Method for formation of hardmask elements during a semiconductor device fabrication process

Abstract

In semiconductor device fabrication processes which include the formation of hardmask elements 17 including Al2O2, unwanted Al2O3 is left between the hardmask elements 17. The unwanted Al2O3 includes a layer 9 of Al2O3which is not homogenous across the surface of the structure 3 it overlies, and Al2O3 deposits on the sides of the hardmask elements 17. A method is proposed in which any such unwanted Al2O3 between the hardmask elements 17 is removed by a wet etching step in which the unwanted Al2O3 is exposed to an etchant liquid which etches the Al2O3 at a faster rate than other portions of the structure. This step allows the unwanted Al2O3 to be removed substantially completely without causing significant detriment to those other portions of the structure. Subsequently, an RIE etching step can be performed using the hardmask elements 17 as a mask, without the unwanted Al2O3 obstructing the RIE etching step.

Description

Field of the Invention

[0001] The present invention relates to method of removing alumina (Al2O3) from hardmask elements formed during a semiconductor fabrication process, and in particular a process for forming a semiconductor device including one or more Ferroelectric capacitors.

BACKGROUND OF INVENTION

[0002] Known multilayer FeRAM (Ferroelectric Random Access Memory) and DRAM devices include ferrocapacitors comprising a top electrode layer and a bottom electrode layer separated by a ferroelectric layer. The bottom electrode is connected to lower layers of the devices using polysilicon contact plugs or Tungsten (W) plugs. The ferroelectric materials in FeRAMs and high K materials in DRAM are generally crystallized at a high temperature (600 C or above) in ambient oxygen. During this process, a barrier is required to prevent oxygen diffusion from the ferroelectric capacitor to the contact plug. An Ir (Iridium) based barrier is a good material to efficiently block this oxygen diffusion. Subsequently, the barrier layer is removed by etching using respective hardmask elements to cover each of the ferrocapactitors. Typically, the hardmask layer may be TEOS, and an alumina layer may be provided as an interlayer between the bottom electrode and the hardmask.

[0003] FIG. 1(a) to 1(b) shows a known process for forming hard masks. The initial structure is as shown in FIG. 1(a). It includes a layer 1 of TEOS (Tetraethyl Orthosilicate), which may overlie other layers including electronic components. Above the TEOS layer 1 is a barrier layer 3 which includes a lower barrier layer 5 of Ir (or Ir and lrO2) and thickness 120 nm having the function of stopping oxygen damage to the plugs, and an upper barrier layer 7 of Pt and thickness 10 nm. Above the barrier layer 3 is a layer 9 of Al2O3, which may have a thickness of 20 nm. Above the alumina layer 9 is an dTEOS (dilute TEOS) layer 11 of thickness 100 nm. The dTEOS layer 11 is covered with a patterned mask 13.

[0004] During a subsequent RIE (reactive ion etching) step referred to as “ hard mask opening”, the structure is transformed into that shown in FIG. 1(b), mask elements 13 have been removed, and the dTEOS layer 11 and Al2O3 layer 9 have been partially removed. The remaining portions of the layer 11 and the Al2O3 portions beneath them constitute the hard mask elements 17. After the mask opening step, part of the etched Al2O3 remains on the sidewalls of the hard mask elements 17 (these deposits are referred to as “fences”), and part of the Al2O3 is on top of the etched layers.

[0005] Note that the exposed parts of the Al2O3 layer 9 do not have a uniform thickness. This is because the thickness of the dTEOS layer 11, and in particular the effectiveness of the RIE machine, and are both inhomogeneous. In particular, the RIE machine may apply an average over-etch of 5%, which is an over-etch of 0% in the area A and 10% in the area B. Assuming that the thickness of the hard mask is 1000 nm, and the typical selectivity of Al2O3 to dTEOS is 10 (i.e. the rate of etching of dTEOS is ten times as fast as that of Al2O3). This will mean that an additional Al2O3 thickness of 10 nm is etched in the area B as compared to the area A. In an inner area A of the structure, the thickness of the Al2O3 layer 9 (in portions not covered by the remaining portions of the dTEOS layer 11) is 20 nm. At the outer area B of the structure, the thickness of the uncovered portions of the Al2O3 layer 9 is 10 nm. The amount of fences also depends upon the position on the surface, so that typically the level of fences is high in the edge area B, whereas there are no fences in the area A.

[0006] At this stage BE (bottom electrode) RIE is carried out, using the hardmasks formed the previous step. FIG. 1(c) shows the structure after 3 minutes of BE etching at an etch rate of 5 nm/min. By this time, the thickness of the uncovered Al2O3 in the area A is 5 nm. However, in the area B all the A1203 has already been removed, and the etching of the lower barrier layer 5 has already begun, By the time that the RIE has been completed, the structure is as shown in FIG. 1(d).

