Methods for manufacturing microstructures

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 the manufacture of microstructures. Typically, microstructures are in the form of micro-electro-mechanical systems (MEMS) that require the removal of one material relative to a substrate or other deposited materials. In particular, the invention relates to an improved method for manufacturing microstructures that utilizes the step of etching silicon dioxide using hydrogen fluoride (HF) vapor.

Background Art

[0002] Isotropic etching of silicon oxide is widely used in semiconductor and MEMS processing, mainly in wafer cleaning and lift-off processes. In the manufacture of microstructures, for example micro-electro-mechanical structures (MEMS) of the type shown in Figure 1 and designated generally by reference numeral 1, an etching process is used to remove sacrificial (i.e., unwanted) regions or layers of material 2. MEMS 1 find applications in inertial measurement, pressure sensing, thermal measurement, microfluidics, optics, and radio frequency communications, and the range of possible applications for these structures continues to grow. A sacrificial layer 2 is first deposited on a substrate 3 during the construction of the MEMS, and is subsequently removed using an etching step, which allows the released structure 4 to operate as designed, for example as a micromirror, accelerometer or microphone. To fabricate reliable structures, the release etching step requires removing the sacrificial layer 2 without etching the surrounding materials. Ideally, the etching of the sacrificial layer 2 should have no effect at all 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. GB2,487,716B. HF vapor etching is a plasma-free chemical etching, with the reaction formula:

[0004] Described by TIFF0007923030000001.tif24170.

[0005] It was found that water (H2O) ionizes HF vapor as described by equation (1), and the ionized HF vapor

[0006] TIFF0007923030000002.tif8170 then etches silicon dioxide (SiO2) with water (H2O) acting as a catalyst. From equation (2), it is also clear that water (H2O) is generated from the etching reaction itself.

[0007] Similar to its use in the fabrication of MEMS1, silicon dioxide layers are also present within semiconductor devices. For this reason, hydrogen fluoride (HF) vapor etching techniques are also known to be used to create air gap structures within multilevel metal structures used in standard semiconductor devices, see, for example, U.S. Patent No. 7,211,496.

[0008] It is universally accepted that, in order to carry out HF vapor etching at usable etching rates greater than, so to speak, 30 nm / min, it is necessary for an aggregated fluid layer 5 to 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 Vaccum Science and Technology A, 10(4) July / Aug 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 aggregated fluid layer 5. European Patent No. EP2046677B1 discloses how to control the formation and composition of the aggregated fluid layer 5 to control the HF vapor etching of silicon dioxide. Precise etching control is achieved by performing HF etching in a vacuum chamber and controlling the chamber pressure, temperature, and gas flow into the chamber. Other parameters that affect HF vapor etching are the composition of the silicon dioxide layer to be 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 a substrate. In these processes, chemical precursors, one a silicon source and the other an oxygen source, react to deposit a silicon dioxide layer onto the substrate. The most common of these processes is plasma CVD (PECVD), because this process allows deposition to be performed at low temperatures, <450°C.

[0010] When depositing oxide layers (e.g., silicon dioxide layer 2 via the PECVD process), impurities may be incorporated into the layer, either intentionally or accidentally. When this is done intentionally, it is known as doping of silicon dioxide layer 2. The use of doping layers (e.g., phosphate silicate glass (PSG) layers and borosilicate glass (BPSG) layers) is also common in semiconductor manufacturing processes because the presence of dopant materials provides superior step coverage, thermal properties, electrical improvements, and barrier performance. However, since the HF vapor etching process is a chemical etching process, the above process etches silicon dioxide but often does not etch impurity substances. Therefore, as doped silicon dioxide is etched, impurity substances become apparent in the condensed fluid layer 5 present during the etching process. As etching progresses, if the impurity substances are not etched themselves by the HF vapor, they will accumulate in the condensed fluid layer 5, provided that what is revealed is not a volatile substance. Depending on their chemical properties and the nature of the impurities, impurities may remain in the aggregated fluid layer 5 until the etching process stops and the aggregated fluid layer 5 evaporates. In this respect, the impurities form residues, which are highly undesirable characteristics in the final MEMS or semiconductor microstructure. [Overview of the project]

[0011] Therefore, an object of embodiments of the present invention is to provide a method for fabricating microstructures that utilizes the step of HF etching a sacrificial layer silicon dioxide, which exhibits a reduction in the level of residue layers or films compared to techniques known in the art.

[0012] According to a first aspect of the present invention, a method for fabricating a microstructure, - Using hydrogen fluoride (HF) vapor to etch the silicon dioxide (SiO2) sacrificial layer, - To remove the residue layer formed when etching the silicon dioxide layer with HF vapor. A method including this is provided.

