Method of fabricating microstructure

Abstract

A method of manufacturing a microstructure having a three-dimensional structure is described. The method includes isotropic vapor etching of the sacrificial material. The etch parameters of the isotropic vapor etch vary over time in response to (or based on) a priori knowledge of the microstructure. The method has many advantages because microstructures can be created more efficiently while minimizing the risk of device failure.

Description

[0001] The present invention relates to a method for fabricating microstructures. Typically, the microstructures are in the form of semiconductor devices or microelectromechanical systems (MEMS) that require material removal relative to a substrate or other deposited material. In particular, the present invention relates to an improved method for fabricating microstructures using an etching step. Background Art

[0002] semiconductor manufacturing

[0003] Semiconductor manufacturing is a highly complex series of processes that use multiple steps to build semiconductor devices. However, at its most basic, it is the same method used to form all microstructured devices.

[0004] Fabricating a semiconductor device (e.g., of the type shown in FIG. 1 , and generally designated by reference numeral 1 ) typically involves first depositing a thin film 2 on a substrate 3 . Next, a photoresist layer 4 is deposited on the thin film 2 . Photoresist layer 4 is patterned using a photographic exposure process, followed by a development and rinse phase. The resulting patterned photoresist layer 4 serves as a mask. An etching process is then used to remove the exposed underlying thin film 2 . This process is then repeated multiple times to construct the semiconductor device 1 .

[0005] The etching process used in semiconductor device manufacturing is a low-pressure plasma process called reactive ion etching (RIE), which is ubiquitous in semiconductor manufacturing. RIE has both chemical and physical components. The chemical component of etching involves rearranging molecular structures by breaking existing bonds and forming new ones. The physical component of etching involves highly directed ion bombardment of the semiconductor wafer surface being etched.

[0006] Figure 1a and Figure 1b Depicted is a semiconductor wafer 1 prior to an RIE etching process, Figure 1c and Figure 1d A semiconductor device 1 produced by the RIE etching process is depicted. A key feature of this method is that the RIE etching process is anisotropic; in other words, the etching is highly directional in the z-direction, thereby imparting essentially the same pattern in the xy plane as the photoresist layer 4 to the underlying film 2. Although the RIE etching process is three-dimensional, the material removed is determined solely by the progression in the z-direction. Therefore, a key variable used to describe the etching process is the etch rate, measured in micrometers per minute, a one-dimensional unit.

[0007] The etching process depends on various parameters, such as gas flow, pressure, temperature, and plasma power. Experiments are performed to optimize these parameters, in other words, to optimize the etching settings, to provide a suitable etching rate. Thin film 2 includes an exposed surface area 5 that is not covered by photoresist layer 4 and is therefore exposed to the etchant. This exposed surface area 5, also known as the etch front, is constant during the anisotropic RIE etching process, so the optimized parameters apply during the RIE etching process. Therefore, in semiconductor manufacturing, the parameters of the etching process generally do not change over time, and the etching process can be considered a one-step process.

[0008] There are some exceptions, as etching parameters may be varied, for example, to overcome a starting surface layer or to fine-tune an existing etching process.

[0009] When etching polysilicon, the initial native oxide layer must be removed from the exposed polysilicon surface. The RIE chemistry is optimized to etch silicon and, therefore, is not optimized to etch through the native oxide that grows on the exposed polysilicon surface. To quickly etch through the native oxide, a breakthrough step is added before the primary polysilicon etch. This so-called breakthrough step biases the etch toward a more physical component, causing ion bombardment to sputter the native oxide from the surface. This two-step etch is a short breakthrough step followed by the main RIE etch, where the process parameters are fixed.

[0010] Deep Reactive Ion Etching (DRIE) uses the Bosch process, as described in U.S. Patent No. US5501893A, in which the etching process cycles between an etching step and a polymerization step. This process causes a deep structure to be formed in the film 2 with vertical edges, such as a groove with a horizontal surface corresponding to the bottom of the groove and a vertical surface corresponding to the side of the groove. In the polymerization step, a polymer layer is applied to the exposed vertical and horizontal surfaces. The polymer layer protects the surface from the influence of the DRIE chemical composition. However, in the DRIE process, the physical component penetrates the polymer layer on the horizontal surface, rather than the polymer layer on the vertical surface. Therefore, the polymerization step effectively increases the unidirectionality of the DRIE process, in other words, the anisotropy.

[0011] Although this DRIE process is a cyclic two-step process, the etching parameters generally do not change during the etching step. It is worth noting that as the DRIE process progresses and the structure deepens, the depth of the trench has an impact on the etching step. It should be noted that, as described in European Patent EP 0822584 B1, the original etching parameters are fine-tuned to ensure that the etching rate remains stable as the etching step proceeds. However, the profile of the structure remains unchanged, so this etching process is still considered one-dimensional.

[0012] MEMS manufacturing

[0013] Fabrication of MEMS devices (such as Figure 2 The method of manufacturing the MEMS device 6 is similar to the method of manufacturing the semiconductor device 1 in that the methods primarily use all the same processing techniques. However, one process currently unique to the manufacture of the MEMS device 6 is a release etch process, also known as a sacrificial etch process.

[0014] In the construction of the MEMS device 6, a sacrificial layer 7 is initially deposited on the substrate 3 and then removed by an etching process, which allows the released structure 8 to function as designed (e.g., as a micromirror, accelerometer, or microphone). In some MEMS devices 6, the etching process is to create a cavity that provides, for example, thermal isolation from the underlying substrate 3.

[0015] The structure of the sacrificial layer 7 to be etched, as well as the interaction between the etchant and the sacrificial layer 7, is highly dependent on the MEMS device 6 being fabricated. There is little commonality between the structures of different MEMS devices 6. In this context, the ideal etching process is isotropic, etching uniformly in all directions. Furthermore, it is desirable that the etchant be non-reactive with other materials within the MEMS device 6, thus exhibiting high selectivity.

[0016] The release etching process was initially performed using wet etching, exposing the sample to a bath of chemical etchant. Obviously, this method offers limited control over the etching process, as it is determined solely by the temperature and etchant concentration. Another problem with wet etching is the risk of stiction. As the liquid is removed, the surfaces of the release structure 8 can be drawn together by capillary action, and if these surfaces come together, there is a very strong attractive force holding them together.