[0007] One function of the Al2O3 layer 9 is to prevent the RIE machine being contaminated by the Pt or Ir during the hard mask opening process (i.e. between FIGS. 1(a) and 1(b)). Another function is to guarantee that none of the dTEOS layer 11 remains at the opened positions after the hard mask opening step. This is because the etching rate of dTEOS is much higher than that of Al2O3.

[0008] However, the inhomogeneity in the Al2O3 causes the following three problems in the BE RIE process. The first is that the fence situation is difficult to control. The fences are partially controlled by the shape of the hard mask. However, the hard mask shape is controlled by the Al2O3 fences, because of the low etching rate of the Al2O3 In area B, due to the fences, 50% of the Al2O3 is etched by the oxide RIE machine (i.e. the etching of the TEOS and Al2O3, as shown in FIG. 1(b)), and 50% of the Al2O3 is etched by the metal RIE machine (etching of the remaining Al2O3, Pt, Ir, TEOS, as shown in FIGS. 1(c) and (d)). By contrast, in the area A the Al2O3 is 100% etched by the metal etching machine. Thus, due to the fences, the BE etching is not uniform.

[0009] The second problem is that the inhomogeneity makes the BE etching hard to control. During the BE etching the etch rate in the area B (where Pt and Ir is being etched) will be 50 nm/min, but the etch rate in area A (where Al2O3 is being etched) is 5 nm/min. Therefore, the location B will start to etch Pt two minutes earlier than area A. This means that the endpoint of the etching is not clear, so that there may be over-etching of the TEOS layer 1. It is known that over-etching may cause a peeling problem.

[0010] The third problem is that the inhomogenity in the Al2O3 thickness increases the total time required by the etching process to ensure that the whole of the desired Al2O3 has been removed. For example, in the case of a structure with a thickness of Ir of 120 nm, the total etching time will be 7 mins. Of this, 4 mins is for the Al2O3, 3 mins is for the Ir, and 2 mins is over-etching.

SUMMARY OF THE INVENTION

[0011] The present inventors have appreciated that it would be advantageous to remove the Al2O3 between the mask elements prior to the BE etching, so as to remove its inhomogeneity,

[0012] The present invention aims to provide a method for removing unwanted Al2O3 as part of a method of formation of hardmask elements in a semiconductor device fabrication process.

[0013] In general terms, the invention proposes that in a wafer formed with a hardmask elements including Al2O3 and unwanted Al2O3 between the elements of the hardmask, a wet etching step should be performed. By “wet etching” is meant a process of etching in which the Al2O3 is removed by exposure to an etchant liquid. The etchant liquid may be such that the Al2O3 is etched at a faster rate than other portions of the structure, so that the unwanted Al2O3 can be removed without causing significant detriment to those other portions of the structure

[0014] More specifically, the invention proposes that in a semiconductor device fabrication process, a method for forming on a structure a hardmask comprising Al2O3 should comprise:

[0015] forming a layer comprising Al2O3;

[0016] forming a mask layer over the layer comprising Al2O3;

[0017] etching portions of the layer comprising Al2O3 which are exposed by the mask layer, to form hardmask elements; and

[0018] performing wet etching to remove Al2O3 between the hardmask elements.

BRIEF DESCRIPTION OF THE FIGURES

[0019] Preferred features of the invention will now be described, for the sake of illustration only, with reference to the following figures in which:

[0020] FIG. 1, which is composed of FIGS. 1(a) to 1(d), shows the steps in a known process of forming a hard mark, and using that harkmask to perform BE etching;

[0021] FIG. 2, which is composed of FIGS. 2(a) to 2(d), shows a method according to the invention; and

[0022] FIG. 3 shows electron microscope photographs taken during the process of FIG. 2.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0023] Referring firstly to FIG. 2(a), the initial structure used in the embodiment of the invention may be exactly as shown in FIG. 1(a) and as described above. Portions of the structure corresponding to those of FIG. 1(a) are given identical reference numerals. That is, the structure shown in FIG. 2(a) comprises a layer 1 of TEOS (Tetraethyl Orthosilicate), which may overlie other layers including electronic components. Above the TEOS layer 1 is a barrier layer 3, which includes a lower barrier layer 5 of Ir and thickness 120 nm and an upper barrier layer 7 of Pt and thickness 1 On. Above the barrier layer 3 are bottom electrode, ferroelelectric and top electrode layers (not shown) and then a layer 9 of Al2O3, which may have a thickness of 20 nm. Above the alumina layer 9 IS an dTEOS layer 11 of thickness 100 nm. The dTEOS layer 11 is covered with a patterned mask 13.

