Microstructure manufacturing method

Abstract

A method for fabricating a microstructure having a three-dimensional structure is described. The method includes performing an isotropic steam etch of a sacrificial material. Etch parameters for the isotropic steam etch are varied as a function of time depending on (or based on) prior knowledge of the microstructure. This method has numerous advantages, including the ability to more efficiently form the microstructure while minimizing the risk of device failure.

Description

[Technical Field]

[0001] The present invention relates to methods for fabricating microstructures, typically in the form of semiconductor devices or microelectromechanical systems (MEMS) that require the removal of material from a substrate or other deposited material. In particular, the present invention relates to improvements in methods for fabricating microstructures that use an etching step. [Background technology]

[0002] Semiconductor Manufacturing Semiconductor manufacturing is a highly complex series of steps that are used to create semiconductor devices, however, the most basic methods are the same as those used to create any microstructured device.

[0003] Fabrication of a semiconductor device, such as that of the type depicted in FIG. 1 and generally designated by the numeral 1, typically involves first depositing a film 2 on a substrate 3. A photoresist layer 4 is then deposited on the film 2. The photoresist layer 4 is patterned using a photolithographic exposure, followed by a development and rinsing step. The patterned photoresist layer 4 acts as a mask. The exposed underlying film 2 is then removed using an etching process. This is then repeated multiple times to construct the semiconductor device 1.

[0004] The etching process employed in the fabrication of semiconductor devices is a low-pressure plasma process called reactive ion etching (RIE), which is widely used in semiconductor manufacturing. RIE has both chemical and physical components. The chemical component of etching is the rearrangement of molecular structure by breaking existing molecular bonds and forming new ones. The physical component of etching is the highly directed bombardment of ions toward the surface of the semiconductor wafer being etched.

[0005] 1a and 1b show the semiconductor wafer 1 before the RIE etching process, and FIGS. 1c and 1d show the semiconductor device 1 after the RIE etching process. A key feature of this method is that the RIE etching process is anisotropic. That is, the etching is highly directional in the z-direction, so that the pattern of the photoresist layer 4 in the xy plane is transferred substantially intact to the underlying film 2. Although the RIE etching process etches in three dimensions, the material removed is defined only by progression in the z-direction. Therefore, the primary variable used to describe the etching process is the etch rate, measured in one-dimensional units, μm / min.

[0006] The etching process depends on various parameters, such as gas flow rate, pressure, temperature, and plasma power. Experiments are performed to optimize the parameters, i.e., the etching setup, to provide an appropriate etching rate. The film 2 includes an exposed surface area 5 that is not covered by the photoresist layer 4 and is exposed to the etchant. Because the exposed surface area 5, also known as the etch front, remains constant throughout the anisotropic RIE etching process described above, the optimized parameters can be applied for the duration of the RIE etching process. Thus, in semiconductor manufacturing, the etching process parameters typically do not change over time, and therefore the etching process can be considered a one-stage process.

[0007] However, there are some exceptions, for example, when the etching parameters are changed to remove an initial surface layer or to fine-tune an existing etching process.

[0008] When etching polysilicon, there is an initial native oxide layer that must be removed from the surface of the exposed polysilicon layer. Because RIE chemistry is optimized for etching silicon, it is not optimized to etch the native oxide that forms on the exposed polysilicon surface. To quickly remove the native oxide, a breakthrough step is added before the main polysilicon etch. In this breakthrough step, the etch is biased to be more physically inclusive, and ion bombardment sputters the native oxide from the surface. This two-step etch involves a short breakthrough step followed by the main RIE etch, with all process parameters fixed.

[0009] Deep reactive ion etching (DRIE) uses the Bosch process, as described in U.S. Patent No. 5,501,893A, in which etching steps and polymer deposition steps are alternately repeated. This process facilitates the formation of deep structures, such as trenches, in the film 2 with vertical edges, with horizontal surfaces corresponding to the bottom of the trench and vertical surfaces corresponding to the sides of the trench. During the polymer deposition step, a polymer layer is applied to the exposed vertical and horizontal surfaces. The polymer layer protects the surface from the chemical elements of the DRIE. However, during the DRIE process, physical elements penetrate the polymer layer on the horizontal surfaces but not on the vertical surfaces. Therefore, the polymer deposition step effectively enhances the unidirectional, or anisotropic, nature of the DRIE process.

[0010] This DRIE process is a repetitive two-step process, but the etching parameters typically do not change during the etching step. Note that as the DRIE process progresses and the structure deepens, the trench depth affects the etching step. As described in European Patent EP 0 822 584 B1, the initial etching parameters are fine-tuned to stabilize the etch rate as the etching step progresses. However, because the profile of the structure does not change, such an etching process is still considered one-dimensional.

[0011] MEMS manufacturing The fabrication of the MEMS device depicted in Figure 2 and generally designated by the numeral 6 is similar to the fabrication of semiconductor device 1, and these fabrication methods share a commonality, primarily in that they all use the same processing techniques. However, one process that is unique to the fabrication of the current MEMS device 6 is the release etch process, also known as the sacrificial etch process.

[0012] In fabricating a MEMS device 6, a sacrificial layer 7 is first deposited on the substrate 3 and then removed in an etching process, thereby releasing a structure 8 that operates as designed, for example as a micromirror, accelerometer, or microphone. In some MEMS devices 6, this etching process creates a cavity that provides, for example, thermal isolation from the underlying substrate 3.

[0013] The structure of the sacrificial layer 7 to be etched and the accessibility of the etchant to interact with the sacrificial layer 7 are highly dependent on the MEMS device 6 being fabricated. There is little commonality between the structures of different MEMS devices 6. The ideal etching process in this case would be an isotropic etch, which etches evenly in all directions. Furthermore, it is desirable for the etchant to be highly selective, not reactive with other materials in the MEMS device 6.

[0014] The release etching process was initially performed using wet etching, where the sample is immersed in a bath of chemical etchant. This method clearly limits control over the etching process, as it is determined simply by the temperature and concentration of the etchant. Another problem that arises with wet etching is the risk of stiction: as the liquid is removed, the surfaces of the release structure 8 can be attracted to each other by capillary action, and once the surfaces come into contact, they stick to each other due to very strong attractive forces.

