Method for manufacturing integrated structures including removing a sacrificial region

Abstract: The method is based on the use of a silicon carbide mask for removing a sacrificial region. In case of manufacture of integrated semiconductor material structures, the following steps are performed: forming a sacrificial region (6) of silicon oxide on a substrate (1) of semiconductor material; growing a pseudo-epitaxial layer (8); forming an electronic circuit (10-13, 18); depositing a silicon carbide layer (21); defining photolithographycally the silicon carbon layer so as to form an etching mask (23) containing the topography of a microstructure (27) to be formed; with the etching mask (23), forming trenches (25) in the pseudo-epitaxial layer (8) as far as the sacrificial region (6) so as to laterally define the microstructure; and removing the sacrificial region (6) through the trenches (25).

Description: The present invention relates to a method for manufacturing integrated structures including removing a sacrificial region. In particular, the method refers to the manufacture of integrated microstructures such as microsensors, microactuators and special micromechanisms. Recently, methods for manufacturing microstructures have been proposed, which are based on process and technology steps typical of the manufacture of integrated circuits and enabling a microintegrated structure and the associated control electronics to be integrated in a single chip. For example, EP-A-0 822 579 describes a method according to which an embedded sacrificial region of silicon oxide is formed above a substrate of monocrystalline silicon; a pseudo-epitaxial layer is then grown, which has, above the embedded sacrificial layer, a polycrystalline structure and elsewhere a monocrystalline structure; the electronic components of the circuitry are integrated in the monocrystalline structure portion of the pseudo-epitaxial layer; trenches are formed in the polycrystalline structure portion of the pseudo-epitaxial layer so as to define the form of the desired microintegrated structure and, through the trenches themselves, the embedded sacrificial region is etched so as to form a suspended mass which can be used as a static, kinematic or dynamic microstructure. The operations of trenching and removing the embedded sacrificial region require two etching processes: a first etching through a thick silicon layer (the pseudo-epitaxial layer has typically a thickness of 8-9 µm) and a second etching using concentrated hydrofluoric acid. These operations are therefore somewhat complex, in particular the second wet etching process with a very aggressive solution gives rise to problems as regards the already exposed metal regions in the pads. In fact tests carried out using a single masking resist layer for defining the microstructure and protecting the pad metal regions have produced negative results, since the machine used for forming the trenches (MXT machine made by AMT) is not sufficiently selective with respect to the resist layer. Also, the use of a double resist mask, a first one defined on the active area nitride with the design of the desired microstructures and a second one covering the entire device, including the pads, except for large windows above the microstructures to be formed, has provided unsatisfactory results. The same problem arises when it is required to remove a sacrificial layer or a region by an etching process which is aggressive and/or has long duration. In fact present resist masks are unable to sufficiently withstand etching operations which have a duration of more than a few minutes and/or a high concentration (for example hydrofluoric acid with a concentration of up to 49%). US-A-4,351,894 discloses a method for selectively etching a silicon dioxide film using a carbide mask. US-A-5,470,797 discloses a method for manufacturing a diaphragm, including the features of the preamble of claim 1, wherein, before forming metal regions and contacts, a sacrificial region is removed below a semiconductor region using a nitride mask. The object of the invention is therefore to improve the known processes so as to allow complete removal of sacrificial regions even when long durations and/or high concentrations of very aggressive solutions are required. According to the present invention, there is provided a method for manufacturing integrated structures, as defined in Claim 1. Hereinafter, a method for manufacturing an integrated structure including a suspended mass formed in a region of polycrystalline silicon grown epitaxially will be described by way of a non-limiting example. However, as discussed below, the invention is not limited to the described example and extends in general to the use of a silicon carbide mask for protecting regions of semiconductor material, metal and other materials used in the microelectronics industry during etching for the removal of sacrificial regions. In the accompanying drawings: Figures 1-5 show cross-sections through a wafer of semiconductor material during different steps of the known method; Figures 6-10 show cross-sections similar to those of Figs. 1-5, during different steps of the method according to the invention. The method described here comprises initial steps which are similar to those of the known method summarised above and described rapidly here with reference to Figs. 1-5 in which the different layers and regions are not shown to scale. Initially, a silicon oxide layer 2 is formed above a substrate 1 of monocrystalline silicon; a silicon nitride layer 3 is deposited above the silicon oxide layer 2 and a phototechnical step and a step of etching nitride in the zone where the desired microstructure is to be formed are carried out. The intermediate structure according to Fig. 1 is thus obtained. Subsequently a step of local oxidation, masked by silicon nitride layer 3 is performed, which results in forming a sacrificial region 6 of silicon oxide having a thickness, for example, of 2 µm (Fig. 2). Then silicon nitride layer 3 is removed and a polycrystalline silicon layer with a thickness for example of 4500 Å is deposited; by a phototechnical and chemical etching step, the polysilicon layer, except for the zone above sacrificial region 6, and silicon oxide layer 2 above substrate 1, are