Method for manufacturing a microstructure

Abstract

To provide a method of producing a microstructure that employs the step of HF-etching a sacrificial layer silicon dioxide that exhibits reduced levels of residual layers or films.SOLUTION: A method of producing a microstructure comprises employing a hydrogen fluoride (HF) vapor to etch a sacrificial layer of silicon dioxide (SiO2), and thereafter removing a residual layer formed when HF-vapor-etching the layer of silicon dioxide.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a method for use in fabricating microstructures, typically in the form of microelectromechanical systems (MEMS) that require the removal of material relative to a substrate or other deposited material. In particular, the invention relates to an improved method for fabricating microstructures that utilizes etching silicon dioxide with hydrogen fluoride (HF) vapor. [Background technology]

[0002] Isotropic etching of silicon oxide is widely used in semiconductor and MEMS processing, primarily in wafer cleaning and release processes. In the fabrication of microstructures, such as microelectromechanical structures (MEMS) of the type shown in FIG. 1 and generally designated by reference numeral 1, etching processes are used to remove sacrificial (i.e., unwanted) regions or layers of material 2. MEMS 1 finds applications in inertial measurement, pressure sensing, thermal measurement, microfluidics, optics, and radio frequency communications, and the range of possibilities for these structures continues to grow. During the construction of MEMS, sacrificial layer 2 is first deposited on substrate 3 and then subsequently removed using an etching step that allows the released structure 4 to function as designed, for example, as a micromirror, accelerometer, or microphone. To fabricate a reliable structure, the release etching step requires the removal of sacrificial layer 2 without etching the surrounding material. Ideally, etching of sacrificial layer 2 should have no effect on the remaining structure.

[0003] One of the most common materials used as the sacrificial layer 2 is silicon dioxide, which is then etched using hydrogen fluoride (HF) vapor, see for example UK Patent No. GB ​​2,487,716B. HF vapor etching is a plasma-less chemical etch, according to the reaction:

[0004] Described by TIFF0007772412000001.tif24170.

[0005] Water (HO) was found to ionize HF vapor, as described by equation (1), and the ionized HF vapor

[0006] TIFF0007772412000002.tif8170 then etches the silicon dioxide (SiO2) with water (H2O) acting as a catalyst. From equation (2), it is also clear that water (H2O) is produced from the etching reaction itself.

[0007] Silicon dioxide layers are also present in semiconductor devices, as are those used in the fabrication of MEMS 1. For this reason, it is also known that hydrogen fluoride (HF) vapor etching techniques are used to create air gap structures in multilevel metal structures used in standard semiconductor devices; see, for example, U.S. Patent No. 7,211,496.

[0008] It is universally accepted that for HF vapor etching to proceed at usable etch rates greater than, say, 30 nm / min, a condensed fluid layer 5 must be present on the surface to be etched; see, for example, Helms et al., "Mechanisms of the HF / H2O vapor phase etching of SiO2," Journal of Vacuum Science and Technology A, 10(4), July / August 1992. Of all the compounds involved in the HF vapor etching process described above, water (H2O) has the lowest vapor pressure and therefore forms the main component of the condensed fluid layer 5. European Patent No. EP 2046677 B1 discloses how control of the formation and composition of the condensed fluid layer 5 is key to managing HF vapor etching of silicon dioxide. Precise etch control is achieved by performing the HF etch in a vacuum chamber and controlling the chamber pressure, temperature, and gas flow to the chamber. Other parameters that affect HF vapor etching are the composition of the silicon dioxide layer being etched and the method of deposition of the silicon dioxide layer.

[0009] Chemical vapor deposition (CVD) processes are commonly used to deposit silicon dioxide (SiO2) onto substrates. In these processes, chemical precursors, one a silicon source and the other an oxygen source, react to deposit a silicon dioxide layer 2 onto the substrate 3. The most common of these processes is plasma-enhanced CVD (PECVD), as this process allows deposition to be carried out at low temperatures, <450°C.

