowledgeable average specialist can instead of the flow-fill oxide for example also doped oxides, boron-silicate glasses, boron-phosphorus-silicate glasses, so-called Low-k materials with low dielectric constant, ozone TEOS, amorphous or use polysilicon.

Knowing the chemical and physical composition of the material of the first layer 50 In the exemplary embodiment, which was the STI oxide, the person skilled in the art can select a suitable etching process with suitable etching parameters in order to achieve a desired etching selectivity for the materials mentioned. With knowledge of the present invention, it is also obvious to use materials other than the STI oxide 50 for filling the trenches 10 and 20 use.

It is particularly advantageous if the etching-selective material of the second layer has an etching selectivity both with respect to the pad nitride and to the first material of the filled trenches 10 and 20 having.

The invention is also not limited to an application for structuring opaque polysilicon 100 comprehensive word lines. Rather, the application of the invention is particularly suitable for structuring higher metal levels. Opaque layers can also include, for example, other electrically conductive materials such as the metals mentioned, in particular copper, aluminum, or metal compounds such as tungsten silicide, etc., doped semiconductor materials, carbon, antireflection layer materials etc. The formation of metal levels is usually associated with a CMP process to be carried out beforehand, by means of which the topography of the alignment marks can be impaired. The invention is consequently particularly advantageously applicable to the structuring of metal levels.

Claims (9)

Method for forming an alignment mark in an essentially opaque layer ( 100 ) on a substrate, comprising the steps: - providing the substrate comprising a surface ( 91 ) and a first ( 10 ) and a second ( 20 ) Trench, the first trench ( 10 ) a first width and a first depth and the second trench ( 20 ) has a second width and a second depth, and wherein the first width is greater than the second width and / or the first depth is greater than the second depth, - applying a first layer ( 50 ) comprising a first material with a thickness such that the second trench ( 20 ) due to its smaller width and / or depth completely and the first trench ( 10 ) is only partially filled due to its greater width and / or depth, - application of a second layer ( 30 ) comprising a second material which has a selectivity in an etching process compared to the first material, - adjusting the thickness of the second layer ( 30 ) in such a way that the first trench ( 10 ) completely through the second layer ( 30 ) is filled, - Chemical-mechanical polishing of the substrate to expose the surface ( 91 ) of the substrate and to form a flat surface over the first ( 10 ) and second ( 20 Trenches, - etching the second material selectively to the first material for partial opening ( 81 ) of the first trench, without using a resist or hard mask, - applying the opaque layer ( 100 ) on the flat surface and in the opening ( 81 ) of the first trench ( 10 ) so that the alignment mark as a recess ( 201 ) the opaque layer above the opening ( 81 ) is formed. A method according to claim 1, characterized in that at the application of the second layer at least one second material from the group comprising: endowed or non-stoichiometric Oxides or nitrides, boron silicate glass, boron phosphorus silicate glass, flow fill oxide, Low-k materials, ozone tetra ethyl ortho silicate, Poly-silicon can be used. Method according to one of claims 1 or 2, characterized in that when the first layer ( 50 ) a material comprising an undoped oxide or an oxynitride is used. A method according to claim 3, characterized in that the first layer ( 50 ) to form shallow trench isolation ( 21 ) is applied in a deposition process at elevated temperatures, in particular a high-density plasma, a chemical vapor deposition or a physically strengthened vapor deposition process, or that the deposition process is followed by a high-temperature process, in particular an annealing, in order to compact the material of the first layer , A method according to claim 4, characterized in that the substrate with a layer ( 1 ) comprising monocrystalline silicon and a further layer comprising a pad nitride arranged on the surface above it ( 40 ) is provided in which two layers ( 1 . 40 ) the first ( 10 ) and the second trench ( 20 ) are formed in each case. A method according to claim 5, characterized in that the second material with an etching selectivity compared to the pad nitride ( 40 ) and the oxide or oxi-nitride of the shallow trench isolation ( 21 ) is selected. Method according to one of claims 1 to 6, characterized in that the opaque layer ( 100 ) a material from the group comprising metals, semiconductor materials, carbon, anti-reflective layer materials, amorphous or polysilicon. A method according to claim 5, characterized in that - the opaque layer ( 100 ) Comprises poly-silicon, - a photosensitive layer ( 70 ) is applied, - the alignment mark is used to align the substrate in a projection device, - the opaque layer ( 100 ) to form word lines for driving active areas on the monocrystalline silicon ( 1 ) is exposed with a structure pattern representing a dynamic memory. Method according to one of the preceding claims, characterized characterized in that the etching is wet-chemical or is carried out in a dry etching process.