[0017] Better process control was achieved using vapor phase etching systems. The first systems using this approach were very basic and employed a pulsed approach, where gas was flowed into a vacuum chamber, the chamber pressure was raised to a predetermined target, and the chamber was maintained at that pressure, with the gas still flowing or completely sealed. This process was then repeated multiple times to complete the etching. Again, there was little control over the etching process, and the etching process typically used the same parameters for all different MEMS device structures.

[0018] One of the most common materials used as sacrificial layer 7 is silicon dioxide, which is etched using hydrogen fluoride (HF) vapor, see for example UK Patent No. GB ​​2,487,716 B. HF vapor etching is a plasma-free chemical etch that etches silicon dioxide isotopically described by the reaction equation:

[0019]

[0020] It was found that water (H2O) ionizes HF vapor as described in equation (1), and then the ionized HF vapor Silicon dioxide (SiO2) is etched, with water (H2O) acting as a catalyst. As is clear from equation (2), the etching reaction itself also produces water (H2O).

[0021] It is generally acknowledged that, in order to carry out HF vapor etching with available etch rate (for example greater than 30 nanometers / minute), need to have condensation fluid layer 9 on the surface to be etched, for example referring to " Vacuum Science and Technology Magazine " A, 10 (4) July / August 1992 (Journal of Vacuum Science and Technology A, 10 (4) July / August 1992) people such as Helms that publish are entitled " Mechanisms of the HF / H O vaporphase etching ofSiO (SiO HF / H O vapor phase etching mechanism) ".Among all compounds relevant to above-mentioned HF vapor etching process, water (H O) has the lowest vapor pressure, therefore forms the basis of condensation fluid layer 9.

[0022] As can be seen from equation (2), H2O is a byproduct of etching, but it also affects the etching rate by contributing to the formed condensation layer 9. When controlling the etching process, the amount of etching that occurs and the amount of H2O that is subsequently produced by the etching must be considered.

[0023] European patent number EP2046677 B1 discloses how to control the formation and composition of condensation fluid layer 9 to be the key of the HF vapor etching of management silicon dioxide.By carrying out HF etching in vacuum chamber, control chamber pressure, temperature and the gas flow rate that flows into chamber, realize accurate etching control.Other parameters that affect HF vapor etching are the composition of etched silicon dioxide layer and deposition method thereof.For example, the silicon dioxide of sacrificial layer 7 is by thermal oxidation or plasma enhanced chemical vapor deposition (PECVD) produces.For identical etching parameters, the density of silicon dioxide is large, and etching process is slow more. Summary of the Invention

[0024] Therefore, an object of embodiments of the present invention is to provide a more efficient method for preparing microstructures compared to the techniques known in the art.

[0025] According to a first aspect of the present invention, there is provided a method of manufacturing a microstructure, the method comprising isotropically vapor etching a sacrificial material, wherein etching parameters of the isotropic vapor etching vary over time in response to (or based on) a priori knowledge of the microstructure.

[0026] Preferably, the a priori knowledge of the microstructure comprises a priori knowledge of the variation of the area and / or position and / or direction of the etch front of the isotropic vapour etch.

[0027] Alternatively or additionally, the a priori knowledge of the microstructure comprises a priori knowledge of the compositional variations of the etched sacrificial material.

[0028] Preferably, the isotropic vapor etching comprises a first phase under a first set of etching parameters within a first time period for etching the sacrificial material at a first volume etch rate.

[0029] Preferably, the isotropic vapor etching further comprises a second stage under a second set of etching parameters within a second time period for further etching the sacrificial material at a second volume etch rate.

[0030] Preferably, the first bulk etch rate and the second bulk etch rate depend on different characteristics of the microstructure.

[0031] Preferably, the first set of etching parameters is different from the second set of etching parameters.

[0032] Optionally, the etching parameters changed between the first stage and the second stage include pressure, gas flow rate, gas flow ratio, gas type and / or temperature.

[0033] Optionally, the first time period is different from the second time period.

[0034] Preferably, the first bulk etch rate and the second bulk etch rate are maintained below a threshold value.

[0035] Optionally, the first and second bulk etching rates are kept below the threshold by fixing the first and second etching parameters from the beginning so that the highest possible etching rate is below the threshold.

[0036] Alternatively, the first and second volume etch rates are maintained below a threshold by varying the first and second etch parameters during the isotropic vapor etch such that the etch rate is below the threshold.

[0037] Alternatively, the first etching parameter may be dynamically varied during the first etching phase. Similarly, the second etching parameter may be dynamically varied during the second etching phase. This dynamic variation of the etching parameter may be considered as fine-tuning the etching parameter to compensate for variations in the etching front.

[0038] Preferably, the first stage of the isotropic vapour etching may correspond to removing the sacrificial material not covered by the mask layer.The sacrificial material is removed mainly in the z-direction.

[0039] Preferably, the second stage of isotropic vapor etching can correspond to the sacrificial material removing under the mask layer.Sacrificial material is mainly removed in the xy direction.The second stage of isotropic vapor etching can be considered as undercut (undercut) stage or release stage.

[0040] Preferably, the isotropic vapor etching further comprises a third stage under a third set of etching parameters within a third time period for further etching the sacrificial material at a third volume etching rate.

[0041] Preferably, the third stage of the isotropic vapor etching can correspond to removing further sacrificial material under the mask layer. The sacrificial material is removed mainly in the xy direction. The third stage of the isotropic vapor etching can be regarded as a deeper undercut stage.

[0042] Alternatively, the second stage of the isotropic vapor etching may correspond to removing sacrificial material having a different density or readiness to etching than the sacrificial material removed in the first stage of the isotropic vapor etching.

[0043] Alternatively, the first stage of the isotropic vapor etch may correspond to removing the sacrificial material in a reaction-limited etch regime.There may be a relatively small amount of sacrificial material to be etched.