[0010] When depositing an oxide layer (e.g., a silicon dioxide layer 2 via a PECVD process), impurities can be incorporated into the layer, either intentionally or accidentally. When this is done intentionally, it is known as doping the silicon dioxide layer 2. The use of doped layers (e.g., phosphosilicate glass (PSG) and borophosphosilicate glass (BPSG) layers) is also common within semiconductor fabrication processes, as the presence of dopant materials provides superior step coverage, thermal properties, electrical improvement, and barrier performance. However, because the HF vapor etching process is a chemical etch, the process etches silicon dioxide but often does not etch materials containing impurity materials. Thus, as the doped silicon dioxide is etched, the impurity materials become apparent in the condensed fluid layer 5 present during the etching process. As the etching progresses, if the impurity materials are not themselves etched by the HF vapor, they will collect within the condensed fluid layer 5 if the apparent materials are not volatile. Depending on the chemical properties of the impurity material, the impurity material may remain in the condensed fluid layer 5 until the etching process is stopped and the condensed fluid layer 5 is evaporated, at which point the impurity material forms residues that are highly undesirable traits in the final MEMS or semiconductor microstructure. Summary of the Invention

[0011] It is therefore an object of embodiments of the present invention to provide a method for fabricating microstructures utilizing HF etching of a sacrificial silicon dioxide layer that exhibits reduced levels of residue layers or films when compared to techniques known in the art.

[0012] According to a first aspect of the present invention, there is provided a method of fabricating a microstructure, comprising the steps of: - utilizing hydrogen fluoride (HF) vapor to etch a sacrificial layer of silicon dioxide (SiO2); - removing the residue layer formed during HF vapor etching of said layer of silicon dioxide; A method is provided that includes:

[0013] The steam etching of the sacrificial layer of silicon dioxide (SiO2) and the removal of the residue layer may be performed sequentially or simultaneously in a common processing chamber, or the steam etching of the sacrificial layer of silicon dioxide (SiO2) and the removal of the residue layer may be performed sequentially in separate processing chambers.

[0014] Optionally, removing said residue layer comprises reacting said residue layer with a first additional gas.

[0015] Most preferably, removing the residue layer comprises removing a silicon-containing residue layer. Alternatively, removing the residue layer comprises removing an ammonium salt-containing residue layer. In yet an alternative embodiment, removing the residue layer comprises removing a carbon-containing residue layer.

[0016] Optionally, removing the residue layer comprises reacting the silicon with hydrogen gas to produce silane (SiH4).

[0017] Alternatively, removing the residue layer can include reacting the silicon with oxygen gas to produce silicon dioxide (SiO2). Removing the residue layer can then further include utilizing hydrogen fluoride (HF) vapor to etch the silicon dioxide (SiO2).

[0018] In a further alternative, removing the residue layer includes reacting the silicon with fluorine gas to produce silicon tetrafluoride (SiF4).

[0019] In a further alternative, removing the residue layer may include etching the silicon with xenon difluoride (XeF2) vapor.

[0020] Removing the residue layer can include reacting the carbon with oxygen gas to produce carbon dioxide (CO) and / or carbon monoxide (CO), or reacting the carbon with hydrogen gas to produce methane (CH), or reacting the carbon with fluorine gas to produce tetrafluoride (CF) and / or hexafluoroethane (CF).

[0021] In a further alternative, removing the residue layer can include heating the ammonium salt to a temperature greater than 160°C.

[0022] The method for fabricating a microstructure may further include utilizing a vacuum system to remove by-products formed when removing the residue layer.

[0023] Most preferably, the microstructure comprises a microelectromechanical system (MEMS), or alternatively, the microstructure comprises a semiconductor device.

[0024] According to a second aspect of the present invention, there is provided a method of fabricating a microstructure, comprising the steps of: - using hydrogen fluoride (HF) vapor to etch a sacrificial layer of silicon dioxide (SiO2); - removing the silicon-containing residue layer formed during HF vapor etching of said layer of silicon dioxide; A method is provided that includes:

[0025] An embodiment of the second aspect of the invention may include one or more features of the first aspect of the invention or an embodiment thereof, or vice versa.