[0015] A greater degree of process control is possible with the use of vapor-phase etching systems. The first systems using this approach were very basic and employed a pulsed approach, in which gas is flowed into a vacuum chamber, the chamber pressure is raised to a preset target value, and the chamber is held at this pressure with the gas flowing or completely sealed. This procedure is repeated multiple times to complete the etch. Again, there is little control over the etch process, and the etch process parameters are typically the same for all the various MEMS device structures.

[0016] One of the most common materials used as the sacrificial layer 7 is silicon dioxide, which is etched using hydrogen fluoride (HF) vapor (see, for example, British Patent GB 2,487,716B). HF vapor etching is a plasma-free chemical etch that etches silicon dioxide isotropically and is represented by the following reaction: TIFF2026504909000002.tif26170

[0017] Water (H2O) ionizes the HF vapor, as shown in equation (1), and then the ionized HF vapor (HF2 - ) etches silicon dioxide (SiO2) with water (H2O) acting as a catalyst. From equation (2), it is clear that water (H2O) is also produced from the etching reaction itself.

[0018] It is widely accepted that HF ​​vapor etching requires the presence of a condensed fluid layer 9 on the surface to be etched in order to proceed at a usable etch rate, e.g., greater than 30 nm / min (see, e.g., the article by Helms et al., entitled "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 phase etching process described above, water (H2O) has the lowest vapor pressure and thus forms the basis of the condensed fluid layer 9.

[0019] As can be seen from equation (2), HO is an etching by-product, but it also affects the etching rate by contributing to the condensed layer 9 that forms. When controlling the etching process, the amount of etching and the amount of HO that is produced must be taken into consideration.

[0020] European Patent EP 2046677B1 discloses how controlling the formation and composition of the condensed fluid layer 9 is crucial for managing HF vapor etching of silicon dioxide. Precise etching control is achieved by performing the HF etch in a vacuum chamber and controlling the chamber pressure, temperature, and gas flow rate into the chamber. Another parameter that influences HF vapor etching is the composition of the silicon dioxide layer being etched and its deposition method. For example, this depends on whether the silicon dioxide of the sacrificial layer 7 is produced by thermal oxidation or plasma-enhanced chemical vapor deposition (PECVD). The denser the silicon dioxide, the slower the etching process will be for the same etching parameters. Summary of the Invention

[0021] It is therefore an object of one embodiment of the present invention to provide a method for manufacturing microstructures more efficiently compared to techniques known in the art.

[0022] According to a first aspect of the present invention, there is provided a method of manufacturing a microstructure, comprising: The method includes performing an isotropic steam etch of a sacrificial material; The etching parameters of the isotropic steam etch are varied as a function of time in response to (or based on) prior knowledge of the topography.

[0023] Preferably, the prior knowledge of the topography includes prior knowledge of variations in area and / or position and / or direction of the etch front of the isotropic steam etch.

[0024] Alternatively or additionally, the prior knowledge of the microstructure includes prior knowledge of the compositional changes of the sacrificial material being etched.

[0025] Preferably, the isotropic steam etch includes a first stage of etching the sacrificial material at a first volumetric etch rate for a first period of time with a first set of etching parameters.

[0026] Preferably, the isotropic steam etch further comprises a second stage in which the sacrificial material is further etched at a second volumetric etch rate for a second period of time with a second set of etching parameters.

[0027] Preferably, the first and second volumetric etch rates depend on different features of the microstructure.

[0028] Preferably, the first set of etching parameters is different from the second set of etching parameters.

[0029] Optionally, the etching parameters that are varied between the first and second stages include pressure, gas flow rate, gas flow ratio, gas species and / or temperature.

[0030] Optionally, the first period of time is different from the second period of time.

[0031] Preferably, the first volumetric etch rate and the second volumetric etch rate are maintained below a threshold value.

[0032] Optionally, the first and second volumetric etch rates are maintained below a threshold value by initially fixing the first and second etch parameters such that the maximum possible etch rate is below the threshold value.

[0033] Alternatively, the first and second volumetric etch rates are maintained below the threshold value by modifying the first and second etch parameters during the isotropic steam etch such that the etch rates are below the threshold value.

[0034] Optionally, the first etching parameters can be dynamically changed during the first etching stage. Similarly, the second etching parameters can be dynamically changed during the second etching stage. Such dynamic changes of etching parameters can be thought of as fine tuning of etching parameters to compensate for changes in the etch front.

[0035] Preferably, the first stage of the isotropic steam etching corresponds to removing the sacrificial material not covered by the mask layer, the sacrificial material being removed mainly in the z-direction.

[0036] Preferably, the second stage of the isotropic steam etch corresponds to removing the sacrificial material below the mask layer, the sacrificial material being removed primarily in the xy direction, and the second stage of the isotropic steam etch can be considered an undercut or release stage.

[0037] Preferably, the isotropic steam etch further comprises a third stage in which the sacrificial material is further etched at a third volumetric etch rate for a third period of time with a third set of etching parameters.

[0038] Preferably, a third stage of the isotropic steam etch corresponds to further removal of the sacrificial material below the mask layer, the sacrificial material being removed primarily in the x and y directions, and the third stage of the isotropic steam etch can be considered a deeper undercut stage.

[0039] Alternatively, the second stage of the isotropic steam etch may correspond to removing a sacrificial material that has a different density or ease of etching compared to the sacrificial material removed in the first stage of the isotropic steam etch.

[0040] Alternatively, the first stage of the isotropic steam etch may correspond to removing the sacrificial material under reaction-limited etching conditions, where the amount of sacrificial material etched is relatively small.

[0041] Preferably, the first etching parameters include a relatively high etchant partial pressure by operating at a lower carrier gas flow rate and a higher chamber pressure.

[0042] Preferably, the second stage of isotropic steam etching may correspond to removing the sacrificial material under transport-limited etching conditions, where the amount of sacrificial material etched is relatively large.

[0043] Preferably, the second etching parameters include a relatively high etchant flow rate.

[0044] Preferably, the first and second stages of the isotropic steam etch involve substantially the same or similar volumetric etch rates. Advantageously, the two-stage process maintains a practical volumetric etch rate as the etch conditions transition from reaction-limited to transport-limited.

[0045] Preferably, the isotropic steam etch comprises multiple stages, each stage corresponding to an incremental change in etching parameters and / or duration.

