Methods for adjusting adhesion strength during sensor processing

The present disclosure is directed to systems and methods for adjusting adhesion strength between materials during semiconductor sensor processing. One or more embodiments are directed to using various surface treatments to a substrate to adjust adhesion strength between the substrate and a polymer. In one embodiment, the surface of the substrate is roughened to decrease the adhesive strength between the substrate and the polymer. In another embodiment, the surface of the substrate is smoothed to increase the adhesive strength between the substrate and the polymer.

BACKGROUND

1. Technical Field

The present disclosure is directed to systems and methods for adjusting adhesion strength between a sensor and a substrate.

2. Description of the Related Art

During sensor fabrication, in some types of devices the sensor is processed while attached to a substrate. The substrate acts as a carrier and provides structural and mechanical support to the sensors during subsequent processing steps. In some cases, flexible sensors are formed from a polymer film, such as polyimide, while the polymer film is attached to the substrate.

BRIEF SUMMARY

The present disclosure is directed to systems and methods for adjusting adhesion strength between materials during semiconductor sensor processing. One or more embodiments are directed to surface treating a substrate to adjust adhesion strength between the substrate and a polymer. In one embodiment, the surface of the substrate is roughened to decrease the adhesive strength between the substrate and the polymer. In another embodiment, the surface of the substrate is smoothed to increase the adhesive strength between the substrate and the polymer.

DETAILED DESCRIPTION

FIGS. 1-2collectively show a glucose sensor system10that includes a sensor12for measuring a person's glucose levels. The sensor12is mounted on an insertion support member13as is shown inFIG. 1. In the illustrated embodiment, the sensor12is a flexible bio-medical sensor, and in some embodiments may be a microelectromechanical system (MEMS) sensor. As shown inFIG. 2, a first end14of the sensor12includes sensing electrodes16. Blood is placed in contact with the sensing electrodes16for sensing various levels, such as glucose levels. A second end18of the sensor12includes contact pads20that are electrically coupled to the sensing electrodes16via conductive traces. The sensing electrodes16may comprise a metal material, and in some embodiments, the sensing electrodes16comprise at least one of chrome and gold. The contact pads20are configured to receive signals indicative of the sensed glucose levels from sensed electrodes16via the traces. The glucose sensor system10may further include a support portion14for supporting the insertion support member13and a transmitter15that is configured to wirelessly provide the signals indicative of the sensed glucose levels to a monitor (not shown).

The sensor12is flexible in that it is deformable in response to a force applied thereto. In that regard, the sensor12may be formed from a flexible material, such as a polymer.

FIG. 3illustrates a slide30that includes a plurality of the flexible sensors12ofFIGS. 1-2secured to a substrate34. The flexible sensors12are formed on a polymer40while the polymer40is secured to the substrate34. The substrate34provides structural and/or mechanical support to the flexible sensors12during subsequent processing. The flexible sensors12are arranged on the slide30in spaced relation to an outer perimeter of the slide30. The outer perimeter defines a cutting edge, in which the slide30was separated, such as by laser cutting, from adjacent slides on a wafer as is well known in the art. In some embodiments, portions of the polymer40may be removed from the substrate34at the perimeter of the slides30to define a cutting line.

FIG. 4shows a partial cross section view of a flexible sensor12on the substrate34as shown inFIG. 3. The cross section is taken adjacent to the traces in the flexible sensor12electrically coupling the bond pads to the sensing electrodes to simplify the image. A lower surface36of the polymer40that forms the flexible sensors12is secured to a surface38of the substrate34. The substrate34is any substrate that is suitably rigid to support the flexible sensors12during downstream processing. In one embodiment, the substrate34comprises glass, such as Alumino Silicate 1737, which is boro-aluminosilicate glass. In other embodiments, the substrate34comprises silicon, metal or other rigid materials.

The polymer40used to form the flexible sensor12may be any polymer40configured to shrink during a curing or baking process. Such polymers40may include silicone compounds, polyimides, biocompatible solder masks, epoxy acrylate copolymers, or the like. In one embodiment, the polymer is polyimide Durimide® 116.

The polymer40may be deposited or formed on the substrate34. In one embodiment, the polymer40is coated on the surface38of the substrate34by spin coating on the wafer. In that regard, the polymer40may be dispensed in a flowable form at a center portion of a wafer of which the substrate34is one of many that comprise the wafer; and while the wafer rotates, the polymer40spreads across the surface38of the wafer due to centripetal force. The polymer40is cured and the flexible sensor12may be formed via downstream processing.