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

[0014] Optionally, removing the residue layer includes reacting the residue layer with a first additional gas.

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

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

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

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

[0019] In a further alternative, removing said residual layer may comprise etching said silicon using xenon difluoride (XeF2) vapor.

[0020] Removing said residual layer may comprise reacting said carbon with oxygen gas to produce carbon dioxide (CO2) and / or carbon monoxide (CO). Alternatively, removing said residual layer comprises reacting said carbon with hydrogen gas to produce methane (CH4). Alternatively, removing said residual layer comprises reacting said carbon with fluorine gas to produce carbon tetrafluoride (CF4) and / or ethane hexafluoride (C2F6).

[0021] In a further alternative, removing said residual layer may comprise heating said ammonium salt to a temperature higher than 160°C.

[0022] The above method of fabricating a microstructure may further comprise using a vacuum pumping system to remove by-products formed when removing said residual layer.

[0023] Most preferably, said microstructure comprises a micro-electro-mechanical system (MEMS). Alternatively, said microstructure comprises a semiconductor device.

[0024] According to a second aspect of the present invention, there is provided a method of fabricating a microstructure, the method comprising: - using hydrogen fluoride (HF) vapor to etch a sacrificial layer of silicon dioxide (SiO2); and - removing a silicon-containing residual layer formed when performing HF vapor etching on said layer of silicon dioxide A method comprising the above steps is provided.

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

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

[0027] Embodiments of the third aspect of the invention may include one or more features of the first aspect, the second aspect of the invention or any embodiments thereof, and vice versa.

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

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

[0030] Various embodiments of the invention will now be described herein, by way of example only, with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] [Figure 1] FIG. 1 is a schematic diagram of HF vapor etching of a MEMS including 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 the HF vapor etching of the MEMS of FIG. 1. [Figure 3] FIG. 3 is a schematic diagram of the MEMS of FIG. 1 subsequent to an HF vapor etching process. [Figure 4] Figure 4 is a flowchart of the method for fabricating a MEMS according to the present invention. [Modes for carrying out the invention]

[0032] Figure 2 shows a schematic representation of an etching apparatus 6 suitable for etching the MEMS 1 shown in Figure 1. It can be seen that the etching apparatus 6 comprises an etching chamber 7 fitted with six input lines 8, 9, 10, 11, 12, and 13, as well as an output vacuum line 14.

[0033] Inside the etching chamber 7 is a temperature-controlled pedestal 15 suitable for holding the MEMS structure 1 to be etched inside the etching chamber 7. Fluid supplied from six input lines 8, 9, 10, 11, 12, and 13 enters the internal volume of the etching chamber 7 via a lid 17 and a fluid injection system 16 installed inside the etching chamber 7.

[0034] The pedestal 15 on which the MEMS1 is installed is controlled by a temperature controller to maintain a pedestal temperature T p This temperature may be set and maintained above room temperature. This temperature may be higher or lower than room temperature, and a specific temperature may be selected to optimize the etching process (typically 5–25°C). In addition, during the etching process, the walls of the etching chamber 7 are heated to approximately 20–70°C.

[0035] Pressure of etching gas inside the etching chamber, P c However, this is monitored by the chamber pressure controller 18. The pressure controller 18 also incorporates a gas flow controller, which is used to provide a means of controlling the pressure inside the etching chamber 7 by controlling the operation of a vacuum system 19 installed on the output vacuum line 14.

[0036] HF steam 20 is supplied controllably to the etching chamber 7 via 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 via a second input line 9. In particular, a liquid fluid controller (LFC) 23 and a vaporizer 24 installed within the second input line 9 are used to generate a controlled level of water vapor from the water storage unit 25. The flow of nitrogen from the nitrogen gas source 26 to the vaporizer 24 is controlled by a second MFC 22. The nitrogen carrier gas is used to transport water vapor into the volume of the etching chamber 7 via the fluid injection system 16.

[0038] The third input line 10, the fourth input line 11, and the fifth input line 12 provide means for connecting additional gas sources 27, 28, and 29, such as hydrogen (H2), oxygen (O2), or fluorine (F2), to the internal volume of the etching chamber 7. Control of the flow of these gases is again provided by the mass flow controller (MFC) 22.

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

[0040] The computer controller 31 is used to automate the adjustment of various components and parameters of the etching chamber 7, such as the supply of nitrogen carrier gas, the supply of HF vapor, the chamber temperature and pressure, etc.