[0046] Preferably, the etchant is HF vapor and the sacrificial material is silicon dioxide. Alternatively, the etchant is XeF2 and the sacrificial material is silicon.

[0047] Additionally, the method may further include monitoring the etching conditions with an etching monitor.

[0048] Preferably, the method may further comprise the step of dynamically controlling the etching process by feedback from an etching monitor and / or prior knowledge of the topography.

[0049] Preferably, the microstructure may be a semiconductor device, a CMOS semiconductor, a MEMS device, a MEMS microphone or a microchannel.

[0050] According to a second aspect of the present invention, there is provided a microstructure produced according to the method of the first aspect of the present invention.

[0051] 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.

[0052] According to a third aspect of the present invention, there is provided a method for manufacturing a microstructure having a three-dimensional structure, comprising: The method includes performing an isotropic steam etch of a sacrificial material; The isotropic vapor etching includes two or more stages in response to changes in the three-dimensional structure of the microstructure, and controls the volumetric etching rate in response to (or based on) changes in the etch front and / or changes in the composition of the sacrificial material.

[0053] Embodiments of the third aspect of the invention may include one or more features of the first and / or second aspect of the invention or embodiments thereof, or vice versa.

[0054] According to a fourth aspect of the present invention, there is provided a method of manufacturing a microstructure, comprising: The method includes performing an isotropic steam etch of a sacrificial material; The isotropic steam etch includes two or more stages, and the volumetric etch rate in each stage is: (a) Changes in the etch front due to changes in the three-dimensional structure of the microstructure, and / or (b) Composition change of the sacrificial material being etched are selected (or pre-selected) depending on (or based on)

[0055] The microstructure preferably comprises a three-dimensional structure. The etch front change and / or composition change is preferably known prior to carrying out the method or is predicted during etching.

[0056] Embodiments of the fourth aspect of the invention may include one or more features of the first and / or second aspect of the invention or embodiments thereof, or vice versa. [Brief explanation of the drawings]

[0057] Various embodiments of the present invention will now be described, by way of example only, with reference to the drawings in which: [Figure 1] FIG. 1 shows (a) a perspective view and (b) a schematic diagram of a semiconductor wafer before a one-dimensional etching process known in the art, and (c) a perspective view and (d) a schematic diagram of a semiconductor device after a one-dimensional etching process known in the art. [Figure 2] FIG. 2 shows a schematic diagram of a MEMS wafer before HF vapor etching as known in the art. [Figure 3] FIG. 3 shows a schematic diagram of an etching apparatus according to the present invention. [Figure 4]Figure 4 shows (a) a perspective view and (b) a schematic diagram of a MEMS wafer before three-dimensional isotropic steam etching according to the present invention, (c) a perspective view and (d) a schematic diagram of a MEMS wafer during the first stage of three-dimensional isotropic steam etching, (e) a perspective view and (f) a schematic diagram of a MEMS wafer during the transition from the first stage to the second stage of three-dimensional isotropic steam etching, and (g) a perspective view and (h) a schematic diagram of a MEMS device after the second stage of three-dimensional isotropic steam etching. [Figure 5] FIG. 5 shows (a) a schematic diagram of a MEMS microphone after a first stage of three-dimensional isotropic steam etching according to the present invention, (b) a schematic diagram of a MEMS microphone after a second stage of three-dimensional isotropic steam etching, and (c) a schematic diagram of a MEMS microphone after a third stage of the three-dimensional isotropic steam etching process. [Figure 6] FIG. 6 shows a schematic diagram of a complementary metal oxide semiconductor (CMOS) sensor before multi-step isotropic steam etching according to the present invention. [Figure 7] FIG. 7 shows plots of (a) XeF2 flow rate as a function of carrier gas flow rate, and (b) XeF2 concentration as a function of carrier gas flow rate, shown in the etcher of FIG. [Figure 8] FIG. 8 shows contour plots of etch rate versus chamber pressure and carrier gas flow rate for (a) reaction-limited and (b) transport-limited etching conditions. [Figure 9] FIG. 9 shows perspective views of a microchannel (a) before isotropic steam etching according to the present invention, (b) during the first stage of reaction-limited isotropic steam etching, and (c) during the second stage of transport-limited isotropic steam etching. [Figure 10] FIG. 10 shows plots of (a) etched volume as a function of time and (b) radius as a function of time in fabricating the microchannel of FIG. [Figure 11]Figure 11 shows the carrier gas (N) flow rate as a function of etching time for a process recipe including steps 1 (gray), 2 (yellow), 3 (orange), 9 (green), 12 (blue), and 15 (pink) for fabricating the microchannel of Figure 9.

[0058] In the following description, like parts are designated by the same reference numerals throughout the specification and drawings. The drawings are not necessarily to scale, and the proportions of certain parts have been exaggerated to more fully illustrate the details and features of embodiments of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0059] The present invention will be described below with reference to FIGS.

[0060] Etching equipment FIG. 3 shows a schematic diagram of an etcher 10 suitable for etching microstructures such as the semiconductor device 1 of FIG. 1 or the MEMS device 6 of FIG. 2. The etcher 10 of FIG. 3 is suitable for a variety of etching processes, although it will be understood that separate etchers may be provided for different etching processes. It can be seen that the etcher 10 comprises an etching chamber 11 to which six input lines 12, 13, 14, 15, 16, 17 and an output vacuum line 18 are connected.

[0061] Within the etching chamber 11 is a temperature controlled pedestal 19 suitable for placing the wafers 1, 6 to be etched within the etching chamber 11. Fluids supplied from six input lines 12, 13, 14, 15, 16, 17 enter the interior volume of the etching chamber 11 via a fluid injection system 20 located within the lid 21 of the etching chamber 11.

[0062] The pedestal 19 on which the microstructures 1, 6 are placed is heated to a pedestal temperature T pThis temperature can be set and maintained at room temperature, but a specific temperature is selected to optimize the etching process (typically between 5 and 25°C). Additionally, during the etching process, the walls of the etching chamber 11 are typically heated to between about 20 and 70°C.

[0063] The pressure of the etchant gas in the etching chamber, P c is monitored by a chamber pressure controller 22. The pressure controller 22 also incorporates a gas flow controller, which is employed to provide a means of controlling the pressure within the etching chamber 11 by controlling the operation of a vacuum pumping system 23 located in the output vacuum line 18.

