Patent Application
20070298525
Referring to
In accordance with one embodiment of the present invention, a plurality of metallic upstanding structures 14 (i.e., structures 14a, 14b, and 14c) may be defined. These structures 14 may be made of material suitable for the growth of bridge-like, single walled carbon nanotubes. Specifically, in some embodiments of the present invention, prior to the formation of the electronic features on the opposite side of the substrate 10, the upstanding structures 14 may be formed on the back side 16.
In some embodiments of the present invention, the structures 14 may be formed directly on the substrate 10. The structures 14 may include pillars in one embodiment of the present invention covered by metal catalyst 15 such as iron, cobalt, or nickel. As an example, the structures 14 may be of a height of about a micron. The structures 14 may be formed, for example, by glancing angled deposition methods. By controlling the substrate 10 rotational motion, including both its angle and velocity, the structures 14 height can be controlled. Although different metal catalysts may be utilized, nickel may be preferred because it may offer lower contact resistance with the nanotubes to be formed subsequently.
Referring to
Carbon nanotubes 20 may then be grown so as to bridge between the structures 14a, 14b, and 14c. In one embodiment, gas phase chemical vapor deposition may be used to grow the carbon nanotubes. In one embodiment of the present invention, methane may be used as a source of carbon for the growth of carbon nanotubes. As a result, the nanotubes extend from one upstanding structure 14 to another. Argon gas may be supplied during the deposition of the carbon nanotubes to reduce oxidation. A pressure of about 500 Torr and a furnace temperature of 800 to 950° C. in a methane environment may be utilized in one embodiment.
In one embodiment, the structures 14a and 14c are reasonably proximate, as are the structures 14a and 14b. However, the structures 14b and 14c are spaced sufficiently far apart that carbon nanotubes are not formed between the structures 14b and 14c. For example, in one embodiment of the present invention, a line through the center line of the structures 14a and 14c intersects a line through the center line of the structures 14b and 14a at approximately right angles. In one embodiment, only the structures 14a and 14c, as well as the structures 14a and 14b, are close enough to form bridging carbon nanotubes 20 (
The structures 14 may be formed, in one embodiment, by depositing a catalyst 15 over a pillar pre-formed on a substrate. For example, the pillars may be silicon or silicon dioxide pillars. The pillars may be formed, for example, by growing or depositing the pillar material, masking, and etching to form the pillars in the desired arrangement. In some embodiments, at least two of the pillars may be aligned with a crystallographic plane of substrate 10 (in the embodiment where the substrate is a crystalline semiconductor).
During the catalyst film deposition, the substrate 10 may be tilted twice about plus and minus 45 degrees to spread the catalyst 15 over the structures 14. The carbon nanotubes 20 later form on the sidewall of structures 14 where the catalyst 15 is present. The catalyst 15 may not completely cover the pillars in some cases.
In some embodiments, an array of pillars (not shown) may be grown, but only some of the pillars may be activated with catalyst. For example, only three pillars may be activated with catalyst so that right angled arrays of carbon nanotubes are formed. The selective activation may be accomplished using masks or selective catalyst deposition. While cylindrically shaped structures 14 are depicted, other shapes may also be used.
As shown in
Because of the angulation between the sets of carbon nanotubes secured between structures 14a and 14c versus those secured between structures 14a and 14b, strain in the nanotubes can be measured in two dimensions. For example, the two sets of carbon nanotubes may be perpendicular to one another. The strain in the nanotubes 20 correspond to the strain in a device under test secured to the nanotubes 20 and structures 14.
In some embodiments, and particularly in embodiments in which the structures 14 are formed directly on a silicon die, the substrate 10 may subsequently be thinned down so that its own thickness does not contribute to changes in stress of the die whose stress is being measured. A thinned down substrate may also be glued onto any polymeric or ceramic surface.
