Patent Application
20130075795
The present invention relates generally to aerogel dielectric layers, and more specifically, to aerogel dielectric layers on integrated circuit chips.
Monolithic microwave integrated circuit (“MMIC”) chips are often mounted on a circuit board by metal or solder posts to form a MMIC chip system. When a MMIC chip is mounted with the circuits facing the circuit board, it is referred to as a “MMIC flip chip system.” The difference in thermal expansion properties between the MMIC flip chip and the circuit board can create mechanical stresses on the metal or solder posts when the system experiences temperature cycles or large thermal fluctuations. These stresses may render the system unreliable.
A conventional solution is to insert an underfill material having desirable mechanical properties between the MMIC flip chip and the circuit board to reduce the stresses on the metal or solder posts. However, due to undesirable dielectric properties, underfill contacting sensitive components of the MMIC flip chip can degrade the electrical performance of the chip.
Prior solutions to this problem include using tall metal or solder posts that provide sufficient flexibility during thermal expansion to alleviate the need for underfill. Another approach is to apply underfill carefully to avoid the sensitive components of the MMIC flip chip. Both of these approaches require non-standard processes and are therefore unavailable or expensive to implement.
Aspects of the invention relate to aerogel dielectric layers on integrated circuit chips. In one embodiment, the invention relates to an aerogel dielectric layer comprising an aerogel, the aerogel dielectric layer being disposed between a circuit and a circuit board. In another embodiment, the circuit is a monolithic microwave integrated circuit (“MMIC”) flip chip.
In one embodiment, the aerogel has a dielectric constant of approximately 2.0 or less and a compression strength of at least approximately 100 psi. In another embodiment, the aerogel has a dielectric constant of approximately 1.5 or less and a compression strength of at least approximately 100 psi. In another embodiment, the aerogel has a density of approximately 0.05 g/cc to 0.30 g/cc, and a compression strength of approximately 100 psi to 400 psi. In yet another embodiment, the aerogel has a dielectric constant of approximately 1.15 to 1.40, a density of approximately 0.10 g/cc to 0.25 g/cc, and a compression strength of approximately 100 psi to 360 psi.
The aerogel may have a loss tangent of approximately 0.002 to 0.007, and the aerogel dielectric layer may be approximately 5 μm to 20 μm thick.
The MMIC flip chip may be fixed to the circuit board by one or more solder bumps or solder posts. In another embodiment, an underfill material may be applied between the MMIC flip chip and the circuit board. In another embodiment, an aerogel dielectric layer is disposed between the MMIC flip chip and the underfill.
The invention relates to a method for preparing an aerogel dielectric layer on a portion of a circuit, the method including depositing a gelling precursor solution on the portion of the circuit, processing the gelling precursor solution to form the dielectric layer comprising an aerogel on the portion of the circuit, mounting the circuit to a circuit board such that the aerogel is between the circuit and the circuit board, and applying an underfill between the circuit and the circuit board, wherein the aerogel is disposed between the portion of the circuit and the underfill.
Referring now to the drawings, embodiments of an aerogel dielectric layer are incorporated into a MMIC flip chip system including a MMIC flip chip mounted to a circuit board. The circuit board may include any of a variety of known board types, including without limitation, printed circuit boards, printed wiring boards, hybrid boards, and the like. For convenience, the term “circuit board” will be used to refer to any of those types of boards with which the invention is used. In the invention, the aerogel dielectric layer is disposed between sensitive components of the MMIC flip chip, for example a field-effect transistor (“FET”), and the circuit board. In MMIC flip chip systems including an underfill between the MMIC flip chip and the circuit board, the aerogel dielectric layer is displayed between sensitive components of the MMIC flip chip and the underfill.
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Adhesion of the underfill to the MMIC chip relies partially on contact between the underfill and the MMIC chip itself. Therefore, in many embodiments, the aerogel dielectric layer does not cover the entire MMIC chip. However, materials having a high dielectric constant, such as underfill, placed adjacent sensitive components of the MMIC chip, such as a FET, tend to degrade performance of the MMIC chip by interfering with electric fields created during its operation. An aerogel having a low dielectric constant reduces the extent of any interference with these electric fields, and therefore reduces any adverse impact on the MMIC chip performance. Performance degradation due to close proximity to a high dielectric constant underfill can be reduced by providing an aerogel dielectric layer having a dielectric constant of approximately 2.0 or less and a thickness of approximately 5 μm to 20 μm disposed between at least those portions of the MMIC chip having sensitive components and the underfill. In one embodiment, the aerogel dielectric layer has a dielectric constant of approximately 1.5 or less.
In one embodiment, the aerogel of the aerogel dielectric layer has a density of approximately 0.05 g/cc to 0.30 g/cc.
The aerogel must be strong enough to support itself in place during the processes leading up to the underfill application. The aerogel must also be strong enough to resist the flow of underfill during the underfill application so that the aerogel remains in place over the sensitive components of the MMIC chip. In one embodiment, the aerogel of the aerogel dielectric layer has a compression strength of at least approximately 100 psi. In another embodiment, the compression strength of the aerogel is approximately 100 psi to 400 psi.
The aerogel dielectric layer may have a loss tangent of approximately 0.002 to 0.007. A low loss tangent is desirable because it reduces the rate at which electromagnetic energy is dissipated.
Table 1 shows properties for three samples of polyimide aerogels. These three samples exhibit a dielectric constant of approximately 1.15 to 1.40, a density of approximately 0.10 g/cc to 0.25 g/cc and a compression strength of approximately 100 psi to 360 psi.
While the above description contains many specific embodiments of the invention, these should not be construed as limitations on the scope of the invention, but rather as examples of specific embodiments thereof. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their equivalents.