[0044] Preferably, the first etching parameters include a relatively high etchant partial pressure by operating at a relatively low carrier gas flow rate and a relatively high chamber pressure.

[0045] Preferably, the second phase of the isotropic vapour etch may correspond to removing the sacrificial material in a transport-limited etch regime.There may be a relatively large amount of sacrificial material to be etched.

[0046] Preferably, the second etching parameters include a relatively high etchant flow rate.

[0047] Preferably, the first and second stages of the isotropic vapor etch comprise substantially the same or similar volume etch rates. Advantageously, the two-stage process maintains a substantial volume etch rate when the etching regime transitions from reaction-limited to transport-limited.

[0048] Preferably, the isotropic vapor etch comprises a plurality of stages, each stage corresponding to an incremental change in etching parameters and / or time period.

[0049] Preferably, the etchant may be HF vapor and the sacrificial material may be silicon dioxide. Alternatively, the etchant may be XeF2 and the sacrificial material may be silicon.

[0050] Additionally, the method may further include monitoring the etching condition using an etching monitor.

[0051] Preferably, the method may further comprise dynamically controlling the etching process by feedback from an etching monitor and / or a priori knowledge of the three-dimensional structure.

[0052] Preferably, the microstructure may be a semiconductor device, a CMOS semiconductor, a MEMS device, a MEMS microphone or a microchannel.

[0053] According to a second aspect of the present invention, there is provided a microstructure manufactured according to the method of the first aspect of the present invention.

[0054] Embodiments of the second aspect of the invention may include one or more features of the first aspect of the invention or its embodiments, and vice versa.

[0055] According to a third aspect of the present invention, a method for manufacturing a microstructure having a three-dimensional structure is provided, the method comprising isotropic vapor etching of a sacrificial material, wherein the isotropic vapor etching comprises two or more stages in response to changes in the three-dimensional structure of the microstructure, the microstructure controlling the volume etching rate in response to (or based on) changes in the etching front and / or changes in the composition of the sacrificial material.

[0056] 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, and vice versa.

[0057] According to a fourth aspect of the invention, there is provided a method for manufacturing a microstructure, the method comprising isotropically vapor etching a sacrificial material, wherein the isotropic vapor etching comprises two or more stages, wherein the volumetric etch rate in each stage is selected (or preselected) in response to (or based on) the following factors:

[0058] (a) changes in the etch front caused by changes in the three-dimensional structure of the microstructure; and / or

[0059] (b) Compositional changes of the etched sacrificial material.

[0060] The microstructure preferably comprises a three-dimensional structure.The variations in the etch front and / or the composition variations are preferably known before performing the method or are anticipated when performing the etching.

[0061] Embodiments of the fourth aspect of the invention may include one or more features of the first and / or second aspects of the invention or embodiments thereof, and vice versa. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Various embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0063] Figure 1 shows:

[0064] (a) perspective view and (b) schematic diagram of a semiconductor wafer prior to a one-dimensional etching process known in the art; and

[0065] (c) Perspective view and (d) schematic diagram of a semiconductor device after a one-dimensional etching process known in the art.

[0066] Figure 2 Shown is a schematic diagram of a MEMS wafer before HF vapor etching as known in the art;

[0067] Figure 3 A schematic diagram of an etching apparatus according to the present invention is shown;

[0068] Figure 4 shows:

[0069] (a) perspective view and (b) schematic diagram of a MEMS wafer before three-dimensional isotropic vapor etching according to the present invention;

[0070] (c) Perspective view and (d) schematic diagram of a MEMS wafer during the first stage of three-dimensional isotropic vapor etching;

[0071] (e) perspective view and (f) schematic view of a MEMS wafer during transition from a first stage to a second stage of three-dimensional isotropic vapor etching; and

[0072] (g) Perspective view and (h) schematic diagram of the MEMS device after the second stage of three-dimensional isotropic vapor etching.

[0073] Figure 5 shows:

[0074] (a) Schematic diagram of a MEMS microphone after a first stage of three-dimensional isotropic vapor etching according to the present invention;

[0075] (b) Schematic diagram of a MEMS microphone after the second stage of 3D isotropic vapor etching; and

[0076] (c) Schematic diagram of the MEMS microphone after the third stage of the 3D isotropic vapor etching process.

[0077] Figure 6 A schematic diagram of a complementary method-oxide-semiconductor (CMOS) sensor prior to multi-stage isotropic vapor etching according to the present invention is shown;

[0078] Figure 7 shows the relationship between (a) XeF2 flow rate and carrier gas flow rate and (b) XeF2 concentration and Figure 3 A graph showing the relationship between carrier gas flow rate changes in etching equipment;

[0079] FIG8 depicts contour plots of etch rate as a function of chamber pressure and carrier gas flow rate for (a) reaction-limited etching regime and (b) transport-limited etching regime;

[0080] Figure 9 depicts a perspective view of the microchannel at the following time

[0081] (a) before performing isotropic vapor etching according to the present invention;

[0082] (b) during the first stage of reaction-limited isotropic vapor etching; and

[0083] (c) During the second stage of transmission-limited isotropic vapor etching.

[0084] FIG10 depicts graphs of (a) etched volume versus time and (b) radius versus time when fabricating the microchannel of FIG9 ;

[0085] Figure 11 The variation of carrier gas (N2) flow rate with etching time in fabricating the microchannel of Figure 9 is depicted. The process recipe includes 1 step (gray), 2 steps (yellow), 3 steps (orange), 9 steps (green), 12 steps (blue), and 15 steps (pink) in carrier gas flow rate.

[0086] In the following description, the same parts are identified by the same reference numerals throughout the specification and drawings. The drawings are not necessarily drawn to scale, and the scale of some parts is exaggerated to better illustrate the details and features of the embodiments of the present invention. DETAILED DESCRIPTION

[0087] Now refer to Figures 3 to 11 To describe the description of the present invention.

[0088] Etching equipment

[0089] Figure 3 A schematic diagram of an etching apparatus 10 suitable for etching a microstructure is shown, such as a semiconductor device 1 or a Figure 2 MEMS devices6. Figure 3 The etching apparatus 10 is suitable for various etching processes. However, it is understood that separate etching apparatuses can be provided for different etching processes. As can be seen, the etching apparatus 10 includes an etching chamber 11 to which are attached six input lines 12, 13, 14, 15, 16, and 17, as well as an output vacuum line 18.