[0026] According to a third aspect of the present invention, there is provided a method of fabricating a microstructure, comprising the steps of: - utilizing hydrogen fluoride (HF) vapor to etch a sacrificial layer of silicon dioxide (SiO2); - removing a residue layer containing ammonium salts formed during HF vapor etching of said layer of silicon dioxide; A method is provided that includes:

[0027] An embodiment of the third aspect of the invention may include one or more features of the first or second aspects of the invention or embodiments thereof, or vice versa.

[0028] According to a fourth aspect of the present invention, there is provided a method of fabricating a microstructure, comprising the steps of: - utilizing hydrogen fluoride (HF) vapor to etch a sacrificial layer of silicon dioxide (SiO2); - removing the carbon-containing residue layer formed during HF vapor etching of said layer of silicon dioxide; A method is provided that includes:

[0029] Embodiments of the fourth aspect of the invention may include one or more features of the first, second or third aspects of the invention or embodiments thereof, or vice versa.

[0030] Various embodiments of the invention will now be described, by way of example only, with reference to the drawings in which: [Brief explanation of the drawings]

[0031] [Figure 1] FIG. 1 is a schematic diagram of HF vapor etching of a MEMS that includes a silicon dioxide layer located between a substrate and a release layer. [Figure 2] FIG. 2 is a schematic diagram of a process chamber system suitable for performing HF vapor etching of the MEMS of FIG. [Figure 3] FIG. 3 is a schematic diagram of the MEMS of FIG. 1 following an HF vapor etching process. [Figure 4] FIG. 4 is a flow chart of a method for fabricating a MEMS in accordance with the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0032] Figure 2 shows a schematic representation of an etching apparatus 6 suitable for etching the MEMS 1 of Figure 1. It can be seen that the etcher 6 comprises an etching chamber 7 to which six input lines 8, 9, 10, 11, 12 and 13, and an output vacuum line 14 are attached.

[0033] Inside the etching chamber 7 is a temperature controlled pedestal 15 suitable for mounting the MEMS structure 1 to be etched inside the etching chamber 7. Fluids supplied from six input lines 8, 9, 10, 11, 12 and 13 enter the interior volume of the etching chamber 7 through a lid 17 and a fluid injection system 16 located inside the etching chamber 7.

[0034] The pedestal 15 on which the MEMS 1 is placed is controlled by a temperature controller to a pedestal temperature T p The etching chamber 7 may be set and maintained at a temperature of 5 to 25°C. This temperature may be higher or lower than room temperature, with the particular temperature being selected to optimize the etching process (typically 5 to 25°C). In addition, during the etching process, the walls of the etching chamber 7 are typically heated to around 20 to 70°C.

[0035] The pressure of the etchant gas inside the etching chamber, P c , is monitored by a chamber pressure controller 18. The pressure controller 18 also incorporates gas flow controllers that are utilized to provide a means of controlling the pressure inside the etching chamber 7 by controlling the operation of a vacuum pumping system 19 installed on the output vacuum line 14.

[0036] HF vapor 20 is controllably supplied to the etching chamber 7 by a first input line 8 through a regulator 21 and a first mass flow controller (MFC) 22 .

[0037] A controlled amount of water is supplied to the etching chamber 7 through a second input line 9. In particular, a liquid fluid controller (LFC) 23 and vaporizer 24 located within the second input line 9 are utilized to generate a controlled level of water vapor from a water reservoir 25. The flow of nitrogen from a nitrogen gas source 26 to the vaporizer 24 is controlled by a second MFC 22. A nitrogen carrier gas is utilized to carry the water vapor into the interior volume of the etching chamber 7 via a fluid injection system 16.

[0038] A third input line 10, a fourth input line 11 and a fifth input line 12 provide means for connecting additional gas sources 27, 28 and 29, such as hydrogen (H), oxygen (O) or fluorine (F), to the interior volume of the etching chamber 7. Control of the flow of these gases is again provided by mass flow controllers (MFCs) 22.