An upper surface of the flexible sensor12may further include an insulation layer (not shown), such as a polymer, formed thereon. It is to be understood that the insulation layer on the upper surface of the flexible sensor12may having openings exposing the sensing electrodes and the bond pads discussed in reference toFIGS. 1-2. It is to be appreciated that the insulation layer on the upper surface may be a different material or the same material as the polymer40that forms the flexible sensor12and secures the flexible sensor12to the substrate34.

After processing the flexible sensor is complete, the individual flexible sensors12may be removed, such as peeled as is shown inFIG. 5, from the substrate34and installed in a device, such as the glucose sensor system10ofFIG. 1. In order to prevent damage to the flexible sensors12during the removal process, adequate control over the adhesion between the polymer40of the flexible sensor12and the substrate34is desired. In particular, lower adhesion may be desired such that the sensor12may be easily removed from the substrate34after downstream processing steps have been completed. In other situations, however, higher adhesion may be desired such that the sensor12remains adhered to the substrate34post downstream processing.

Previous ways for adjusting the adhesion strength between a polymer, such as polyimide, and a substrate are rather limiting. Methods for increasing the adhesive strength of polyimide includes adding an adhesive promoter or by curing the polyimide. However, these methods do not allow adequate control over the adhesive properties that may be needed for various applications. For instance, although adhesive promoters and curing are useful for increasing adhesive strength of the polyimide, they do not reduce the adhesive strength of the polyimide. Furthermore, the temperatures in which an adhesive material can exposed may be limited by properties of the adhesive, such as the materials curing temperature and glass transition temperature.

The adhesive strength of polyimide may also be adjusted (i.e. increased or decreased) by varying the thickness of the polyimide. Although this provides some control over the adhesive strength of the polyimide, semiconductor processing design rules typically defines the thickness of the polyimide, thus preventing further adjustments to the thickness to obtain the necessary adhesive properties for a particular application.

According to embodiments of the present disclosure, prior to applying the polymer40to the substrate34, the substrate34may be surface treated to change the surface condition of the substrate34. In that regard, the adhesion strength between the polymer40and the substrate34may be controlled. In the illustrated embodiment, the surface38of the substrate34is roughened as can be seen inFIG. 5as the flexible sensor12is peeled from the surface38of the substrate34and more clearly shown in the close up view inFIG. 6. In some embodiments, the roughened surface38of the substrate34may be achieved by etching, such as wet etching or plasma etching, the surface38. In one embodiment, the substrate34is etched with sulfuric acid.

The inventors unexpectedly discovered that by changing the surface condition of the substrate34, different adhesion strengths between the substrate34and the polymer40can be achieved. In particular, the inventors realized that by roughening the surface38of the substrate34, adhesion strength between the substrate34and the polymer40is substantially reduced. It was also learned that by smoothing the surface38of the substrate34, the adhesion strength between the substrate34and the polymer40may be increased.

One embodiment of the present disclosure will now be described. In this embodiment, the substrate is boro-aluminosilicate glass and a surface of the substrate was roughened via wet etch processing. The substrate will be referred to hereafter as “the treated substrate.” The surface of the treated substrate was roughened using a mixture of sulfuric acid, deionized water, and hydrogen peroxide, where the mixture's temperature was increased to 130° C. After treatment, the treated substrate had an average roughness value (Ra) of ˜1.0 nanometers and a root mean squared roughness value (Rq) of ˜1.33 nanometers. In this embodiment, the polyimide was applied to the surface of the treated substrate to a thickness of about 20 microns ±3, the stack was cured at a temperature of 325° C. ±10 for about 30 minutes, and flexible sensors were formed thereon. The flexible sensors were then peeled from the treated substrate at a peel speed of 1 inch per minute. The adhesion strength between the polyimide and the treated substrate was determined using the following equation:

The peel force was measured to be 20 grams/force (gf) and was compared to the peel force measurement from a substrate that did not undergo surface treatment (referred to herein after as “untreated substrate”). The surface of the untreated substrate had a Ravalue of ˜0.65 nanometers and Rqvalue of ˜0.94 nanometers. The peel force of the untreated substrate was 400 gf.

The results, over 10 times reduction in peel force for the treated substrate over the untreated substrate, were unexpected. It is generally expected that when a flowable adhesive is applied to a roughened surface, the strength of the adhesion increases because the amount of surface area in contact with the adhesive increases. That is, the adhesive flows into the peaks and valleys making up the roughed surface and thus spreads across a greater surface area, than when the surface area is substantially smooth, without peaks and valleys. In this case, however, the adhesion strength unexpectedly decreased. The inventors discovered that because the polyimide shrank during the curing process, the amount of surface contact the polyimide had with the substrate also decreased thereby reducing the adhesion strength between the polyimide and the substrate.

The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. For instance, although the described embodiments are directed to flexible sensors, it is to be understood that the sensors may also be rigid. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.