The nanotubes 20 may be electrically coupled to an external strain gauge (not shown) using metal lines, as shown in
The wire or electrical connection 26 may be connected to a strain gauge. When the nanotubes are strained, a voltage change across the nanotubes is proportional to the strain experienced by the nanotubes.
In order to measure the strain on a semiconductor die, the nanotubes 20, shown in
As indicated in
Other structures 14 may also be utilized to grow bridge-like carbon nanotubes, including telephone pole and soccer goal oriented office staples. Literally, upstanding office staples may be utilized by securing them to silicon wafers using an appropriate adhesive such as carbon tape. The staples may have their points upstanding (“telephone poles”) or inverted (“soccer goal”) and extending into the substrate.
Then, carbon nanotubes may be grown using chemical vapor deposition and a furnace at 1373 Kelvin under about 100 mTorr vacuum. To 0.02 g/ml solution of ferrocene in 10 milliliters of hexane, two volume percent thiophene is added. The hexane may act as the source of carbon and the ferrocene acts as a catalyst for gas diffusion formation of carbon nanotubes. The solution may be heated to 150° C. and then introduced into a horizontal quartz tube furnace at an average rate of about 0.1 milliliters per minute for ten minutes. Thiophene is known to promote the formation of single-walled carbon nanotubes in H2 atmosphere, whereas multi-walled carbon nanotubes are found to grow predominantly in the absence of H2 atmosphere. Single-walled carbon nanotubes or multi-walled carbon nanotubes can be used by controlling the nanotube growth conditions via controlling the H2 concentration in the furnace [No H2 atmosphere will give multi-walled carbon nanotubes, whereas H2 atmosphere may promote the single-walled carbon nanotube growth]. Although the recipe and numbers recited above are recommended to grow carbon nanotubes, the growth conditions are not necessarily limited to this recipe or these numbers, but rather is inclusive of those.
Then the whole die, including the nanotubes 20, may be coated with the die attach 28, followed by the die attach cure. Stress changes during the cure process may be measured by various nanotubes 20. The structure 14 height can be controlled to a few microns, such as 10 to 15 microns, so that the structure is less than about 25 microns in total height.
Referring next to
An organic or other substrate 17 may have a die 35 secured thereto which includes structures 14, pads 22, and metal lines 24. The mold compound 30 may then be added over the top to form a semiconductor package 36.
In some embodiments, the nanotubes 20 may be highly accurate stress indicators. Of course, a stress indicator is correspondingly also a strain indicator. Because they have anisotropic characteristics in the length dimension and have very small dimensions transversely to the length dimension, high special resolutions may be obtained with carbon nanotubes. For example, the carbon nanotubes may be 1 to over 10 microns in length and less than 2 to 30 nanometers in diameter.
Because they tend to be atomically relatively perfect and chemically stable, carbon nanotubes can be more reliable as sensors than metallic structures of the same dimensions. Moreover, due to their anisotropic nature, nanotubes can potentially measure the stress tensors on the die. In some embodiments, the state of stresses, at locations that are separated by distances as small as a few microns to a few hundred nanometers, may be measured. In some cases, spatial resolution of a half micron may be possible.
The nanotube 20 to metal structure 14 contact resistance can be improved using various strategies. In one embodiment, electron beam irradiation at the nanotube 20/structure 14 junction may be used. As another alternative, small dots of solder may be deposited on the structure 14/nanotube 20 joint, followed by reflow.
Referring to
As a result, the effect of stress on its operation may be monitored before the die is sold, as well as after the die is sold, in some cases. All that is necessary, in some embodiments, to record the stress is to attach a measuring apparatus to the nanotubes 20 via the pads 22. However, it may also be possible to measure the stress “on die,” for example, using circuitry formed on the die front side.
References throughout this specification to “one embodiment” or “an embodiment” mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation encompassed within the present invention. Thus, appearances of the phrase “one embodiment” or “in an embodiment” are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be instituted in other suitable forms other than the particular embodiment illustrated and all such forms may be encompassed within the claims of the present application.
While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.