removed. The remaining portion of the polycrystalline silicon layer, denoted at 7 in Fig. 3, therefore forms the seed for a next epitaxial growth step. If envisaged by the process, steps of implanting doping species for forming embedded structures and/or junction isolating regions using top/bottom technique are performed in a manner known per se and not shown. Then an epitaxial growth is performed, forming a pseudo-epitaxial layer 8 having a thickness of 8 µm for example, including a polycrystalline region 8' above sacrificial region 6 and a monocrystalline region 8" elsewhere. The wafer indicated at 9 in Fig. 4 is thus obtained. Then, standard process steps for forming electronic components, including forming conducting and isolating regions inside monocrystalline region 8" as well as conductive, connecting and isolating regions above wafer 9, are performed; for example, in accordance with Fig. 5, pockets 10, 11 forming part of a MOS transistor 12 and a bipolar transistor 13 shown schematically are formed in monocrystalline region 8". A first dielectric layer 15 (for example BPSG - Boron Phosphorous Silicon Glass) is deposited; metal areas are formed and contact pads are defined, one whereof being visible in Fig. 6, at 18; a second dielectric layer 16 (for example PSG - Phosphorous Silicon Glass) and then a passivating layer 17 (for example, oxynitride) are deposited; then, by a special phototechnical and chemical etching step, portions of second dielectric layer 16 and of passivating layer 17 are removed from region of pad 18 (as can be seen on the left in Fig. 5) as well as in the zone above a microstructure to be formed (above polycrystalline region 8'). First 'dielectric layer 15 is deliberately left above polycrystalline region 8' in order to protect a zone where a microstructure must be formed, obtaining the structure according to Fig. 5 which is then subjected to electrical tests envisaged for wafers. Then, a barrier layer 20, for example oxide obtained from TEOS with a thickness of 1000 Å, is deposited; by a phototechnical and chemical etching step, barrier layer 20 is then removed so as to form a window 20a in a zone where the microstructure must be formed; simultaneously also the exposed portions of first dielectric layer 15 are removed, providing the intermediate structure according to Fig. 6. Then, initially a first masking layer 21, comprising silicon carbide, with a thickness for example of 7500 Å, followed by a second masking layer 22, preferably comprising oxide obtained from TEOS, with a thickness for example of 4000 Å, are deposited. A phototechnical step for defining the microstructure is then performed using a resist mask; thus, parts of first and second masking layers 21 and 22 corresponding to a desired lithographic form for the microstructure are then removed. Masking layers 22 and 21 therefore form a hard mask 23. Then, using hard mask 23 just obtained, the silicon of polycrystalline region 8' is etched in order to form trenches laterally defining the desired microstructure. The chemical etching step is interrupted automatically on silicon oxide sacrificial region 6. At the end of etching, the structure according to Fig. 8, in which the trenches in the polycrystalline region 8' are indicated by 25, is thus obtained. Afterwards, silicon oxide forming sacrificial region 6 is removed by etching with concentrated hydrofluoric acid; during this step, the oxide forming second masking layer 22 is also removed. The structure according to Fig. 9 is thus obtained, wherein, instead of sacrificial region 6, there is now an air gap 26 defining at the bottom the suspended mass (indicated by 27) of the microstructure. First masking layer 21 is then removed by exposing to a sulphur hexafluoride solution (SF 6 ) which removes silicon carbide from the surface of polycrystalline region 8' and barrier layer 20; during this step, barrier layer 20 protects silicon nitride passivating layer 17 which would be etched by the silicon carbide etching mixture. Finally, barrier layer 20 is removed, providing the final structure according to Fig. 10. In the described method, barrier layer 20, as shown, has the function of providing an end point of silicon carbide removal etching, but must also be removed beforehand from the zone where the microstructure is to be formed, so as to avoid infiltration of hydrofluoric acid at the interface between polycrystalline region 8' and first silicon carbide masking layer 21 during removal of sacrificial region 6; consequently, the mask used for removing barrier layer 20 (and first dielectric layer 15, Fig. 6) must define a window 20a with such dimensions as to expose the microstructure zone, but covering passivating layer 17 on all sides. Second masking layer 22 has the function of allowing correct formation of trenches 25, even when the etched area (not covered by hard mask 23) represents a significant portion of the total area of the wafer (more than about 5%) or there are zones with a high concentration of exposed area; in fact, under these conditions, in the absence of second masking layer 22, there could be problems of etched material re-deposition. Second masking layer 22 therefore ensures good trench etching, without complicating the process steps or creating problems of removal, since, as explained above, it is removed completely by hydrofluoric acid during etching of sacrificial region 6. However, if the abovementioned area conditions do not occur or if etching technique ensures trench correct etching irrespective of the existing topography, it is possible to dispense with second masking layer 22 and perform both etching of the trenches and etching for removing the sacrificial region using silicon carbide layer alone. The use of silicon carbide for protecting the wafer functionally important exposed regions (pad metal regions) during the steps of forming the trenches and removing the sacrificial region allows aggressive chemical etching solutions to be used, such as hydrofluoric acid, ensuring excellent etching profiles and avoiding damage to important parts of the integrated devices, despite the need to remove the material (silicon oxide) to a depth and in quantities which are unusual for the electronics industry. In particular, the silicon carbide mask is able to ensure excellent