[0039] Xenon difluoride (XeF2) vapor 30 is controllably supplied to the etching chamber 7 by a sixth input line 13 through a second regulator 21 and a mass flow controller (MFC) 22.

[0040] A computer controller 31 is utilized to automate the regulation of various components and parameters of the etching chamber 7, such as the nitrogen carrier gas supply, the HF vapor supply, the chamber temperature and pressure, and the like.

[0041] In order to allow the described method of etching to proceed, it is necessary to obtain diagnostics that allow accurate monitoring of the condensed fluid layer 5. As explained above, the physical properties of the condensed fluid layer 5 directly affect the etch rate on the MEMS 1, and therefore monitoring the etch rate provides a direct diagnostic of the physical properties of the condensed fluid layer 5.

[0042] In practice, the etch rate may be monitored in a number of ways, for example, by monitoring the level of by-products produced, by monitoring the wafer etching directly, or by monitoring changes in chamber conditions.

[0043] FIG. 3 depicts a schematic representation of the MEMS 1 of FIG. 1 following an HF vapor etching process performed inside the etcher 6 of FIG. 2. As can be seen, once the HF process is complete, the silicon dioxide layer 2 is removed so that the release structures 4, typically formed from silicon or aluminum, are free to move relative to the substrate 3 and to do so as needed. Unfortunately, a residue layer, generally designated by the reference numeral 32, is present on the exposed surface of the MEMS 1. The residue layer 32 has the appearance of a random distribution of particle residue.

[0044] Of the chemical precursors utilized within the PECVD processes discussed above to deposit silicon dioxide layer 2, the two most commonly used silicon sources are silane (SiH4) or tetraethylorthosilicate (TEOS), formally named tetraethoxysilane, (Si(OC2H5)4).

[0045] When the silicon source is silane (SiH4), the oxygen source is typically nitrous oxide. In this case, applicants have found that nitrogen can be incorporated into the silicon dioxide layer, so that when etched with HF vapor, the resulting residue layer 32 contains ammonium salts.

[0046] When TEOS is utilized as the silicon source, applicants have found that subsequent etching of the MEMS 1 with HF vapor produces a residue layer 32 that contains carbon impurities.

[0047] Applicant has also found that the residue layer 32 produced during HF vapor etching of MEMS 1 often contains silicon impurities, regardless of the silicon source. The presence of silicon impurities within residue layer 32 may be the result of PECVD conditions that produce a silicon-rich silicon dioxide layer 2. In this case, when the HF vapor etches silicon dioxide layer 2, the HF vapor will not etch the silicon contaminants, and therefore the silicon contaminants remain as residue. To date, silicon-based residue layer 32 has been completely overlooked in the art because the presence of silicon within any post-etch analysis of MEMS 1 has always been attributed by those skilled in the art to the fact that silicon is located in other regions of the device, e.g., substrate 3.

[0048] Various methods for fabricating microstructures, for example semiconductor devices or MEMS 1, including removing the residue layer 32 according to the present invention will now be described with reference to FIG.

[0049] The process involves HF vapor etching of the sacrificial silicon dioxide layer 2, as described in detail above with respect to Figures 1-3. The process then involves removing the residue layer 32 from the microstructure by utilizing one or more of the following techniques:

[0050] When the residue layer 32 includes silicon, a third input line 10 may be utilized to connect a hydrogen gas source 27 to the etching chamber 7. The hydrogen gas may be ionized, for example, by a remote plasma system, before being supplied to the etching chamber 7. Alternatively, the hydrogen gas may be ionized within the etching chamber 7 itself. The silicon in the residue layer 32 then reacts with the hydrogen to produce silane (SiH4). Because silane (SiH4) is a volatile material, the silane may simply be pumped out of the etching chamber 7 by a vacuum system 19.

