Posted: May 25, 2018
Contributing Author: Sinjin Dixon-Warren, PhD.
The Bosch Deep Reactive Ion Etch (DRIE) patents continue to play a critical role in Micro-Electro-Mechanical Systems (MEMS) technology. The MEMS industry would not exist without this seminal invention. The Bosch DRIE process enables the formation of micro-mechanical structures in the surface of a silicon or polysilicon layer and is critical to the formation of MEMS devices, including inertial sensors and microphones.
Franz Laermer and Andrea Schilp at Bosch filed their seminal US patent in the United States in on November 27, 1993 (US5501893A). Figure 1 shows the front page of US5501893A, entitled Method of anisotropically etching silicon. The patent was given a foreign application priority date of December 5, 1992, based on an earlier German patent (DE4241045C1). The US patent has now expired, since it has been more than 20 years since the patent was filed; however, TechInsights continues to see evidence that the invention is being widely used in the MEMS industry.
In this article, we will review the essential features of the Bosch invention, and will show examples of where this invention is likely still being used in current MEMS technology.
It is worth noting that Bosch filed their invention as a method patent. The other types of patents, according to US law, are machine patents, article of manufacture patents and composition of matter patents. Method patents are generally more difficult to support through reverse engineering, since there may be other equivalent methods available that result in an indistinguishable final product. Fortunately, the Bosch DRIE process results in artifacts that are observable in the final product, and hence the patent would likely be supportable by reverse engineering through inference from the observed features in the product.
The independent claim 1 of the granted patent describes the essentials of the Bosch invention.
What is claimed is:
1. A method of anisotropic plasma etching of silicon to provide laterally defined recess structures therein through an etching mask employing a plasma, the method comprising:
a. anisotropic plasma etching in an etching step a surface of the silicon by contact with a reactive etching gas to removed material from the surface of the silicon and provide exposed surfaces;
b. polymerizing in a polymerizing step at least one polymer former contained in the plasma onto the surface of the silicon during which the surfaces that were exposed in a preceding etching step are covered by a polymer layer thereby forming a temporary etching stop; and
c. alternatingly repeating the etching step and the polymerizing step.
In plain language, the Bosch process describes a method for etching a trench through a mask hole into a silicon substrate by repeated application of two process steps. First, an anisotropic reactive etching gas is used to etch the silicon for a short period of time. Dependent claim 15 of the patent indicates that the reactive gas is a mixture of SF6 and Ar. Second, a polymerizing etching gas is used to deposit a polymer material onto the etched surface. Dependent claim 16 of the patent indicates that the polymerizing etching gas will be a hydrofluorocarbon. The two steps are repeated and the deposited polymer protects the sidewalls from additional etching, thus resulting in essentially vertical sidewalls. Microscopic examination of the sidewalls, however, reveals undulations or “scalloping,.” the presence of which can be used to infer that the Bosch DRIE process was used. It would likely not be possible to support claims 15 and 16, which relate to the composition of the gases used for the etch and polymerization steps in the process, since normally the polymer and other residues will be removed at the end of the etch process.
The Bosch BMI055 is a compact 6-axis inertial sensor targeted at the mobile and handheld consumer market. TechInsights’ analysis shows that the 3mm x 4.5mm x 1mm BMI055 package contains four silicon die, the CMG130P MEMS 3-axis gyroscope and its associated BAG160BD ASIC and the CMA232M MEMS 3-axis accelerometer and its associated BAA255B ASIC, as shown in Figure 2.
The two MEMS dies are formed using the Bosch DRIE process. Figure 3 and Figure 4 show optical and infrared photographs of the CMA232M MEMS 3-axis accelerometer die found inside the Bosch BMI055. The proof mass of the CMA232M is mounted on a central torsion spring. The device can capacitively sense Y-axis motion by in-plane deflection of the proof mass in the Y-direction. It can sense X-axis motion by in-plane twisting of the proof mass and Z-axis motion by out-of-plane twisting of the proof mass. The device features X, Y and Z interdigitated capacitive sense structures.
Figure 5 presents a plan-view SEM image of the X-axis sense capacitors. These capacitors are wired to sense torsional rotation of the proof mass around the Z-axis of the device. The MEMS structures were fabricated by DRIE of a layer of deposited polysilicon using a mask that was deposited over the surface of the wafer. Figure 6 and Figure 7 show tilt-view SEM images of the X-axis sense capacitors. The DRIE scalloping is quite clearly seen on the sidewalls of the etches structures.
A cross-sectional view of the X-axis sense capacitors is presented in Figure 8. The polysilicon capacitor beams are about 20 µm tall and 2.5 µm wide with highly vertical sidewalls. The scalloping artifacts are visible on the sidewalls of the beams. Figure 9 shows a detailed view of the top of a pair of X-axis sense capacitors. The scalloping features are about 400 nm tall, suggesting that about 50 repeated cycles of the Bosch DRIE etch process were required to etch the 20 µm tall features.
TechInsights has observed scalloping on the vertical sidewalls of MEMS features from virtually all the major MEMS vendors. Figure 10 through Figure 12 show examples of DRIE scalloping artifacts found on products from STMicroelectronics, Invensense and Kionix. TechInsights has also seen DRIE scalloping artifacts on the side walls of Through Silicon Vias (TSVs), as shown in Figure 13.
TechInsights provide patent categorization as service to our clients. We generally rank patent based on two criteria. The first is supportability through reverse engineering and the second is use in the industry. A supportable patent must contain claim elements that would be detectable through reverse engineering. The seminal Bosch DRIE patent US5501893A would likely get a medium rating for supportability, since it describes process features, such as etching of silicon and gas chemistry, that would not be easily supportable by reverse engineering; however, it does produce the scalloping on the sidewalls that is detectable. It would get a high rating for use by industry.
In conclusion, this survey of recent MEMS reverse engineering results shows that the seminal Bosch DRIE patent continues to have wide applicability in the MEMS industry. This is an example of benefits of the patent system. The patent provided protection for Bosch for their invention for twenty-year period, in exchange for disclosure of the details of the invention. The patent has now expired and the invention is now available freely for anyone to use. There are, of course, likely follow on patents which further improve the invention, and these patents may still be in force. According to Google Patents there are 551 patents that cited US5501893A, many which have not yet expired. TechInsights’ business is largely about helping our clients defend their active patents from infringement and assisting with licensing when infringement is found.
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