[0064] HF vapor 24 is controllably supplied to etching chamber 11 by first input line 12 via regulator 25 and first mass flow controller (MFC) 26 .

[0065] A controlled amount of water is supplied to the etching chamber 11 by the second input line 13. In particular, a liquid fluid controller (LFC) 27 and vaporizer 28 disposed within the second input line 13 are employed to generate a controlled level of water vapor from a water reservoir 29. The flow of nitrogen from a nitrogen gas source 30 to the vaporizer 28 is controlled by the second MFC 26. A nitrogen carrier gas is used to transport the water vapor into the interior volume of the etching chamber 11 via the fluid injection system 20.

[0066] A third input line 14, a fourth input line 15, and a fifth input line 16 provide means for connecting additional gas sources 31, 32, 33, such as hydrogen (H), oxygen (O), or fluorine (F), to the interior volume of the etching chamber 11. Control of the flow rates of these gases is likewise achieved by mass flow controllers (MFCs) 26.

[0067] Xenon difluoride (XeF) vapor is controllably supplied to the etching chamber 11 by a sixth input line 17, which includes a XeF bubbler 34 and a nitrogen gas source 30. The XeF bubbler 34 contains XeF crystals. As nitrogen gas passes over the XeF crystals, the XeF sublimes and is carried by the nitrogen gas into the etching chamber 11. A mass flow controller (MCF) 26, in conjunction with a pneumatic valve 25a, controls the supply of nitrogen gas to the XeF bubbler 34, the supply of nitrogen gas to the etching chamber 11, and the supply of nitrogen gas containing XeF to the etching chamber 11. The pumping speed of the MCF 26 and / or the vacuum pumping system 23 can be controlled, for example, by a pump control valve, to maintain a set operating pressure with the etching chamber 11.

[0068] A computer controller 35 is used to automate the regulation of various components and parameters of the etching chamber 11, such as the supply of nitrogen carrier gas, HF vapor, chamber temperature and pressure, etc.

[0069] When fabricating a semiconductor device 1 such as that shown in FIG. 2 , the applicant has discovered that the surface area of ​​the sacrificial layer 7 exposed to the etchant, i.e., the etch front, can affect the etching rate. For example, the area and / or the position and / or the orientation of the etch front can affect the etching rate. Specifically, an etch front with a large area etches faster than an etch front with a small area. This is because an etch front with a large area generates more HO by-products, which contribute to the formation of the condensation layer 9.

[0070] As the sacrificial etch progresses, the etch front changes depending on the structure being etched. The change in etch front can be very abrupt. The characteristics of the etch front, such as the area and / or position and / or direction of the etch front, can change. As the etch front changes, the etch rate also changes. Therefore, even if the parameters of the etching process remain unchanged, the etch rate will change depending on the surface being etched and the variations in the silicon dioxide materials encountered, such as their density and ease of etching.

[0071] As the etch front area decreases or the silicon dioxide material becomes denser, the etch rate can decrease to a level where it is impractical to continue. As the etch front area increases or the etch front encounters less dense oxide, the etch rate increases. If the etch rate becomes very high, it can cause problems such as stiction and metal corrosion.

[0072] It is clear that to achieve reliable etching, the etch rate must be properly controlled as the etch proceeds through the structure. However, operating the etch process in one dimension and considering the etch rate in microns per minute is inefficient for three-dimensional structures such as the release structure 8 of the MEMS device 6. In other words, performing the etch process in a single time step without considering pre-defined, known, or predicted microstructural changes, i.e., changes in the etch front and / or changes in the composition of the sacrificial material, that would cause the etch rate to vary is inefficient.

[0073] Etching Method The present invention relates to a method for producing a microstructure having a three-dimensional structure, various examples of which are depicted in Figures 4 to 11 and will be described in detail below.

[0074] All of these examples generally relate to methods that include isotropic steam etching of a sacrificial material, the isotropic steam etching including two or more temporal stages, with the volumetric etch rate for each stage being selected (or preselected) in response to (or based on) prior knowledge of the microstructure. Specifically, the prior knowledge of the microstructure can include (a) prior knowledge of changes in the area and / or position and / or direction of the isotropic steam etch etch front, and / or (b) prior knowledge of changes in the composition of the sacrificial material being etched. The terms prior knowledge and pre-determined are used interchangeably herein. Specifically, the terms refer to information that is known before the isotropic steam etching begins, by knowing what the microstructure is.

[0075] The volumetric etch rate is determined and can be controlled by etching parameters. Therefore, the method can also be described using etching parameters. Specifically, isotropic steam etching involves varying etching parameters of the isotropic steam etch as a function of time in response to (or based on) prior knowledge of the microstructure. In this specification, the terms chamber parameters and etching parameters are used interchangeably.

[0076] The method of the present invention is significantly different from standard semiconductor RIE etching processes, such as those described above. RIE etching processes are one-dimensional, e.g., the area and direction of the etch front do not change as etching progresses. In contrast, the method of the present invention is three-dimensional. Therefore, the method of the present invention must be viewed and described in a completely different way than methods known in the art.

[0077] Importantly, the isotropic steam etching process is highly dependent on the three-dimensional structure. As the etching process progresses, the etch front and / or density of the sacrificial material change. As a result, etching parameters optimized for the first stage of etching may not be optimal for the second stage of etching, with each stage corresponding to etching of a different feature of the three-dimensional structure. Ideally, to achieve an etching process optimized for complete release of the microstructure and / or to maintain a practical etch rate, the etching parameters should change as the etching process progresses to accommodate the changes in the structure.

[0078] The method of the present invention is carried out in a vacuum chamber 11 of an etching apparatus 10, as shown in FIG. 3. All etching parameters are precisely controlled. Sample temperature, gas flow rates, and chamber pressure are all precisely controlled, and as the etching process progresses, adjustments can be made to optimize the setup of the vacuum chamber 11 and maximize etching. The conditions of the vacuum chamber 11 are controlled using software, and the control parameters of the vacuum chamber 11 are set using a series of control steps. This series of control steps is typically referred to as a process recipe. In summary, the chamber parameters or setup describe the etching process at a single point in time or at the same stage, while the process recipe describes how the chamber parameters change from the beginning to the end of the etching process.