protection of the layers and underlying regions also in the case of etching using hydrofluoric acid for a duration of up to 1 hour at a concentration of up to 49%. Basically, it has been shown that use of silicon carbide allows the requirements of micromachining to be combined with those of microelectronics, achieving excellent results. In the method described above, the thickness of silicon carbide layer 21 depends essentially on evenness of the wafer prior to deposition of the silicon carbide layer itself. In particular, the abovementioned thickness (7500 Å) reliably prevents infiltration of hydrofluoric acid in weak points of the silicon carbide layer itself when discontinuities are present, and may therefore be reduced significantly in case of wafers with an even surface. Finally it is obvious that numerous modifications and variations may be made to the method described and illustrated herein, all of which falling within the scope of the invention, as defined in the accompanying claims. In particular, the use of silicon carbide as a mask for etching a sacrificial region with a very aggressive acid is also applicable to processes other than that described above; for example, the invention is applicable to processes using SOI substrates, in which the silicon oxide layer comprised between the two wafers of monocrystalline silicon forms (partly) the sacrificial region (see, for example, EP-A-0 822 398) or the sacrificial region is made using different techniques, for example is formed by deposited oxide or by oxide regions formed in special recesses of the substrate. The present method is also applicable to suspended structures formed by polycrystalline silicon deposited above sacrificial oxide regions in turn formed above the substrate, at the end of the process for forming circuitry electronic components; or else when the sacrificial region is obtained by oxidating porous silicon. The silicon carbide mask can be used also for removing embedded regions of material different from silicon oxide and in case of an etching agent other than hydrofluoric acid, for example in case of suspended structures of metal material, in which the sacrificial region comprises polymer material which is removed by an alkaline agent, such as oxygen plasma.

Claims: A method for manufacturing integrated structures, comprising the steps of: forming a wafer (9) including at least a semiconductor material substrate (1), a semiconductor material layer (8) and a sacrificial region (6) between said substrate and said semiconductor material layer; forming an etching mask (23) above said wafer (9); and removing said sacrificial region (6) using said etching mask (23); characterized in that said etching mask (23) comprises silicon carbide and in that the method further comprises the steps of: before the step of forming an etching mask, forming a barrier layer (20) over said semiconductor material layer (8), said barrier region having an opening (20a) in a zone of said semiconductor material layer (8) where a microstructure is to be defined; after said step of forming an etching mask, defining said microstructure in said semiconductor material layer. A method according to Claim 1, characterized in that said sacrificial region (6) comprises silicon oxide.. A method according to Claim 1 or 2, characterized in that said step of removing said sacrificial region (6) is performed by chemical etching with hydrofluoric acid. A method according to any one of the preceding claims, characterized in that : said etching mask (23) is formed above said semiconductor material layer (8) and contains a topography of the microstructure (27) to be formed; using said etching mask (23), trenches (25) are formed in said semiconductor material layer (8) so as to laterally define said microstructure, said trenches extending as far as said sacrificial region (6); and said sacrificial region (6) is removed through said trenches (25). A method according to any one of the preceding claims, characterized in that said step of forming the etching mask (23) comprises the step of forming a silicon carbide layer (21) and defining photolithographycally said silicon carbide layer. A method according to any one of the preceding claims, characterized in that said barrier layer (20) covers selectively electronic components and contact pads (18) formed on said wafer. A method according to any one of the preceding claims, characterized in that said step of forming a barrier layer (20) comprises the steps of depositing a barrier layer above said wafer (9) and removing selective portions of said barrier layer. A method according to any one of the preceding claims, characterized in that said barrier layer (20) comprises silicon oxide deposited from TEOS. A method according to any one of Claims 5 to 8, characterized in that , after said step of forming a silicon carbide layer (21) and prior to said step of defining photolithographically, the step of forming a masking layer (22) is performed and said step of photolithographically defining also comprises the step of selectively removing portions of said masking layer. A method according to Claim 9, characterized in that said masking layer (22) comprises silicon oxide. A method according to Claim 10, characterized in that said silicon oxide is deposited from TEOS. A method according to any one of the preceding Claims, characterized in that said step of forming a wafer (9) comprises the steps of: forming an oxide region (6) on said substrate; growing epitaxially said second semiconductor material layer (8). A method according to any one of the preceding Claims, characterized in that , prior to said step of forming an etching mask (23), the following steps are performed: forming integrated electronic components (10-13) in said layer of semiconductor material (8); forming a contact pad (18) of electrically conducting material above said wafer (9); forming a passivating layer (16, 17) above said wafer (9) ; creating openings in said passivating layer (16, 17) above said pad (18).

Inventors: CASTOLDI LAURA, FERRERA MARCO, GELMI ILARIA, MONTANINI PIETRO, CASTOLDI, LAURA, FERRERA, MARCO, GELMI, ILARIA, MONTANINI, PIETRO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1998
Main classification: G01L9
IPC codes: H01L21, G01P15, B81B3, G01C19, B81C1, H01L49