[0090] Within the etch chamber 11 is a temperature controlled pedestal 19 adapted to position the wafers 1, 6 to be etched within the etch chamber 11. Fluids supplied from six input lines 12, 13, 14, 15, 16, and 17 enter the interior volume of the etch chamber 11 via a fluid injection system 20 located within a lid 21 of the etch chamber 11.

[0091] The base 19 where the microstructures 1 and 6 are located can be set and maintained at a base temperature T by a temperature controller. p The temperature may be higher or lower than room temperature, with the specific temperature being selected to optimize the etching process (typically 5-25° C.). Furthermore, during the etching process, the walls of the etching chamber 11 are heated, typically to around 20-70° C.

[0092] The pressure of the etchant gas in the etching chamber is P c is monitored by the chamber pressure controller 22. The pressure controller 22 also incorporates a gas flow controller to provide a means of controlling the pressure within the etch chamber 11 by controlling the operation of a vacuum pumping system 23 located on the output vacuum line 18.

[0093] HF vapor 24 is controllably supplied to the etching chamber 11 from the first input line 12 through a regulator 25 and a first mass flow controller (MFC) 26 .

[0094] A controlled amount of water is supplied to the etching chamber 11 through the second input line 13. In particular, a liquid fluid controller (LFC) 27 and an evaporator 28 located within the second input line 13 are employed to generate controlled levels of water vapor from a water reservoir 29. Nitrogen flow from a nitrogen source 30 to the evaporator 28 is controlled by a second MFC 26. Nitrogen carrier gas is used to deliver the water vapor to the interior volume of the etching chamber 11 through the fluid injection system 20.

[0095] The third input line 14, the fourth input line 15 and the fifth input line 16 are provided for supplying additional gas sources 31, 32 and 33 (such as hydrogen (H2), oxygen (O2) or fluorine (F2) ) ) means connected to the interior volume of the etch chamber 11. Control of the flow rates of these gases is again provided by mass flow controllers (MFCs) 26.

[0096] Xenon difluoride (XeF2) vapor is controllably supplied to the etching chamber 11 through a sixth input line 17, which includes a XeF2 bubbler 34 and a nitrogen source 30. The XeF2 bubbler 34 includes XeF2 crystals. When nitrogen passes through the XeF2 crystals, the XeF2 sublimes and is carried into the etching chamber 11 by the nitrogen. A mass flow controller (MCF) 26 controls the following in conjunction with a pneumatic valve 25a: the nitrogen supply to the XeF2 bubbler 34; the nitrogen supply to the etching chamber 11; and the supply of nitrogen containing XeF2 to the etching chamber 11. The pumping rate of the vacuum pumping system 23 and / or the MCF 26 can be controlled, for example, by a pump control valve to maintain the set operating pressure of the etching chamber 11.

[0097] A computer controller 35 is used to automatically adjust various components and parameters of the etching chamber 11, such as the supply of nitrogen carrier gas, HF vapor, chamber temperature and pressure, etc.

[0098] In manufacturing Figure 2 When the semiconductor device 1 is shown, the applicant has discovered that the surface area of ​​the sacrificial layer 7 exposed to the etchant (in other words, the etch front) affects the etch rate. For example, the area and / or position and / or direction of the etch front can affect the etch rate. Specifically, an etch front with a large area etches faster than an etch front with a small area because an etch front with a larger area produces more H2O byproducts, which in turn contributes to the condensation of the layer 9.

[0099] As the sacrificial etch proceeds, the etch front will change depending on the structure being etched. These changes in the etch front can be quite dramatic. Properties of the etch front, such as its area, position, and / or direction, can change. As the etch front changes, so too can the etch rate. Thus, while the etching process parameters remain constant, the etch rate will vary depending on changes in the surface being etched or in the silicon dioxide material encountered (e.g., density or etch readiness).

[0100] If the area of ​​the etch front decreases or the silicon dioxide material becomes denser, the etch rate may slow to a point where it can no longer continue. If the area of ​​the etch front increases or the etch front encounters a less dense oxide, the etch rate will increase. Very high etch rates can lead to problems such as stiction and metal corrosion.

[0101] Clearly, the etch rate must be controlled to produce a robust etch as it etches through the structure. However, for three-dimensional structures, such as the release structure 8 of the MEMS device 6, it is inefficient to operate the etching process in a one-dimensional manner and consider the etching rate in micrometers per minute. In other words, it is inefficient to operate the etching process in a single time phase without considering predefined, known, or expected changes in the microstructure (i.e., changes in the etch front and / or changes in the composition of the sacrificial material that will change the etch rate).

[0102] Etching method

[0103] The present invention relates to a method for manufacturing a microstructure having a three-dimensional structure. Figure 11 Various examples of this approach are described in detail below.

[0104] In general, all of these examples relate to a method, which comprises isotropic vapor etching of sacrificial material. This isotropic vapor etching comprises two or more time stages, in which the volume etching rate in each stage is selected (or preselected) in response to (or based on) a priori knowledge of microstructure. Specifically, the a priori knowledge of microstructure can comprise: the a priori knowledge of the variation of the area and / or position and / or direction of the etching leading edge of (a) the isotropic vapor etching, and / or the a priori knowledge of the composition variation of (b) the etched sacrificial material. The terms a priori knowledge and predetermined are used interchangeably in this article. Specifically, these terms refer to information known before the isotropic vapor etching begins, by virtue of knowing what microstructure is.

[0105] Volume etch rate is determined by etching parameter, and can be controlled by etching parameter.Therefore, the method also can be represented by etching parameter.Specifically, isotropic vapor etching comprises the etching parameter of the isotropic vapor etching of response to (or based on) prior knowledge of microstructure, time-varying.Term chamber parameter and etching parameter are used interchangeably in this article.

