Patent Application
20090081828
The present invention relates to microelectromechanical systems (MEMS). More particularly, the present invention relates to MEMS fabrication methods.
Microelectronic and microelectromechanical devices, such as microelectronic integrated circuits (ICs) and MEMS devices, not only offer the advantages attendant to miniaturization, but also afford improvements over the performance of macro scale devices, which generally range in size from tens to hundreds of millimeters (mm). Additionally, MEMS devices may exploit principles that work exclusively on a micro scale, which generally ranges in size from a micrometer (μm, or one-millionth of a meter) to a millimeter. MEMS technology has already been applied to various electromechanical devices, including pressure and inertia sensors, micro-fluidics devices, radio frequency (RF) and optical devices, such as switches, mechanical resonators, phase shifters, etc., and so on.
MEMS devices employ three-dimensional, movable (and/or fixed) mechanical structures, such as cantilevers, membranes, cavities, channels, etc., that are machined using micro-fabrication techniques. Specifically, MEMS devices typically combine surface and/or bulk micro-machined actuating and/or sensing elements with electronic signal processing circuits on a single chip (or die). MEMS technology provides many benefits when compared to macro scale piezoelectric and capacitive devices, such as low cost, stable sensitivity, high reliability, ease of use, etc., as generally noted above.
Microelectronic ICs are solid, compact, and lack these three-dimensional mechanical structures. Consequently, many of the techniques developed for fabricating microelectronic ICs are not readily adaptable to MEMS device fabrication. For example, batch processing of microelectronic IC wafers enables these manufacturers to significantly scale down the size and cost of these devices. However, batch processing of MEMS wafers is difficult and prone to lower yields because the three-dimensional mechanical structures are susceptible to damage caused by the singulation process, which may include dicing, sawing, scribing, drilling, etc., of the wafer. Coating the three-dimensional mechanical structures after they have been released from the substrate, but before the wafer is singulated, is not desirable for several reasons, including the inducement of stiction failures by the subsequent cleaning step. Additionally, releasing each MEMS device (or die) after the wafer is singulated is also not desirable because this would effectively eliminate the benefits derived from batch processing. Accordingly, a method for fabrication of a MEMS device that releases the three-dimensional structure before wafer singulation, and without a post-singulation cleaning step, is highly desirable.
Embodiments of the present invention provide methods for singulating microelectromechanical systems (MEMS) die from a wafer. A plurality of MEMS devices are formed on the top surface of a wafer, and a plurality of intersecting scribe lanes are then formed, on the bottom surface of the wafer, to define a plurality of dies, each including at least one MEMS device. The intersecting scribe lanes penetrate the wafer to a depth of about 80%, and the wafer is cleaved along the scribe lanes to separate each of the plurality of dies from the wafer.
The above and other advantages of this invention will become more apparent by the following description of invention and the accompanying drawings.
a depicts a top surface of a MEMS wafer, according to an embodiment of the present invention.
b and 1c depict a top view of a MEMS device, according to an embodiment of the present invention.
a depicts a bottom surface of a MEMS wafer, according to an embodiment of the present invention.
b depicts a cross-sectional view A-A′ of the MEMS wafer of
Embodiments of the present invention provide a fabrication method for MEMS devices that advantageously minimizes MEMS die separation force and stress, shortens fabrication cycle time and provides lower cost, higher performance and higher die yields than existing singulation techniques. Many prior art IC die singulation techniques separate individual dies from silicon (Si) wafers by scribing the top surface of the wafer to a certain depth and then applying a force adjacent to these lines to cleave the wafer into individual IC dies. This technique is possible, generally, because Si, and particularly (100) oriented Si, has cleavage planes parallel to the major flats of the wafer. However, MEMS die sizes are on the order of few millimeters on a side. Consequently, this process becomes difficult because greater force is required to cleave the smaller MEMS die, which introduces undesirable stresses in the singulated MEMS device.
Conventional IC dicing, using a dicing saw, for example, typically requires that the top surface of the wafer be protected by a photoresist layer that is removed after dicing by wet chemistry, for example. As discussed above, a MEMS die contains devices that typically include structure that is supported only along the edges, such as cantilevers or diaphragms, which precludes the use of wet chemistry. While it may be possible to remove a protective photoresist layer by dry etching, all of the residue may not be removed from the top surface, which, of course, contaminates the MEMS device. Similarly, scribing the top surface of a wafer with a laser also introduces debris, and the removal of any protective photoresist layer would have the same problems described for conventional IC dicing.