[0051] Alternatively, a fourth input line 11 may be utilized to connect an oxygen gas source 28 to the etching chamber 7. The oxygen gas may be ionized, for example, by a remote plasma system, before being supplied to the etching chamber 7. Alternatively, the oxygen gas may be ionized within the etching chamber 7 itself. The silicon in the residue layer 32 then reacts with the oxygen to produce silicon dioxide (SiO2). The HF vapor etching process described above may then be repeated to remove the residue layer 32 in a manner similar to that described with respect to the sacrificial silicon dioxide layer 2. The by-products of this second HF vapor etching process may again simply be pumped out of the etching chamber 7 by the vacuum system 19.

[0052] When the residue layer 32 includes silicon, a fifth input line 12 may be utilized to connect a fluorine gas source 29 to the etching chamber 7. The fluorine gas may be ionized, for example, by a remote plasma system, before being supplied to the etching chamber 7. Alternatively, the fluorine gas may be ionized within the etching chamber 7 itself. The silicon in the residue layer 32 then reacts with the fluorine to form silicon tetrafluoride (SiF4). Because silicon tetrafluoride (SiF4) is a volatile material, the silicon tetrafluoride may simply be pumped out of the etching chamber 7 by the vacuum system 19.

[0053] A fourth technique that may be utilized to remove residue layer 32 when the residue layer includes silicon is to perform a xenon difluoride (XeF2) vapor etching process inside etching chamber 7. Here, a sixth input line 13 may be utilized to connect xenon difluoride (XeF2) vapor 30 to etching chamber 7. Applicant owns European Patent Nos. EP 1,766,665 B1 and EP 2,480,493 B1, both of which disclose techniques for etching silicon with xenon difluoride (XeF2) vapor that may be implemented by adapting etching apparatus 6 of FIG. 2.

[0054] It will be recognized that the above-described methods for removing the residue layer 32 when the residue layer includes silicon can be problematic because there are typically other exposed areas of silicon material that form the functional MEMS or semiconductor device. Therefore, any of the above-described techniques for removing the residue layer 32 would be expected to also remove the silicon in the peripheral region. However, applicants have discovered that when the residue layer includes silicon, the rate of etching of the residue layer 32 is generally much greater than for any silicon found in the peripheral region of the device. As a result, the above-described techniques may be performed before any significant etching of the peripheral silicon regions occurs. Applicants believe that the reason for these significant differences in silicon etching rates is due to the fact that the silicon in the exposed residue layer 32 is not a well-structured solid but instead comprises an amorphous, highly porous structure.

[0055] When residue layer 32 contains carbon, a fourth input line 11 may be utilized to connect an oxygen gas source 28 to etching chamber 7. The oxygen gas may be re-ionized, for example, by a remote plasma system, before being supplied to etching chamber 7. The carbon in residue layer 32 then reacts with the oxygen to produce carbon dioxide (CO) and / or carbon monoxide (CO). Because carbon dioxide (CO) and carbon monoxide (CO) are both volatile substances, they may both simply be pumped out of etching chamber 7 by vacuum system 19.

[0056] Alternatively, when residue layer 32 contains carbon, third input line 10 may be utilized to provide a hydrogen gas supply to etching chamber 7. The hydrogen gas may be ionized, for example, by a remote plasma system, before being supplied to etching chamber 7. Alternatively, the hydrogen gas may be ionized within etching chamber 7 itself. The carbon in residue layer 32 then reacts with the hydrogen to produce methane (CH4). Because methane (CH4) is a volatile substance, the methane may simply be pumped out of etching chamber 7 by vacuum system 19.

[0057] Alternatively, when residue layer 32 contains carbon, fifth input line 12 may be utilized to provide a fluorine gas supply to etching chamber 7. The fluorine gas may be ionized, for example, by a remote plasma system, before being supplied to etching chamber 7. Alternatively, the fluorine gas may be ionized within etching chamber 7 itself. The carbon in residue layer 32 then reacts with the fluorine to form tetrafluoride (CF4) and / or hexafluoroethane (CF2F6). Because tetrafluoride (CF4) and hexafluoroethane (CF2F6) are both volatile materials, they may simply be pumped out of etching chamber 7 by vacuum system 19.