[0106] The method according to the present invention is very different from the standard RIE etching process for semiconductors, such as those described above. The RIE etching process is one-dimensional; for example, the area and direction of the etch front do not change as the etching proceeds. However, the method according to the present invention is three-dimensional. Therefore, the method according to the present invention needs to be viewed and described in a very different way from that known in the art.

[0107] It is worth noting that the isotropic vapor 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 will change. Therefore, the optimized etching parameters for the first stage of etching will not be optimal for the second stage of etching, where different stages correspond to etching different features of the three-dimensional structure. Ideally, in order to achieve an optimized etching process with complete microstructure release and / or maintain a practical etching rate, the etching parameters must be varied as the etching process progresses to adapt to the changes in the structure.

[0108] According to the method of the present invention, Figure 3 The etching process is carried out in the vacuum chamber 11 of the etching apparatus 10 shown. The etching parameters are precisely controlled. The sample temperature, gas flow rate and chamber pressure are precisely controlled and can be adjusted to optimize the settings of the vacuum chamber 11 to maximize the etching effect during the etching process. The conditions of the vacuum chamber 11 are controlled using software, wherein the vacuum chamber 11 control parameters are set using a series of control steps. This series of control steps is generally referred to as a process recipe. In summary, the chamber parameters or settings represent the etching process at a single point in time or a uniform phase, while the process recipe represents how the chamber parameters change from the beginning to the end of the etching process.

[0109] It should be understood that there are many vapor isotropic etchings that are relevant to the present invention. The control of these different etchings can be optimized by changing chamber parameters (also referred to as etching parameters). For different etchings, chamber parameters may be different. As an example, we have described the situation of two kinds of specific etchings below, i.e. HF vapor etching and XeF2 etching.

[0110] First, in the context of HF vapor etching, the etch rate is set by creating and controlling a condensation layer that forms on the exposed surface 5 of the sacrificial material 7. The least volatile compound in the chamber is H2O, and the formation of the condensation layer is related to the vapor pressure of H2O.

[0111] Temperature is a difficult control parameter to change quickly and is usually set throughout the etching process.

[0112] The gases used in the etching process, such as HF, N2, and H2O, have a ratio that determines the etching rate at a given pressure. Gas flow rates can be changed quickly and are used to make small changes in the etching rate. Mass flow controllers (MFCs) 26 are used to precisely control the gas flow rates.

[0113] The main control parameter is pressure. The pressure is precisely controlled by the vacuum pumping system 23 on the output vacuum line of the etching chamber 11, especially the throttle valve. The pressure is gradually changed to the target value and then precisely controlled.

[0114] Example 1 - SOI Wafer Etching

[0115] The common substrate 3 used to fabricate MEMS devices 6 is a silicon-on-insulator (SOI) wafer. There are different fabrication methods for creating SOI wafers, but they all form a single-crystal silicon layer 4 on top of an oxide layer 7, with a silicon substrate 3 underneath. The top layer 4 of silicon is patterned, thus acting as a mask when creating MEMS devices 6. The oxide layer 7 is a sacrificial layer that is etched to release structures 8.

[0116] Figure 4a and Figure 4b The initial structure of the MEMS device shown is identical to the arrangement for a standard semiconductor RIE etch as shown in Figure 1. However, as the oxide etch proceeds, a very different etch process occurs.

[0117] The etching process shown in Figure 4 is an isotropic vapor 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 portion, and the etching occurs in three dimensions. The etch front changes during the etching process. Specifically, the area, position, and direction of the etch front change. Figure 4c and Figure 4d The graph shows that the area of ​​the etch front increases due to the increased area of ​​the oxide layer exposed to the etchant. This has an impact on etching because the etch rate increases as the etch proceeds.

[0118] The etch rate needs to be kept below a certain threshold to ensure that problems such as stiction or metal corrosion do not occur. This can be achieved by controlling the etch parameters in two ways.

[0119] First, the chamber pressure can be set so that as the etching process proceeds, the highest etch rate encountered is below a threshold at which problems occur.

[0120] Alternatively, the etching process can be initiated at a higher chamber pressure to achieve a relatively high etch rate. As the etching process progresses, the etch rate increases as the etch front expands into the microstructure 6. The chamber parameters are modified to keep the maximum etch rate below a threshold where problems may occur. This can be achieved by reducing the chamber pressure, which will return to a value very similar to the simpler case described above. Alternatively, the gas flow rate can be modified again to ensure that the etch rate does not exceed the target value.

[0121] like Figure 4c and Figure 4d As shown, the resulting etched oxide layer 7 is not a direct replica of the mask layer 4 described above. The shape and pattern of the etched oxide layer 7 is similar to the mask layer 4, but it shrinks in the xy direction. In other words, the voids created by the etching process are enlarged.

[0122] As the areas of oxide layer 7 not protected by mask layer 4 are etched away, the underlying silicon substrate 3 is exposed and the etch front begins to change again. The exposed surface of oxide 7 in the xy plane has disappeared and the etching process continues, etching oxide 7 directly below mask layer 4, in other words, undercutting mask layer 4, see Figure 4e and Figure 4f As for the etching front, the area of ​​the etching front is reduced, the etching front direction transitions from along the z-axis to the xy plane, and the etching front position transitions to below the mask layer 4. Therefore, the etching rate will be lower.

[0123] During the undercut etch phase, the etch rate decreases simply because the structure and properties of the etch front have changed. Therefore, the chamber parameters (i.e., etch settings or etch parameters) can also be changed to match the new conditions encountered by 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.

[0124] The undercut etching is continued until the target state of the microstructure 6 device is achieved, for example, the structure 8 is released, as shown in FIG. Figure 4g and Figure 4h shown.

[0125] The etching process clearly has two distinct phases: a first phase in which material is removed primarily in the z-direction with an initial large opening area, and a second phase in which material is removed primarily in the lateral xy-direction with a very different undercut. These two etching phases require at least two very different etching settings (i.e., etching parameters) to match the area of ​​the structure 8 being etched.