[0041] In order for the etching method described above to proceed, it is necessary to obtain a diagnosis that allows for accurate monitoring of the aggregated fluid layer 5. As described above, the physical properties of the aggregated fluid layer 5 directly affect the etching rate on the MEMS1, and therefore a direct diagnosis of the physical properties of the aggregated fluid layer 5 can be obtained by monitoring the etching rate.

[0042] In practice, the etching rate can be monitored in numerous ways, for example, by monitoring the level of by-products generated, by directly monitoring wafer etching, or by monitoring changes in chamber conditions.

[0043] Figure 3 shows a schematic representation of the MEMS1 of Figure 1 following the HF vapor etching process performed inside the etching apparatus 6 of Figure 2. As can be seen, once the HF process is complete, the silicon dioxide layer 2 is removed so that the exfoliation structure 4, typically formed from silicon or aluminum, can move freely against the substrate 3 and, if necessary, act in that manner. Unfortunately, a residue layer, commonly represented by reference no. 32, is present on the exposed surface of the MEMS1. The residue layer 32 has the appearance of a random distribution of particulate residue.

[0044] Of the chemical precursors used in the PECVD process discussed so far to deposit silicon dioxide layer 2, the two most commonly used silicon sources are silane (SiH4) or tetraethyl orthosilicate (TEOS), formally named tetraethoxysilane, (Si(OC2H5)4).

[0045] When the silicon source is silane (SiH4), the oxygen source is usually nitrous oxide. In this case, the applicant found that nitrogen can be incorporated into the silicon dioxide layer, and therefore, when etched using HF vapor, the resulting residue layer 32 contains ammonium salts.

[0046] When TEOS is used as a silicon source, the applicant found that subsequent etching of MEMS1 using HF vapor generates a residue layer 32 containing carbon impurities.

[0047] The applicant also found that the residue layer 32 formed during HF vapor etching of MEMS1 often contains silicon impurities regardless of the silicon source. The presence of silicon impurities within the residue layer 32 may be a result of PECVD conditions that produce a silicon-rich silicon dioxide layer 2. In this case, when the HF vapor etches the silicon dioxide layer 2, the HF vapor will not etch the silicon contaminants, and therefore the silicon contaminants remain as residue. Until now, the presence of silicon within the scope of any post-etching analysis of MEMS1 has always been considered by those skilled in the art to be due to the fact that silicon is located in other areas of the device, e.g., within the substrate 3, and therefore the silicon-based residue layer 32 has been completely overlooked in the art.

[0048] Various methods for fabricating a microstructure, such as a semiconductor device or MEMS1, which include removing the residue layer 32 according to the present invention, will be described here with reference to Figure 4.

[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 contains silicon, a third input line 10 may be used to connect a hydrogen gas source 27 to the etching chamber 7. Before being supplied to the etching chamber 7, the hydrogen gas may be ionized, for example, by a remote plasma system. Alternatively, the hydrogen gas may be ionized inside the etching chamber 7 itself. The silicon in the residue layer 32 then reacts with the hydrogen to produce silane (SiH4). Since silane (SiH4) is a volatile substance, it may simply be exhausted out of the etching chamber 7 by a vacuum system 19.

[0051] Alternatively, a fourth input line 11 may be used 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 inside 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 for the sacrificial silicon dioxide layer 2. The byproducts of this second HF vapor etching process may be simply exhausted again to the outside of the etching chamber 7 by a vacuum system 19.

[0052] When the residue layer 32 contains silicon, a fifth input line 12 may be used 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 inside the etching chamber 7 itself. The silicon in the residue layer 32 then reacts with fluorine to produce silicon tetrafluoride (SiF4). Since silicon tetrafluoride (SiF4) is a volatile substance, it may simply be exhausted out of the etching chamber 7 by a vacuum system 19.

[0053] A fourth technique that may be used to remove the residue layer 32 when the residue layer contains silicon is to perform a xenon difluoride (XeF2) vapor etching process inside the etching chamber 7. Here, a sixth input line 13 may be used to connect xenon difluoride (XeF2) vapor 30 to the etching chamber 7. The applicant owns European Patent Nos. EP1,766,665B1 and EP2,480,493B1, both of which disclose a technique for etching silicon using xenon difluoride (XeF2) vapor, which can be performed by adapting the etching apparatus 6 of Figure 2.

[0054] It will be recognized that the methods described above for removing the residue layer 32 when the residue layer contains silicon may involve problems, as there are typically other exposed areas of silicon material forming the working MEMS1 or semiconductor device. Therefore, any of the techniques described above for removing the residue layer 32 would also be expected to remove silicon in the aforementioned peripheral areas. However, the applicant has found that the etching rate of the residue layer 32 is generally much greater when the residue layer contains silicon than for any silicon found in the peripheral areas of the device. As a result, the above techniques may be performed before any significant etching of the peripheral silicon areas occurs. The applicant believes the reason for these significant differences in silicon etching rates is the fact that the silicon in the exposed residue layer 32 is not a well-structured solid, but rather contains an amorphous, highly porous structure.