[0058] When residue layer 32 contains ammonium salts, an alternative technique must be utilized. Since ammonium salts are known to decompose at temperatures >160° C., applicants have realized that by utilizing a heating element inside etching chamber 7 to heat MEMS 1, the ammonium salts can then be vaporized and subsequently pumped out of etching chamber 7 by vacuum system 19.

[0059] The above-described method for forming MEMS has significant advantages over those systems known in the art that utilize an HF vapor etching step to remove a silicon dioxide (SiO2) sacrificial layer in that it provides a means for reducing or removing the residue layer formed as a by-product of this process. Most importantly, the above-described method provides a heretofore unrecognized means for removing silicon-based residue layers.

[0060] Although the techniques described above have been specifically described with reference to MEMS, the techniques described above apply to alternative microstructures (e.g., semiconductor devices) in which the fabrication of the microstructures utilizes an HF vapor etching step to remove a silicon dioxide (SiO2) sacrificial layer.

[0061] In the above discussion, removal of residue layers containing silicon, carbon, and ammonium salts has been described separately above. However, those skilled in the art will recognize that two or more of these impurities may be present within a single silicon dioxide (SiO2) sacrificial layer. The techniques described above may be applied simultaneously or sequentially to a microstructure to remove such complex residue layers formed on the microstructure when HF vapor is utilized to etch the silicon dioxide (SiO2) sacrificial layer. Furthermore, one or more of the techniques described above may be applied simultaneously with the steam etching of the microstructure.

[0062] The above-described HF vapor etching and residue layer removal have been further described as occurring within a common process chamber. Those skilled in the art will further recognize that an alternative embodiment would move the microstructure from the first process chamber in which the HF vapor etching is performed to a second processing chamber to perform one or more of the above-described techniques for residue layer removal.

[0063] A method for fabricating a microstructure is provided that includes utilizing hydrogen fluoride (HF) vapor to etch a sacrificial layer of silicon dioxide (SiO2), and then removing a residue layer formed during the HF vapor etching of the silicon dioxide. The residue layer may include silicon, ammonium salts, or carbon, and various techniques are disclosed for removing such layers. These techniques may be applied to the microstructure simultaneously or sequentially. The described method therefore fabricates a microstructure that exhibits reduced levels of residue when compared to techniques known in the art.

[0064] The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The described embodiments were chosen and described in order to best explain the principles of the invention and its practical application, and thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as may be suited to the particular uses contemplated. Therefore, further modifications or improvements may be incorporated without departing from the scope of the invention as defined by the appended claims.

Claims

1. 1. A method for manufacturing a microstructure, comprising: Silicon dioxide (SiO ２ Utilizing hydrogen fluoride (HF) vapor to etch the sacrificial layer of Once the HF vapor etching is complete, the subsequent step of removing the solid residue layer formed during the HF vapor etching of the layer of silicon dioxide is Including, The solid residue layer that is removed comprises carbon, and removing the solid residue layer comprises: reacting the carbon with hydrogen gas to produce methane (CH4); or reacting the carbon with fluorine gas to produce tetrafluoride (CF4) and / or hexafluoroethane (C2F6); A method comprising:

2. Silicon dioxide (SiO ２ 2. The method of manufacturing a microstructure according to claim 1, wherein said steam etching of said sacrificial layer and said removal of said solid residue layer are performed sequentially in a common processing chamber.

3. Silicon dioxide (SiO ２ 2. The method of claim 1, wherein the steam etching of the sacrificial layer and the removal of the solid residue layer are performed sequentially in separate processing chambers.

4. 4. The method for manufacturing a microstructure according to claim 1, further comprising utilizing a vacuum system to remove by-products formed when removing the solid residue layer.

5. The method of manufacturing a microstructure according to any one of claims 1 to 4, wherein the microstructure comprises a micro-electro-mechanical system (MEMS).

6. The method of manufacturing a microstructure according to claim 1 , wherein the microstructure comprises a semiconductor device.

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