[0126] In short, Figure 4a and Figure 4b A MEMS wafer 6 is depicted before isotropic vapor etching. Figure 4c and Figure 4d The MEMS wafer 6 is depicted during a first stage of isotropic vapour etching. Figure 4e and Figure 4f The process of the MEMS wafer 6 transitioning from the first stage to the second stage of isotropic vapor etching is depicted. Figure 4g and Figure 4h The MEMS device 6 is depicted after a second stage of isotropic vapour etching.

[0127] It should be understood that even within these two etching phases, the etch front is changing, so the etching process can be further fine-tuned to compensate for the changing etch front. Therefore, the etching parameters can be considered to be time-varying.

[0128] Furthermore, it will be appreciated that an intermediate stage, or transition stage, may be required during the etching process to transition between two etching stages, as this may require further controlled changes.

[0129] Example 2 - MEMS Microphone

[0130] 5 depicts a MEMS microphone 36, prior to a release etch, having a 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 polysilicon layer 37 and the second polysilicon layer 38. The MEMS microphone 36 also includes a second oxide layer 40 located below the second lower polysilicon layer 38.

[0131] First polysilicon layer 37 includes a plurality of pores 41 through which the etchant can enter first oxide layer 39. In other words, plurality of pores 41 expose surface area 5 of first oxide layer 39 to the etchant, thereby defining an etch front. To release structure 8 of MEMS microphone 36, the release etch removes material from the etch front of first oxide layer 39 in the z-direction and then undercuts first polysilicon layer 37 in the xy-direction. Thus, the area, position, and direction of the etch front change as the etch proceeds.

[0132] After etching, the two polysilicon layers 37 and 38 are free to move. However, if the release etch is not controlled, stiction may occur, and MEMS microphone 36 may not operate as intended. MEMS microphone 36 also includes a metal pad 42. If the release etch is not controlled, metal pad 42 may corrode, which would be detrimental to the operation of MEMS microphone 36.

[0133] This structure 8 of the MEMS microphone 36 can be etched in a single stage, in other words using one process setup with constant parameters. However, there are advantages to changing the etching to a three-stage process.

[0134] The first stage involves etching the MEMS microphone 36 at a high etch rate to ensure a uniform etch initiation across the MEMS microphone 36 before subsequently releasing the structure 8 . Figure 5a The MEMS microphone 36 is depicted after the first stage. For optimal throughput, the first stage should be maximized, but due to the risk of stiction, care must be taken to avoid over-etching and prematurely releasing the structure 8. In the first stage, the direction of the etch front is primarily along the z-axis. As the etch begins to extend laterally in the xy plane, the area of ​​the etch front increases.

[0135] The second phase involves changing the etching parameters and then performing a release phase of the etching process at a lower etching pressure to achieve a slower etching rate. This second phase is performed until the structure 8 of the MEMS microphone 36 is completely released and the etching front moves to undercut the first polysilicon layer 37, as shown in FIG. Figure 5b In the second stage, the area of ​​the etching front is reduced, the etching front direction is in the xy plane, and the etching front position transitions below the polysilicon layer 37.

[0136] The third stage involves changing the etching parameters again and then performing the final stage of the etching process at a higher pressure to achieve a faster etching rate. This third stage results in lateral etching in the xy plane, further undercutting the first polysilicon layer 37, as shown in FIG. Figure 5c It is worth noting that Figure 5c As shown, the second oxide layer 40 is also etched. In this third stage, the position of the etch front advances further below the polysilicon layer 37, requiring different etching parameters to maintain the etch rate.

[0137] Advantageously, this three-step process is faster and more efficient than a single-step process when manufacturing MEMS microphones, while minimizing the risk of failure (i.e., failure due to stiction). The etching settings (i.e., 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.

[0138] Example 3 - Etching Interlevel Dielectric in CMOS Multi-Level Metal Devices ( InterlevelDielectric, ILD)

[0139] MEMS devices have been fabricated using standard complementary metal oxide semiconductor (CMOS) processing to produce sensors using metallized portions of the structure. Examples of such sensors 43 are Figure 6 As shown, it can be seen to include an inter-level dielectric (ILD) layer 44 and a metal layer 45. The functionality of the sensor 43 relies on the removal of the inter-level dielectric (ILD) layer 44 comprising silicon dioxide by HF vapor etching.

[0140] The manufacturing process of the CMOS sensor 43 is designed and optimized to produce high-quality electronic devices, thereby providing the best electronic performance for these devices. In the continuous effort to produce higher-quality devices, the dielectric constant (k) of the ILD layer should be as low as possible, especially in the lower metal levels of the CMOS sensor 43. Compared with a standard oxide film with the same etching parameters, the silicon dioxide of the ILD layer 44 with a low k tends to be easier to etch, in other words, easier to etch.

[0141] As the etching process progresses through the various ILD layers 44 of the CMOS sensor 43, there comes a point where the lower ILD layer 44 begins to etch. At this point, the etch rate of the lower ILD layer 44 will be higher, and if the etch rate becomes too high, problems may arise. To maintain high throughput, the etching parameters must be adjusted to maintain the desired edge rate.

[0142] Instead of manufacturing the CMOS sensor 43 with a single-step process (which would require a low etch rate to avoid any problems and would therefore be slow), it is advantageous to employ a multi-step process depending on which ILD layer 44 is being etched. Thus, the etch settings (i.e., etch parameters) are changed to increase or decrease the etch rate for etching the ILD layer 44 accordingly, resulting in a more efficient process.

[0143] Example 4 - XeF2 Etching of Microchannels

[0144] XeF2 is a vapor that etches silicon isotropically with higher selectivity for silicon than other materials such as silicon oxide, silicon nitride, aluminum, and photoresist.

[0145] The etching rate is controlled by the XeF2 partial pressure. The higher the XeF2 partial pressure, the higher the etching rate.

[0146] The XeF 2 source material is solid and sublimes to provide XeF 2 vapor 34 . Figure 3 The etching apparatus 10 shown uses a solid source bubbler to contain the source material, and a carrier gas flow rate transports XeF2 vapor 34 to the processing chamber. The XeF2 flow rate is determined by the carrier gas flow rate. Figure 7a The relationship between XeF2 flow rate and carrier gas flow rate is plotted. It can be seen that XeF2 increases with increasing carrier gas flow rate. However, the relationship between XeF2 flow rate and carrier gas flow rate is nonlinear. Figure 7b The figure shows the variation of the XeF2 flow rate to carrier gas concentration ratio with the carrier gas flow rate. Although the relationship between the XeF2 flow rate and the carrier gas flow concentration ratio is consistent, it is nonlinear. It can be seen that the XeF2 flow concentration ratio is inversely proportional to the carrier gas flow rate.