[0055] When the residue layer 32 contains carbon, a fourth input line 11 may be used to connect an oxygen gas source 28 to the etching chamber 7. The oxygen gas may be re-ionized, for example, by a remote plasma system, before being supplied to the etching chamber 7. The carbon in the residue layer 32 then reacts with the oxygen to produce carbon dioxide (CO2) and / or carbon monoxide (CO). Since both carbon dioxide (CO2) and carbon monoxide (CO) are volatile substances, they may be simply exhausted out of the etching chamber 7 by a vacuum system 19.

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

[0057] Alternatively, when the residue layer 32 contains carbon, a fifth input line 12 may be used to supply fluorine gas 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 inside the etching chamber 7 itself. The carbon in the residue layer 32 then reacts with fluorine to produce tetrafluoride (CF4) and / or hexafluorethane (C2F6). Since both tetrafluoride (CF4) and hexafluorethane (C2F6) are volatile substances, they may simply be exhausted out of the etching chamber 7 by the vacuum system 19.

[0058] When the residue layer 32 contains ammonium salts, alternative techniques must be used. Since ammonium salts are known to decompose at temperatures >160°C, the applicant has realized that by using a heating element inside the etching chamber 7 to heat the MEMS 1, the ammonium salts can then be vaporized and subsequently exhausted out of the etching chamber 7 by a vacuum system 19.

[0059] The method described above for forming MEMS has significant advantages over those systems known in techniques 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 byproduct of this process. Most importantly, the method described above provides a means for removing silicon-based residue layers that were previously unrecognized.

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

[0061] In the above discussion, the removal of residue layers containing silicon, carbon, and ammonium salts has been described separately above. However, it will be recognized by those skilled in the art 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 microstructures to remove such complex residue layers formed on the microstructure when HF vapor is used to etch the silicon dioxide (SiO2) sacrificial layer. Furthermore, one or more of the techniques described above may be applied simultaneously with the vapor etching of the microstructure.

[0062] The HF vapor etching and residue layer removal described above have been further described as being performed inside a common process chamber. It will be further recognized by those skilled in the art that in an alternative embodiment, the microstructure will be moved from a first process chamber in which HF vapor etching is performed inside to a second processing chamber in order to perform one or more of the techniques described above for the removal of the residue layer.

[0063] A method is provided for fabricating a microstructure, which involves using hydrogen fluoride (HF) vapor to etch a sacrificial layer of silicon dioxide (SiO2), and then removing the residue layer formed during the HF vapor etching of the silicon dioxide. The residue layer may contain 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 method described thus fabricates a microstructure that exhibits a reduction in the level of residue compared to these techniques known in the art.

[0064] The foregoing description of the invention is presented for illustrative and explanatory purposes and is neither exhaustive nor does it limit the invention to the exact form disclosed. The embodiments described have been selected and described in order to best illustrate the principles and practical uses of the invention, thereby enabling those skilled in the art to best utilize the invention with various modifications suitable for various embodiments and specific conceivable uses. Therefore, further modifications or improvements may be incorporated without deviating from the scope of the invention as defined by the claims set forth herein.

Claims

1. A method for manufacturing microstructures, Silicon dioxide (SiO ２ Using hydrogen fluoride (HF) vapor to etch the sacrificial layer, Once the HF vapor etching is complete, the next step is to remove the solid residue layer formed during the HF vapor etching of the silicon dioxide layer. Includes, A method wherein the solid residue layer to be removed contains an ammonium salt produced by nitrogen incorporated into the sacrificial layer of silicon dioxide (SiO₂) reacting with HF vapor.

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

3. Silicon dioxide (SiO ２ A method for manufacturing a microstructure according to claim 1, wherein the vapor etching of the sacrificial layer and the removal of the solid residue layer are carried out sequentially in separate processing chambers.

4. A method for producing a microstructure according to any one of claims 1 to 3, wherein removing the solid residue layer comprises heating the ammonium salt to a temperature higher than 160°C.

5. A method for producing a microstructure according to any one of claims 1 to 4, further comprising using a vacuum system to remove by-products formed when removing the solid residue layer.

6. A method for manufacturing the microstructure according to any one of claims 1 to 5, wherein the microstructure comprises a micro-electromechanical system (MEMS).

7. A method for manufacturing the microstructure according to any one of claims 1 to 6, wherein the microstructure includes a semiconductor device.

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