Embodiments of the present invention provide methods for singulating a MEMS die from a wafer by scribing the bottom surface of the wafer to depth of about 80% and then cleaving the wafer along the scribe lanes by applying a force to the top surface of the wafer, which advantageously maintains the cleanliness of the top surface of the wafer, and, therefore, the MEMS devices. A preferred embodiment uses a laser to form intersecting scribe lanes on the bottom surface of a wafer, while additional embodiments advantageously accommodate unique, protruding MEMS die geometries, even in single crystalline substrates, such as Si and gallium arsenide (GaAs).
a depicts a top surface of a MEMS wafer, according to an embodiment of the present invention. Wafer 100 is formed from a suitable substrate material, such as Si or GaAs. Preferably, (100) oriented Si is employed, for the reasons noted above. Prior to forming the MEMS devices on top surface 102, wafer 100 may be mounted to carrier 104 in order to facilitate handling, processing, etc. Generally, any number of MEMS devices may be formed on the top surface 102 of wafer 100, using a variety of techniques, such as, for example, bulk micromachining, wet etching, dry etching, surface micromachining, deep reactive ion etching micromachining and micro-molding, etc. Preferably, the MEMS devices are formed in a symmetric lattice or grid arrangement to more easily comport with the inventive die singulation techniques discussed herein. Exemplary MEMS devices include structures on the order of about 100 μm in height.
Two different MEMS devices are depicted in
As discussed above, wafer 100 may be mounted on a carrier 104, such as, for example, a 6″ Si carrier wafer that has been cut as a doughnut, which supports wafer 100 along its edges. Carrier 104 protects the top side 102 of wafer 100 from any scratches, which eliminates the need to apply photoresist in order to protect the MEMS devices located on top side 102. Advantageously, all debris from the scribing process are deposited on the bottom surface 202 of wafer 100, which further protects the MEMS structures, and their attendant critical surfaces, on the top side 102 of wafer 100.
a depicts a bottom surface of a MEMS wafer, according to an embodiment of the present invention. Intersecting scribe lanes 210, 220 are scribed into wafer 100 to a relative depth of about 80% of the thickness of wafer 100. In a preferred embodiment, a Nd-YAG laser (e.g., Laser Corp. Model 4024) operating at 1064 nm, forms scribe lanes 210, 220. In one example, with the laser power set to 0.85 Watts (average) and Q-switched at 2.0 kHz, a four inch wafer can be scribed to a depth of 80% using cutting gas SF6 with a flow rate 10 SCFH, at a feed rate of 0.9 inches per second, in 10 passes. The scribing process is preferably computer controlled, thereby allowing precise alignment of scribe lanes 210, 220 on bottom surface 202 with respect to the layout of MEMS device 110, 120 on top surface 102. For example, optical registration marks may be provided for this purpose. This alignment is indicated in
For convenience, scribe lanes 210 are denoted “vertical” lanes, while scribe lanes 220 are denoted “horizontal” lanes. Of course, this nomenclature is arbitrary and not intended to limit the invention in any manner. Scribe lanes 210 include vertical lanes “1” through “10,” while scribe lanes 220 include horizontal lanes “a” through “k”. As clearly shown in
In another embodiment, scribe lanes 210, 220 conform to protrusions extending from the MEMS device.
An exemplary application for the present inventive technique is the fabrication of a mass imaging spectrograph on a chip. This small, portable, inexpensive MEMS-based instrument can be used, inter alia, to detect and identify dangerous chemical and biological molecules locally and in real-time, rather than at a remote location, such as a laboratory.
In another embodiment, the base wafer may contain an ionizer along with portions of the ion optics and the ion collector on a single die. On a single 6 inch silicon wafer, many such base die may be fabricated. The pump and lid die may be fabricated separately, on 4 inch wafers, for example. The detector array is a CMOS-based design, and may be fabricated on 8 inch wafers, for example. After singulating these die, individual parts are inspected and assembled on the base chip using a flip-chip soldering technique. This hybridized assembly is then joined on a mounting substrate to provide connection to power supplies and a controlling microprocessor.
While this invention has been described in conjunction with specific embodiments thereof, many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth herein, are intended to be illustrative, not limiting. Various changes may be made without departing from the true spirit and full scope of the invention as set forth herein.