[0147] In the absence of etching, for a given chamber pressure, the XeF2 partial pressure is higher at lower carrier gas flows. However, when the sample is etched, the amount of etching has a significant impact on the etching settings (ie, the parameters within the chamber 11).

[0148] When there is a small amount of silicon to be etched, higher XeF2 partial pressures can be achieved by operating at lower carrier gas flows and higher chamber pressures, resulting in higher etch rates. This etch is reaction limited. Figure 8a The relationship between chamber pressure, carrier gas flow rate, and etch rate for reaction-limited XeF2 etching is shown.

[0149] When a large amount of silicon is being etched, the XeF2 flowing into the chamber is consumed relatively quickly, and the XeF2 partial pressure is determined by the rate at which the XeF2 flows into the chamber. In this case, etching is transport-limited, and for higher carrier gas flows, the etch rate is higher, and the effect of chamber pressure is much smaller. Figure 8b The relationship between chamber pressure, carrier gas flow rate, and etch rate for transport-limited XeF2 etching is depicted.

[0150] As previously mentioned, it will be appreciated that the configuration of the processing chamber depends largely on the microstructure being etched.

[0151] Microchannels 46 can be etched in silicon 47 using XeF2 vapor. Before continuing with the XeF2 etch to form microchannels 46, silicon 47 is initially etched to form trenches 48 having polymer sidewalls 49, as shown in FIG. Figure 9a shown.

[0152] The XeF2 vapor etches the exposed silicon 47 at the bottom of the trench 48, from which the etching front area expands to form a microchannel 46, as shown in FIG. Figure 9b and Figure 9c As shown. XeF2 etching is a purely chemical isotropic vapor etching process. 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 formed microchannel 46.

[0153] If XeF2 vapor etching is performed under constant etching parameters, the volume of silicon etched is linearly related to time, such as Figure 10a In other words, the same volume of silicon is etched in the same unit of time, and the channel continues to expand.

[0154] However, if Figure 10b As shown, the variation of the etch channel radius with time (in other words, the one-dimensional etch rate) will not be linear. Figure 10b This gives the impression that the etching process is slowing down, but this is not actually the case since the volume of material removed by etching increases with the radius of the microchannel 46 .

[0155] As will be appreciated, it is advantageous to characterize etching as volumetric etching over time. Therefore, a more relevant measurement is the etch rate in cubic micrometers per minute, which is a three-dimensional etch rate.

[0156] As etching continues, the channel becomes larger and the area of ​​the etch front increases with the radius. However, the area of ​​the etch front increases at a faster rate than the radius, which has a significant impact on XeF2 etching.

[0157] In this example, the etching rate is mainly determined by the XeF2 partial pressure, so the higher the partial pressure, the higher the etching rate. The XeF2 partial pressure in the chamber is controlled by gas flow and chamber pressure. It is also highly affected by the exposed area of ​​silicon 47 (that is, the relative size of the etching front). Since the etching front is relatively small, the etching rate is limited by reaction, and the etching rate is optimized by high etching pressure and low XeF2 flow. Since the etching front has a relatively large area, the etching rate is limited by transmission, and the etching rate is optimized by higher XeF2 flow. In other words, as the etching channel becomes larger, etching changes from one etching regime to another.

[0158] It will be appreciated that if the etching process is run under constant parameters, while etching will continue, the process will not be optimized for variations in the etching process (ie, the evolution of the microchannels 46 and the etch front).

[0159] Conversely, for a given overall etch process time, by defining given parameter start and end values ​​and incremental changes Δ (delta), the total number of steps n, as well as the individual step times, can be determined. This way, the step parameters can be varied or gradually changed over time throughout the etch process. In addition to the etch parameters of interest, the actual step times themselves can also be varied or gradually changed throughout the etch process for additional benefits.

[0160] Vapor-phase etching of bulk silicon or silicon substrates using XeF2 typically requires long etch times, particularly when the etching is a three-dimensional volume. By using a gradual change in the etching parameters, much higher etch rates can be achieved than with conventional single-step processes, helping to reduce etch times. Another benefit is that this can be achieved without using more expensive XeF2 etch precursors.

[0161] In this example, as the microchannel 46 becomes larger, the etching state changes, and the etching rate depends more on the amount of XeF 2 that can be introduced into the chamber 11 . Figure 11 The results show how various etching recipes, specifically the carrier gas flow rate, can be increased over time, resulting in higher XeF2 flows and, in turn, higher etch rates. As the carrier gas flow rate increases, the etching process can be optimized to reduce the overall etching time.

[0162] Variations in etching parameters (ie, specific etching recipes) can be programmed using the software of the computer controller 35. This functionality allows the user to flexibly and efficiently manage the etching process.

[0163] Etch Monitors and Software

[0164] As an optional feature, the etching apparatus 10 may include an etch monitor 50. Observing the conditions within the chamber 11 facilitates setting optimal etching parameters. Understanding the microstructure to be etched and the initial etching conditions, the etch monitor 50 can be used to measure the progress of the etching, verify the etching phase or stage, and appropriately adjust the etching conditions at each stage. The etch monitor 50 can be used to continuously adjust the etching conditions throughout the etching process and at different stages of the etching.

[0165] Etch monitor 50 may take the form of a detector that measures the amount of infrared light passing through processing chamber 11. The amount of light detected is then correlated with the various gas molecules in chamber 11. Knowing the precise amounts of the different gases in chamber 11 is related to the etch rate of the device.

[0166] The etch monitor 50 in conjunction with software can be used to determine when a change in etch conditions is required and move the recipe to the next step.