[0079] It will be appreciated that there are many gas-phase isotropic etches that are relevant to the present invention. Control of these various etches can be optimized by varying chamber parameters (also referred to as etch parameters). The chamber parameters may be different for different etches. For example, conditions for two specific etches, namely, an HF vapor etch and a XeF2 etch, are described below.

[0080] First, in the case of HF vapor etching, the etch rate is set by the creation and control of a condensed layer that forms on the exposed surface 5 of the sacrificial material 7. The least volatile compound in the chamber is HO, and the formation of the condensed layer is related to the vapor pressure of HO.

[0081] Temperature is a control parameter that is very difficult to change quickly and is generally set throughout the etching process.

[0082] The gases used in the etching process are, for example, HF, N2, and HO, and the ratio of these gases determines the etch rate at a particular pressure. Gas flow rate changes can be implemented quickly and can be used to make small etch rate changes. The gas flow rates are precisely controlled using mass flow controllers (MFCs) 26.

[0083] The main control parameter is pressure, which is precisely controlled by the vacuum pump system 23 and in particular the throttle valve on the output vacuum line of the etching chamber 11. The pressure is gradually increased to the target value and then precisely controlled.

[0084] Example 1 - Etching of SOI wafers A common substrate 3 used in the fabrication of MEMS devices 6 is a silicon-on-insulator (SOI) wafer. While there are various methods for fabricating SOI wafers, they all involve forming a single-crystal silicon layer 4 on an oxide layer 7, with a silicon substrate 3 underneath. The top silicon layer 4 is patterned and acts as a mask for creating the MEMS device 6. The oxide layer 7 is a sacrificial layer that is etched to release the structure 8.

[0085] The initial structure of the MEMS device shown in Figures 4a and 4b is the same as the standard semiconductor RIE etch configuration shown in Figure 1. However, as the oxide etch proceeds, a completely different etching process occurs.

[0086] The etching process shown in Figure 4 is an isotropic steam etching process, as opposed to the anisotropic process shown in Figure 1. In other words, the etching process shown in Figure 4 includes a lateral component, meaning that the etching proceeds in three dimensions. The etch front changes during the etching process. Specifically, the area, position, and direction of the etch front change. Figures 4c and 4d show how the area of ​​the etch front increases due to an increase in the area of ​​the oxide exposed to the etchant. This affects the etching because the etch rate increases as the etching proceeds.

[0087] To avoid problems such as stiction and metal corrosion, the etch rate must be kept below a certain threshold, which can be achieved by controlling the etch parameters in two ways.

[0088] First, the chamber pressure can be set so that the maximum etch rate that occurs during the course of the etching process is below a threshold at which problems occur.

[0089] Alternatively, the etching process can be started at a higher chamber pressure to obtain a relatively high etch rate. As the etching process progresses, the etch rate increases as the etch front expands into the microstructure 6. Chamber parameters are modified to keep the maximum etch rate below a threshold where problems occur. This can be achieved by lowering the chamber pressure, which returns to values ​​very similar to the simple scenario described above. Alternatively, the gas flow rates can be modified again to prevent the etch rate from exceeding the target value.

[0090] As can be seen in Figures 4c and 4d, the etched oxide layer 7 is not a direct copy of the overlying mask layer 4. The shape and pattern of the etched oxide layer 7 is similar to that of the mask layer 4, but is scaled down in the x and y directions. In other words, the voids created by the etching process have been enlarged.

[0091] The etch front begins to change again as the areas of oxide film 7 not protected by mask layer 4 are etched away, exposing the underlying silicon substrate 3. The exposed surface of oxide film 7 in the xy plane disappears, and the etching process continues by etching the oxide film 7 directly below mask layer 4, i.e., undercutting mask layer 4 (see Figures 4e and 4f). As for the etch front, its area decreases, its direction shifts from along the z-axis to along the xy plane, and its position shifts below mask layer 4. As a result, the etching rate decreases.

[0092] During the undercut etching stage, the etch rate has decreased simply because the structure and etch front characteristics have changed. Therefore, the chamber parameters, i.e., the etch setup or etch parameters, can also be changed to suit the new conditions encountered in the etch. The pressure can be increased to increase the etch rate. Similarly, the gas flow ratio can be changed to change the etch rate. Alternatively, both the chamber pressure and the gas flow ratio can be changed.

[0093] The undercut etching continues until the target state of the microstructure 6 device is achieved, for example until structure 8 is released, as shown in Figures 4g and 4h.

[0094] This etching process has two distinct stages: the first is an initial etch in a large open area that removes material primarily in the z-direction, and the second is a completely different undercut etch that removes material primarily laterally (x-y directions). These two etching stages require at least two completely different etch setups, i.e., etch parameters, to match the area of ​​the structure 8 to be etched.

[0095] In summary, Figures 4a and 4b show the MEMS wafer 6 before isotropic steam etching. Figures 4c and 4d show the MEMS wafer 6 during the first stage of isotropic steam etching. Figures 4e and 4f show the MEMS wafer 6 during the transition from the first stage to the second stage of isotropic steam etching. Figures 4g and 4h show the MEMS device 6 after the second stage of isotropic steam etching.

[0096] It will be appreciated that the etch front changes even within these two etching stages, and the etching process can be further fine-tuned to compensate for this changing etch front, and therefore the etching parameters can be thought of as changing as a function of time.

[0097] Furthermore, it will be appreciated that an intermediate stage (also called a transition stage) may be required in the etching process, as a more controlled change may be required to transition between two etching stages.

[0098] Example 2 - MEMS microphone 5 shows a MEMS microphone 36 having a structure 8 before a release etch, the structure 8 comprising a first upper polysilicon layer 37, a second lower polysilicon layer 38, and a first oxide layer 39 sandwiched between the first and second polysilicon layers 37, 38. The MEMS microphone 36 comprises a second oxide layer 40 below the second lower polysilicon layer 38.

[0099] The first polysilicon layer 37 includes a plurality of holes 41 through which the etchant can access the first oxide layer 39. That is, the plurality of holes 41 exposes a surface region 5 of the first oxide layer 39 to the etchant, thereby defining an etch front. To release the structure 8 of the MEMS microphone 36, a release etch removes material from the etch front of the first oxide layer 39 in the z-direction and then undercuts the first polysilicon layer 37 in the x- and y-directions. Thus, the area, position, and direction of the etch front change as the etch progresses.

