Microstructures

Details Microstructures Microstructures

2001-02-27

2002-08-20

Christianson, Keith (Department: 2813)

Semiconductor device manufacturing: process

Miscellaneous

C438S119000, C438S455000, C438S054000, C257S682000

Reexamination Certificate

active

06436853

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to miniaturized devices, referred to as microelectronic devices or microdevices, including those used in integrated circuits, and the invention further relates to methods for fabricating assemblies which contain such microdevices.
BACKGROUND OF THE INVENTION
The fabrication of modern, high-speed microelectronic devices includes a number of intricate and costly processing and fabrication steps which are conducted on a very small, microscopic scale. Various types of such microelectronic devices, as well as microelectronic integrated circuits (IC's) which incorporate as many as thousands of such devices, are fabricated and mass produced on silicon wafers. Each silicon wafer generally comprises an array of numerous electrically-isolated individual integrated microelectronic circuits. Each individual circuit on the wafer typically has numerous fabricated pads which are located proximate to the bulk of the circuit and which are electrically connected to the individual circuit itself. The pads serve as electrical interfaces for routing electrical current through the individual microelectronic circuit. Once a silicon wafer and the array of circuits embedded thereon is completely fabricated, the wafer is then generally sliced apart to thereby physically separate the array of circuits into individual circuits. Each individual circuit on its separated portion of the previously-whole silicon wafer is generically referred to as a “chip.” In this separated chip form, an individual circuit can then be wire-bonded into, for example, a plastic or ceramic package and sealed therein for general electrical and thermal insulation purposes. The pads of the packaged chip are commonly wirebonded to electrical leads mounted on the outside of the package to provide electrical access to the packaged chip via the external leads.
Presently, free-standing micro-structure devices called MEMS (Micro-Electrical Mechanical Systems) are gaining ever-growing popularity in the microelectronics industry. Such MEMS devices may include, for example, a micro-accelerometer, a micro-mechanical filter, a pressure sensor, a gyroscope, or a micro-resonator. In light of such popularity, manufacturers are now attempting to fabricate composite devices which integrate both microelectronic integrated circuits and MEMS together on the same chip. However, due to the unique nature of MEMS devices, more reliable packaging processes and methods need to be developed to simultaneously accommodate both microelectronic integrated circuits and MEMS so that composite devices can be mass-produced commercially.
More particularly, many MEMS devices, by their very nature, must be encapsulated and hermetically sealed within a microshell in order to operate properly. For example, a MEMS device such as a pressure sensor or an accelerometer has movable micro-mechanical parts which must be permitted to move for the MEMS to operate properly. Such MEMS require hermetic encapsulation within a microshell to prevent contaminants, such as dust, from interfering with the MEMS device performance. Furthermore, due to the fact that a typical MEMS device has a size on the order of 10
−6
to 10
−3
meter, the fabrication and precise positioning of a microshell over a MEMS device to thereby encapsulate the device can prove to be a significant challenge, for such a small scale environment can require microfabrication and precision positioning on the order of 10
−7
to 10
−9
meter.
One prior art process method for hermetically encapsulating and thereby protecting a MEMS device (in this instance, a microresonator) is described in U.S. Pat. No. 5,589,082, incorporated herein in its entirety by reference (see FIG.
1
). The initial steps in the process include standard surface micromachining steps which ultimately produce a comb-shaped resonator MEMS device mounted on a silicon wafer substrate (see FIG.
1
(
a
)). Since the fingers of the comb-shaped resonator must be protected so that they are free to move to help operate the resonator correctly, a hermetically-sealed microshell needs to be formed about the resonator. Thus, next, a thick layer of PSG (phosphorus doped glass) is deposited on the silicon substrate so that the PSG surrounds and covers the MEMS device, thereby defining the area to be sealed by a microshell (see FIG.
1
(
b
)). Then, a thin layer of PSG is deposited, patterned, and etched to form etch channels (see FIG.
1
(
c
)). Next, a thin layer silicon nitride is deposited over the thick layer of PSG to define a microshell, and etch holes are thereafter precisely defined in the thin layer of silicon nitride (see FIG.
1
(
d
)). Thereafter, all PSG within the microshell is etched away in a concentrated HF gas bath via the etch holes. After rinsing in water and in methanol, the silicon wafer and its encapsulated MEMS device is dried using a supercritical CO2 process. Finally, a relatively thin layer of nitride is deposited to thereby hermetically seal the MEMS resonator device within its microshell, and contact pads are thereafter etched open to provide electrical access to the MEMS device (see FIG.
1
(
e
)). In this manner, the MEMS device is protected from contaminants, and electrical access is also provided.
This particular MEMS encapsulation process method, as briefly described hereinabove and more fully explained in U.S. Pat. No. 5,589,082, is operable for the very narrow purpose intended, but a simpler and more versatile process method is instead desired. In particular, a process method which hermetically encapsulizes a MEMS device while at the same time is highly compatible with standard process methods used to fabricate microelectronic integrated circuits is highly desirable, for such would enable composite devices to be mass-produced commercially.
Another prior art process method for encapsulating a MEMS device is described in U.S. Pat. No. 5,576,251, incorporated herein in its entirety by reference, wherein two substrates are fused together to form a protective covering for a MEMS device. A bonding material is interposed between the two substrates, and the temperature of the bonding material is raised to about 950° C. for about 30 minutes during which time fusion bonding of the two substrates occurs, and chemical reactions remove gas from the cavity between the two substrates, thereby creating a vacuum and a hermetically sealed enclosure about the MEMS device. In another prior art process method, described in U.S. Pat. No. 5,668,033, global heating is utilized to bond two substrates together within an oven. In this particular method, the entire MEMS structure must be heated in an oven to a temperature sufficient to form the bond between the two substrates. Such a temperature, however, often has undesirable and damaging effects on the MEMS device. In still another prior art process method, described in U.S. Pat. No. 4,625,561, the method therein also teaches bonding by global heating in a furnace. One common aspect of the particular prior art encapsulation and bonding methods alluded to hereinabove is that a high temperature is required to facilitate the process of bonding. As a result, such methods are not suitable for use in situations involving microelectronic integrated circuits and/or MEMS which cannot tolerate exposure to such high temperatures.
In general, micromachining and microfabrication process techniques which are typically used to produce MEMS devices and/or microelectronic integrated circuits are very costly. In addition to being costly to produce, MEMS devices and microelectronic integrated circuits are typically temperature sensitive and can be permanently damaged if exposed to high temperatures. Thus, the integration of a MEMS device with a microelectronic integrated circuit device must be carefully executed so as to not permanently damage any expensive temperature-sensitive device. In light of such, a common critical concern in fabricating a composite device is how to integrate a free-standing MEMS device, which requires a high-temperature bondi

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