[0167] By gaining a more detailed understanding of the structure etch and combining it with feedback from the etch monitor to precisely control process parameters, the algorithm can fully control the etch process. Initial conditions are set to optimize the initial etch. The etch monitor 50 continuously observes the conditions in chamber 11, and the software model uses this feedback to determine the optimal chamber 11 conditions for chamber 11 at that moment. The software maintains control of the etch to complete the etch with etch conditions optimized for the entire etch.

[0168] The method for producing a microstructure according to the invention has many advantages compared to methods known in the art.

[0169] The key advantage is that isotropic vapor etching of three-dimensional structures is optimized for that three-dimensional structure. In other words, isotropic vapor etching is considered in three dimensions rather than one. The different stages of isotropic vapor etching can control the volume etch rate, which varies due to changes in the etch front and / or the composition of the sacrificial material.

[0170] In practice, isotropic vapor etching can be advantageously used to more efficiently create microstructures while minimizing the risk of device failure. For example, when releasing the three-dimensional structure of a MEMS device, the volume etch rate can be slowed down to avoid stiction, and then increased after the three-dimensional structure is released to reduce fabrication time. As another example, when fabricating microchannels, the volume etch rate can be varied to accommodate the transition from a reactive to a transport etch regime.

[0171] A method for fabricating a microstructure having a three-dimensional structure is described. The method includes isotropic vapor etching of a sacrificial material. Etching parameters of the isotropic vapor etching are varied over time in response to (or based on) prior knowledge of the microstructure. This method has numerous advantages, including the ability to more efficiently create the microstructure while minimizing the risk of device failure.

[0172] The foregoing description of the present invention has been presented for illustrative and descriptive purposes and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The embodiments described were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suitable for the particular use contemplated. Therefore, further modifications and improvements may be made without departing from the scope of the invention as defined by the appended claims.

Claims

1. A method for manufacturing a microstructure, The method comprises isotropically vapor etching a sacrificial material, in, Etching parameters of the isotropic vapor etch are varied over time in response to a priori knowledge of the microstructure.

2. The method for manufacturing a microstructure according to claim 1, wherein: The prior knowledge of the microstructure includes prior knowledge of the change in the area and / or position and / or direction of the etching front of the isotropic vapor etching.

3. The method for manufacturing a microstructure according to claim 1 or 2, wherein: The a priori knowledge of the microstructure includes a priori knowledge of the compositional variations of the sacrificial material being etched.

4. A method for producing a microstructure according to any one of the preceding claims, wherein: Described isotropic vapor etching comprises: etching the sacrificial material at a first volume etch rate in a first stage under a first set of etching parameters within a first time period; and A second stage under a second set of etching parameters within a second time period is used to further etch the sacrificial material at a second volumetric etching rate.

5. The method for manufacturing a microstructure according to claim 4, wherein: The first bulk etch rate and the second bulk etch rate depend on different characteristics of the microstructure.

6. The method for manufacturing a microstructure according to claim 4 or 5, 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, wherein: The etching parameters changed between the first stage and the second stage include pressure, gas flow rate, gas flow ratio, gas type and / or temperature.

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

9. The method for manufacturing a microstructure according to any one of claims 4 to 8, wherein: The first bulk etch rate and the second bulk etch rate remain below a threshold value.

10. The method for manufacturing a microstructure according to claim 9, wherein: The first and second bulk etching rates are kept below the threshold by fixing the first and second etching parameters from the beginning such that the highest possible etching rate is below the threshold.

11. The method for manufacturing a microstructure according to claim 9, wherein: The first and second volume etch rates are maintained below a threshold by varying the first and second etch parameters during the isotropic vapor etch such that the etch rate is below a threshold.

12. The method for manufacturing a microstructure according to any one of claims 4 to 11, wherein: The first etching parameter is dynamically changed during the first etching phase, and / or the second etching parameter is dynamically changed during the second etching phase.

13. The method for manufacturing a microstructure according to any one of claims 4 to 12, wherein: The first phase of the isotropic vapor etching corresponds to the removal of the sacrificial material not covered by the mask layer.

14. The method for manufacturing a microstructure according to any one of claims 4 to 13, wherein: The second stage of the isotropic vapor etching corresponds to the removal of the sacrificial material beneath the mask layer.

15. The method for manufacturing a microstructure according to any one of claims 4 to 14, wherein: The isotropic vapor etching further includes a third stage under a third set of etching parameters within a third time period for further etching the sacrificial material at a third volume etch rate.

16. The method for manufacturing a microstructure according to claim 15, wherein: The third stage of the isotropic vapor etching can correspond to further removal of the sacrificial material beneath the mask layer.

17. The method for manufacturing a microstructure according to any one of claims 4 to 12, wherein: The second stage of the isotropic vapor etching corresponds to removing sacrificial material having a different density or etch readiness than the sacrificial material removed in the first stage of the isotropic vapor etching.

18. The method for manufacturing a microstructure according to claim 4, wherein: The first phase of the isotropic vapor etching can correspond to the removal of sacrificial material in a reaction-limited etching state.

19. The method for manufacturing a microstructure according to claim 18, 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. The method for manufacturing a microstructure according to claim 18 or 19, wherein: The second phase of the isotropic vapor etching can correspond to removing the sacrificial material in a transmission-limited etching regime.

21. The method for manufacturing a microstructure according to claim 20, 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, wherein: The first and second stages of the isotropic vapor etch include substantially the same or similar volume etch rates.

23. A method for producing a microstructure according to any one of the preceding claims, wherein: The isotropic vapor etch comprises a plurality of stages, each stage corresponding to an incremental change in etching parameters and / or time period.

24. A method for producing a microstructure according to any one of the preceding claims, wherein: The etchant is HF vapor and the sacrificial material is silicon dioxide, and / or the etchant is XeF2 and the sacrificial material is silicon.

25. The method of manufacturing a microstructure according to any one of the preceding claims, further comprising: The etching condition is monitored using an etching monitor, and the etching process is dynamically controlled by feedback from the etching monitor and / or prior knowledge of the microstructure.

26. A method for producing a microstructure according to any one of the preceding claims, wherein: The microstructure can be a semiconductor device, a CMOS semiconductor, a MEMS device, a MEMS microphone or a microchannel.

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

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