[0100] After etching, the two polysilicon layers 37, 38 are free to move. However, if the release etch is not properly controlled, stiction may occur, and the MEMS microphone 36 may not operate as intended. Additionally, the MEMS microphone 36 further includes a metal bond pad 42. If the release etch is not properly controlled, corrosion may occur on the metal bond pad 42, which may adversely affect the operation of the MEMS microphone 36.

[0101] This structure 8 of the MEMS microphone 36 can be etched in a single step, i.e. using a single process setup with constant parameters. However, there are also advantages to changing the etching to a three-step process.

[0102] The first stage involves etching the MEMS microphone 36 at a high etch rate to ensure uniform etching across the entire MEMS microphone 36 prior to the release of the structure 8. Figure 5a shows the MEMS microphone 36 after this first stage. To optimize throughput, this first stage should be maximized, but care must be taken because over-etching can result in premature release of the structure 8 and risk of stiction. During this first stage, the etch front is primarily oriented along the z-axis. The area of ​​the etch front increases as the etch begins to expand laterally in the xy plane.

[0103] The second stage involves changing the etching parameters to perform the release stage of the etching process at a lower etching pressure and slower etching rate. This second stage is performed until the structure 8 of the MEMS microphone 36 is completely released and the etch front moves to undercut the first polysilicon layer 37, as shown in FIG. 5b. During this second stage, the area of ​​the etch front decreases, the direction of the etch front lies in the xy plane, and the position of the etch front moves below the polysilicon layer 37.

[0104] The third stage involves again changing the etching parameters to perform the final stage of the etching process at a higher pressure to increase the etch rate. In this third stage, the first polysilicon layer 37 is further undercut by etching laterally in the xy plane, as shown in Figure 5c. Note that the second oxide layer 40 is also etched, as seen in Figure 5c. In this third stage, the etch front position advances further below the polysilicon layer 37, requiring different etching parameters to maintain the etch rate.

[0105] Advantageously, this three-step process for fabricating MEMS microphones allows for faster and more efficient manufacturing while minimizing the risk of failure due to stiction compared to single-step processes. The etching setup, or parameters, are varied to increase or decrease the etching rate depending on the level of control required for the structural features of the MEMS microphone being etched.

[0106] Example 3 - Inter-layer dielectric (ILD) etching for CMOS multi-level interconnect devices MEMS devices are fabricated using standard complementary metal oxide semiconductor (CMOS) processing to fabricate sensors using the metal wiring portions of the structure. An example of such a sensor 43 is shown in Figure 6, which can be seen to comprise an interlayer dielectric (ILD) layer 44 and a metal layer 45. The function of sensor 43 relies on the removal of the interlayer dielectric (ILD) layer 44, which comprises silicon dioxide, by HF vapor etching.

[0107] The manufacturing process for CMOS sensor 43 is designed and optimized to produce high quality electronic devices and provide the best electronic performance for those devices. In the ongoing effort to produce higher quality devices, the dielectric constant (k) of the ILD must be as low as possible, especially important in the lower wiring levels of CMOS sensor 43. Silicon dioxide, an ILD layer 44 with a low k, is much easier to etch, i.e., tends to etch more readily, compared to standard oxide films with the same etching parameters.

[0108] As the process of etching the various ILD layers 44 of a CMOS sensor 43 progresses, there comes a point where etching of the underlying ILD layer 44 begins. At this point, the etch rate of the underlying ILD layer 44 becomes high, and if the etch rate becomes too high, problems can occur. To maintain high yields, the etch parameters must be adjusted to maintain the desired etch rate.

[0109] Instead of fabricating CMOS sensor 43 with a single-step process, which requires a slow etch rate to avoid problems and results in a slow process, it is advantageous to employ a multi-step process depending on which ILD layer 44 is being etched. In this way, the etch setup, i.e., the etch parameters, are changed to increase or decrease the etch rate depending on which ILD layer 44 is being etched, resulting in a more efficient process.

[0110] Example 4 - XeF in a microchannel 2 etching XeF2 is a vapor that isotropically etches silicon with high selectivity over other materials such as silicon oxide, silicon nitride, aluminum, and photoresist.

[0111] The etching rate is controlled by the XeF2 partial pressure: the higher the XeF2 partial pressure, the faster the etching rate.

[0112] The XeF2 source is a solid and sublimes to provide XeF2 vapor 34. The etching apparatus 10 shown in FIG. 3 uses a solid source bubbler to contain the source and a carrier gas flow to transport the XeF2 vapor 34 to the process chamber. The flow rate of the XeF2 is determined by the carrier gas flow rate. FIG. 7a shows the XeF2 flow rate as a function of the carrier gas flow rate. As shown, XeF2 increases with increasing carrier gas flow rate. However, the relationship between the XeF2 flow rate and the carrier gas flow rate is not linear. FIG. 7b shows the concentration ratio of the XeF2 flow rate to the carrier gas flow rate as a function of the carrier gas flow rate. The relationship between the XeF2 flow rate and the concentration ratio of the carrier gas flow rate is consistent, but not linear. As shown, the concentration ratio of the XeF2 flow rate is inversely proportional to the carrier gas flow rate.

[0113] In the absence of etching, for a given chamber pressure, the lower the carrier gas flow rate, the higher the XeF2 partial pressure. However, when etching a sample, the amount of etching has a significant effect on the etching setup, i.e., the parameters in the chamber 11.

[0114] When the amount of silicon to be etched is small, a higher etch rate can be achieved by increasing the XeF2 partial pressure and lowering the carrier gas flow rate. This etching is reaction-limited. Figure 8a shows the relationship between chamber pressure, carrier gas flow rate, and etch rate for reaction-limited XeF2 etching.

[0115] When the amount of silicon being etched is large, the XeF2 flowing into the chamber is consumed relatively quickly, and the XeF2 partial pressure is determined by the rate at which XeF2 flows into the chamber. In this case, etching is transport-limited, and the etch rate increases with increasing carrier gas flow rate, with the effect of chamber pressure being significantly less. Figure 8b shows the relationship between chamber pressure, carrier gas flow rate, and etch rate for transport-limited XeF2 etching.