[0024] Likewise, the first step of the hardmask forming method is as in the known method described above, to give a structure shown in FIG. 2(b) which is identical to that of FIG. 1(b), and in which the mask elements 13 have been removed, and the dTEOS layer 11 and Al2O3 layer 9 have been. partially removed. The remaining portions of the layer 11 and the Al2O3 portions beneath them constitute the hard mask elements 17. Part of the etched Al2O3 remains as fences on the sidewalls of the hard mask, and part of the Al2O3 is on top of the etched layers.

[0025] At this point, however, the method according to the invention proposes that the top surface of the structure shown in FIG. 1(b) should be treated with a wet etching step using an etchant liquid. This may be a spin etching technique, i.e. one in which the wafer is rotated about an axis perpendicular to its surface (i.e. a vertical axis as shown in FIG. 2(b)) while the etchant liquid is applied to the surface to be etched. The etchant liquid may include hydrofluoric acid (HF), and more specifically may be dilute hydrofluoric acid (DHF). For example, using an HF concentration of under 5% in the case of the dimensions of the structure given above and an Al2O3 layer 9 which was formed by room temperature sputtering using O2 or Ar and an Al2O3 target (although this may alternatively be formed by atomic layer deposition, ALD), we have found that spin etching for 1 min using a 1% HF solution is able to substantially completely remove the Al2O3 at the open areas (i.e. apart from the Al2O3 which is part of the hardmnask elements 17), while removing only a small amount of dTEOS. Thus, the method forms the structure shown in FIG. 2(c).

[0026] This is illustrated in FIG. 3, which shows as FIGS. 3(a) and 3(b) two electron microscope views of a structure shown in FIG. 2(b) before the wet-eching process is carried out. FIGS. 3(c) and 3(d) are corresponding views of a structure as shown in FIG. 2(c) after the wet etching is carried out for 1 min using 1% HF. As can be seen, the Al2O3 fences (shown in the oval on FIG. 3(b)) are removed completely in FIG. 3(d).

[0027] Once the hardmask has been completed as shown in FIG. 2(c), the structure shown in FIG. 2(d) can then be obtained by BE etching using conventional techniques. For example, as in the conventional method, the hardmask may be used in a BE RIE etching process, to give the result shown in FIG. 2(d), in which the Pt and Ir layers 5, 7 and the upper portions of the TEOS layer 1 are removed except under the hardmask elements The upper surface of the TEOS layer 1 can be substantially even across the entire surface of the wafer. Note that, although not shown in FIGS. 2(a) to 2(d), the masking elements 17 cover respective ferroelectric capacitors above the barrier layer 3. In this case, the TEOS layer 1 and the structure beneath it may include lower layers including electronic components electrically connected to the ferroelectric capacitors using (polysilicon) contact plugs.

[0028] Although only a single embodiment of the invention has been described in detail, various variations are possible within the scope of the invention as will be clear to a skilled reader. In particular, the etchant liquid used may be different from the DHF described above. Also, just as many methods are known which employ hardmask etching techniques in the fabrication of semiconductor devices, so the embodiments of the present invention exist in which a liquid etching step is added to the known techniques prior to a BE etching process using a hardmask.

Claims

1. In a semiconductor device fabrication process, a method for forming on a structure a hardmask comprising Al2O3 , the method comprising: forming a layer comprising Al2O3; forming a mask layer over the layer comprising Al2O3: etching portions of the layer comprising Al2O3 which are exposed by the mask layer, to form hardmask elements; and performing wet etching to remove Al203 between the hardmask elements.

2. A method according to claim 1 in which the wet etching is performed by spin wet etching.

3. A method according to claim 1 in which the wet etching is performed using dilute hydrofluoric acid.

4. A method according to claim 1 in which the layer comprising Al2O3 is formed by sputtering using an Al2O3 target or by atomic layer deposition.

5. A method for manufacturing a ferroelectric capacitor comprising the steps of: forming a substructure of the capacitor having a contact plug passing therethrough for electrically connecting a bottom electrode of the capacitor to an underlying active layer; depositing over the substructure the bottom electrode including a barrier layer intermediate therebetween having a composition including Iridium; depositing over the bottom electrode a ferroelectric layer such that the diffusion of oxygen from the ferroelectric layer to the contact plug is inhibited by the intermediate barrier layer; depositing over the ferroelectric layer a top electrode; forming a hardmask over the top electrode by a method according to claim 1; and etching portions of the top electrode, ferroelectric layer, bottom electrode and barrier layer not covered by the hardmask.