[0116] As previously mentioned, it will be appreciated that the process chamber setup will be highly dependent on the topography to be etched.

[0117] Microchannels 46 can be etched into silicon 47 using XeF2 vapor. Before proceeding with the XeF2 etch to form microchannels 46, silicon 47 is initially etched to form trenches 48 with polymer sidewalls 49, as shown in Figure 9a.

[0118] The XeF2 vapor etches the exposed silicon 47 at the bottom of the trench 48, from which the etch front area expands to form the microchannel 46, as shown in Figures 9b and 9c. XeF2 etching is a purely chemical isotropic vapor etch. The etching process is carried out in a vacuum chamber with controlled temperature, gas flow, and chamber pressure. The progress of the etching can be measured by the increase in the radius of the microchannel 46 that is formed.

[0119] When XeF2 vapor etching is performed with constant etching parameters, the volume of silicon etched over time is linear, as shown in Figure 10a, i.e., the same volume of silicon is etched in the same time unit, and the channel continues to expand.

[0120] However, as shown in Figure 10b, the change in radius of the etched channel, i.e., the one-dimensional etch rate, is not linear with time. Figure 10b gives the impression that the etching process is slowing down, but this is not the case because the volume of material removed by etching increases with the radius of the microchannel 46.

[0121] It will be appreciated that it is advantageous to characterize etching in terms of volume etched per time. Therefore, a more appropriate measure of etch rate is the three-dimensional etch rate in μm. 3 / min.

[0122] As etching progresses, the channel becomes larger and the area of ​​the etch front increases with increasing radius. However, the area of ​​the etch front increases at a faster rate than the radius, which has a significant effect on the XeF2 etching.

[0123] In this example, the etch rate is primarily determined by the XeF2 partial pressure, with higher partial pressures resulting in faster etch rates. The XeF2 partial pressure in the chamber is controlled by the gas flow rate and chamber pressure. It is also significantly affected by the exposed area of ​​silicon-47, i.e., the relative size of the etch front. When the etch front is relatively small, the etch rate is reaction-limited, and a high etch pressure and a low XeF2 flow rate optimize the etch rate. When the etch front area is relatively large, the etch rate is transport-limited, and a significant increase in the XeF2 flow rate optimizes the etch rate. In other words, as the etch channel becomes larger, the etch rate changes from one etch condition to another.

[0124] It will be appreciated that if the etching process is run with constant parameters, etching will proceed, but the process will not be optimized to accommodate variations in the etching process, i.e., the evolution of the microchannels 46 and etch front.

[0125] Instead, for a given total etch process time, the total number of steps n and the time for each step can be determined by specifying the start and end values ​​of certain parameters and the incremental delta of change. In this way, step parameters can be varied or stepped up or down over time during etching. In addition to the etch parameters of interest, the actual step times themselves can also be varied or stepped up or down throughout the etch process, potentially providing additional benefits.

[0126] Typically, gas-phase etching of bulk silicon or silicon substrates using XeF2 requires long etching times, especially for three-dimensional volumetric etching. By gradually varying the etching parameters, it is possible to achieve much higher etching rates than conventional single-step processes, facilitating the reduction of etching times. Furthermore, this effect can be achieved without using large amounts of expensive XeF2 etching precursors.

[0127] In this example, the etching conditions change as the microchannel 46 becomes larger, and the etch rate becomes more dependent on the amount of XeF2 that can be introduced into the chamber 11. Figure 11 shows various etch recipes, specifically how the carrier gas flow rate increases as a function of time, resulting in higher XeF2 flow rates and higher etch rates. As the number of carrier gas flow rate increments increases, the etch process can be optimized to reduce the overall etch time.

[0128] Changes in etching parameters, i.e., specific etching recipes, can be programmed in the software of the computer controller 35. This feature gives the user flexibility to efficiently manage the etching process.

[0129] Etch Monitor and Software As an optional feature, etcher 10 may include an etch monitor 50. Observing conditions within chamber 11 is advantageous in setting optimal etch parameters. By knowing the topography to be etched and the initial etch conditions, etch monitor 50 can be used to measure the progress of the etch, determine which stage or phase the etch is in, and appropriately adapt the etch conditions during each stage. The etch monitor 50 can be used to continuously adapt the etch conditions throughout the entire etch and for individual stages of the etch.

[0130] Etch monitor 50 can take the form of a detector that measures the amount of infrared light that is transmitted through process chamber 11. The amount of light detected is related to the various gas molecules within chamber 11. Knowing the exact amounts of the various gases present within chamber 11 is related to the etch rate of the device.

[0131] The etch monitor 50 can be combined with software to determine when the etch conditions need to be changed to move the recipe to the next step.

[0132] With a more detailed understanding of the structural etch and precise control of the process parameters combined with feedback from the etch monitor, the etch process can be fully controlled by the algorithm. Initial conditions are set to optimize the starting etch. The etch monitor 50 continuously observes the state of the chamber 11, and this feedback is used by the software model to determine the optimal chamber 11 conditions for the chamber 11 at that time. The software maintains control of the etch and completes it with optimized etch conditions throughout the etch.

[0133] The method for manufacturing microstructures according to the present invention has numerous advantages over methods known in the art.

[0134] A key advantage is that the isotropic steam etching of three-dimensional structures is optimized accordingly. In other words, the isotropic steam etching is considered in a three-dimensional manner, rather than a one-dimensional manner. The various stages of the isotropic steam etching allow for control of the volumetric etching rate, which varies due to variations in the etch front and composition of the sacrificial material.

[0135] In fact, isotropic vapor etching can be advantageously used to more efficiently form microstructures while minimizing the risk of device failure. For example, the volumetric etch rate can be slowed to avoid stiction when releasing the 3D structure of a MEMS device, and then increased after the 3D structure is released to shorten fabrication time. As another example, when fabricating microchannels, the volumetric etch rate can be modified to accommodate the change in etching conditions from reaction-limited to transport-limited.

[0136] A method for fabricating a microstructure having a three-dimensional structure is described. The method includes performing an isotropic steam etch of a sacrificial material. Etch parameters for the isotropic steam etch are varied as a function of time depending on (or based on) prior knowledge of the microstructure. This method has numerous advantages, as it allows for more efficient formation of the microstructure while minimizing the risk of device failure.

[0137] The foregoing description of the present 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 have been chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling those skilled in the art to best utilize the invention in various embodiments and modifications suited to the particular use envisioned. Accordingly, further changes or modifications may be incorporated without departing from the scope of the invention as defined by the appended claims.

Claims

1. A method for manufacturing a microstructure, comprising: performing an isotropic steam etch of the sacrificial material; 10. A method for fabricating a microstructure, wherein etching parameters of the isotropic steam etch are varied as a function of time in response to prior knowledge of the microstructure.

2. 2. The method for manufacturing a microstructure according to claim 1, 10. A method for manufacturing a microstructure, wherein the prior knowledge about the microstructure includes prior knowledge about changes in area and / or position and / or direction of an etch front of the isotropic steam etch.

3. 3. The method for manufacturing a microstructure according to claim 1 or 2, 10. A method for manufacturing a microstructure, wherein the prior knowledge of the microstructure includes prior knowledge of compositional changes in a sacrificial material being etched.

4. 10. A method for producing a microstructure according to any one of the preceding claims, comprising: The isotropic steam etching a first stage of etching the sacrificial material at a first volumetric etch rate for a first period of time with a first set of etching parameters; and a second step of further etching the sacrificial material at a second volumetric etch rate for a second period of time with a second set of etching parameters.

5. 5. The method for manufacturing a microstructure according to claim 4, The method of manufacturing a microstructure, wherein the first and second volumetric etch rates depend on different features of the microstructure.

6. 6. The method for manufacturing a microstructure according to claim 4 or 5, The method for fabricating a microstructure, wherein the first set of etching parameters is different from the second set of etching parameters.

7. The method for manufacturing a microstructure according to any one of claims 4 to 6, A method for manufacturing a microstructure, wherein the etching parameters that are changed between the first and second stages include pressure, gas flow rate, gas flow rate ratio, gas species, and / or temperature.

8. The method for manufacturing a microstructure according to any one of claims 4 to 7, The method for manufacturing a microstructure, wherein the first period of time is different from the second period of time.

9. The method for manufacturing a microstructure according to any one of claims 4 to 8, The method for fabricating a microstructure, wherein the first volumetric etch rate and the second volumetric etch rate are maintained below a threshold value.

10. 10. The method for manufacturing a microstructure according to claim 9, A method for manufacturing a microstructure, characterized in that the first and second volumetric etch rates are maintained below the threshold by initially fixing the first and second etching parameters so that the maximum possible etch rate is below the threshold.

11. 10. The method for manufacturing a microstructure according to claim 9, wherein the first and second volumetric etch rates are maintained below the threshold by varying the first and second etching parameters during the isotropic steam etch such that the etch rates are below the threshold.

12. The method for manufacturing a microstructure according to any one of claims 4 to 11, 10. A method for manufacturing a microstructure, wherein the first etching parameters are dynamically changed during the first etching stage and / or the second etching parameters are dynamically changed during the second etching stage.

13. The method for manufacturing a microstructure according to any one of claims 4 to 12, 10. A method for manufacturing a microstructure, wherein the first step of isotropic steam etching corresponds to removing sacrificial material not covered by a mask layer.

14. The method for manufacturing a microstructure according to any one of claims 4 to 13, 10. A method for manufacturing a microstructure, wherein the second stage of isotropic steam etching corresponds to removing sacrificial material underneath the mask layer.

15. The method for manufacturing a microstructure according to any one of claims 4 to 14, a third stage in which the isotropic steam etch further etches the sacrificial material at a third volumetric etch rate for a third period of time with a third set of etching parameters.

16. 16. The method for manufacturing a microstructure according to claim 15, The method for manufacturing a microstructure, wherein the third stage of the isotropic steam etching can correspond to further removing sacrificial material underneath the mask layer.

17. The method for manufacturing a microstructure according to any one of claims 4 to 12, 1. A method for manufacturing a microstructure, characterized in that the second stage of the isotropic steam etching corresponds to the removal of a sacrificial material having a different density or ease of etching compared to the sacrificial material removed in the first stage of the isotropic steam etching.

18. 5. The method for manufacturing a microstructure according to claim 4, 10. A method for manufacturing a microstructure, wherein the first stage of the isotropic steam etching can correspond to removing sacrificial material under reaction-limited etching conditions.

19. 19. The method for manufacturing a microstructure according to claim 18, 10. A method for fabricating a microstructure, wherein the first etching parameters include a relatively high etchant partial pressure by operating at a lower carrier gas flow rate and a higher chamber pressure.

20. 20. The method for producing a microstructure according to claim 18 or 19, The method for fabricating a microstructure, wherein the second stage of isotropic steam etching can correspond to removing sacrificial material under transport-limited etching conditions.

21. 21. The method for manufacturing a microstructure according to claim 20, The method for fabricating a microstructure, wherein the second etching parameters include a relatively high etchant flow rate.

22. The method for manufacturing a microstructure according to any one of claims 18 to 21, 10. A method for fabricating a microstructure, wherein the first and second stages of isotropic steam etching comprise substantially the same or similar volumetric etch rates.

23. 10. A method for producing a microstructure according to any one of the preceding claims, comprising:

10. A method for fabricating a microstructure, wherein the isotropic steam etching comprises a plurality of stages, each stage corresponding to an incremental change in etching parameters and / or duration.

24. 10. A method for producing a microstructure according to any one of the preceding claims, comprising: The etchant is HF vapor, the sacrificial material is silicon dioxide, and / or the etchant is XeF ２ 2. A method for producing a microstructure, characterized in that the sacrificial material is silicon.

25. 10. A method for producing a microstructure according to any one of the preceding claims, comprising:

10. A method for fabricating a microstructure, which may further include monitoring etching conditions with an etching monitor and dynamically controlling the etching process via feedback from the etching monitor and / or prior knowledge of the microstructure.

26. 10. A method for producing a microstructure according to any one of the preceding claims, comprising: A method for manufacturing a microstructure, wherein the microstructure is a semiconductor device, a CMOS semiconductor, a MEMS device, a MEMS microphone, or a microchannel.

27. A microstructure produced according to the method of any one of claims 1 to 26.

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