Patent Application
20080038922
The example embodiments are best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Wherever applicable and practical, like reference numerals refer to like elements.
The terms ‘a’ or ‘an’, as used herein are defined as one or more than one.
The term ‘plurality’ as used herein is defined as two or more than two.
In the following detailed description, for purposes of explanation and not limitation, specific details are set forth in order to provide a thorough understanding of example embodiments according to the present teachings. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of apparati, materials and methods known to one of ordinary skill in the art may be omitted so as to not obscure the description of the example embodiments. Such apparati, methods and materials are clearly within the scope of the present teachings.
In the illustrative embodiments, the etch-stop layer 102 is a photoimagable epoxy. The photoimagable epoxy comprises an epoxyfunctional resin adapted for curing by an action of a cation-producing photoinitiator. In certain embodiments, the etch-stop layer 102 is a negative photoresist commercially available from MicroChem Corporation of Newton, Mass. USA and sold under the tradename SU-8 and progeny thereof. This photoresist is also described in U.S. Pat. No. 4,882,245, the disclosure of which is specifically incorporated herein by reference. In other embodiments, the etch-stop layer 102 may be a positive photoresist, such as benzocyclobutene (BCB), which is well known to one of ordinary skill in the art.
The etch-stop layer 102 is spun-on the surface of the first side 103 using standard technique. In certain embodiments, the thickness of the layer 102 is in the range of approximately 2.0 μm to approximately 10.0 μm. In other embodiments, the thickness may be as great as 50.0 μm. In yet other embodiments the thickness may greater than 50.0 μm.
After the etch-stop layer 102 is spun-on, a softbake step is carried out to remove some of the solvents. Next, as shown in
In certain illustrative embodiments, SU-8 may be disposed over the second side 104 and used to form the etch mask 105. As noted previously, SU-8 is a negative photoresist, and thus can be patterned without the need of another photoresist patterning step. Accordingly, SU-8 or other like-materials can be used as the etch-mask to decrease the complexity and the time of processing the substrate 101. The patterning of the SU-8 layer or similar material to form the etch mask 105 is also carried out by known methods.
After the etch mask 105 is formed, an etching step is carried out to form openings 106 in the substrate as shown in
The DRIE process may be a cryogenic etching method or a time multiplexed or pulsed etching method (known as the Bosch method). The method may include a three-step sequence using SF6 for etching and C4F8 for passivating. As these DRIE methods and materials are known to those skilled in the art, details are omitted to avoid obscuring the description of the embodiments.
As noted, in illustrative embodiments, SU-8 and like materials may be used as the etch-stop layer 102. SU-8 and like materials are highly resistant to SF6/C4F8 and other materials typically used in DRIE processes, allowing the etch-stop layer 102 can be comparatively thin, while providing good clearing of substrate etch. Moreover, the use of these materials as etch-stop layer 105 materials substantially prevents punch-through problems that plague certain known etch-stop materials used in DRIE.
As will be appreciated by one of ordinary skill in the art, a comparatively thin etch-stop layer is useful in MEMS applications in order to achieve desired structures, features and feature dimensions. Beneficially, even though the etch-stop layer 102 of the illustrative embodiments can be as thin as approximately 2.0 μm to approximately 10.0 μm, the selectivity to etching of the substrate 101 is comparatively high, allowing for precision in the forming of the features in the substrate. For example, according to representative embodiments the selectivity was SU-8 to Si was greater than approximately 100:1.
As shown in
In accordance with representative embodiments, layers of SU-8 having a thickness in the range of approximately 5.0 μm to approximately 50.0 μm were provided as etch-stop layer 102. The etch-stop layer 102 spanned openings 106, which were circular openings having diameters approximately 500.0 μm and greater. Etch-stop layers of SU-8 having the thicknesses noted above were found to provide suitable structural strength for many applications. In one specific embodiment, an etch-stop layer 102 of SU-8 having a thickness of approximately 10.0 μm to approximately 15.0 μm disposed over an opening 106 required pressure of 90 psi to be broken. It is emphasized that these quantitative examples are merely representative.
Moreover, because the etch-stop layer 105 of the illustrative embodiments is structurally strong, it is generally not pliable. Thus, at locations 107 where it is unsupported by the substrate 101, the etch-stop layer 105 does not appreciably ‘sag’ into or otherwise substantially enter the opening 106.
In many applications this is a useful result. For example, in microfluidic applications laminar flow is realized by comparatively smooth surfaces in the channel of fluid flow such as formed by the walls of the opening 106 and a comparatively flat membrane formed by the etch-stop layer 105 of the example embodiments. By contrast, the flow characteristics of fluid are altered by ‘sagging’ membrane formed by known etch-stop layers such as polyimide or Poly Di-Methyl Siloxane (PDMS) that are not as structurally robust. Among other problems, this can lead to undesirable turbulent fluid flow.
In their function as etch-stop layer 102, SU-8 and like materials of the illustrative embodiments are substantially uncured and substantially unexposed. As noted previously, SU-8 and like materials are photolithographically patternable. In the present embodiment, after the openings 106 are formed, the etch-stop layer 102 is exposed to light of a suitable wavelength through a mask (not shown). After exposure, the etch-stop layer 102 is developed with appropriate developing chemical to provide a photolithographic pattern 201.
Accordingly, the etch-stop layer 102 is adapted to provide two functions: an etch-stop and a resist pattern for subsequent processing. In addition, and unlike many other known etch-stop layers, which are not photolithographically patternable, no additional processing is required to provide the resist pattern. To this end, in order to etch known etch-stop layers in a desired pattern, the etch-stop layer must be coated with a photoresist; the resist must be exposed and developed; and likely stripped after etching or other processing is completed. Thus, because the etch-stop layer 102 of the illustrative embodiments can be photolithographically patterned, an extra masking step can be avoided. Beneficially, the use of SU-8 and like materials for the etch-stop layer 102 makes patterning more efficient and less costly than other etch-stop layers.
In connection with illustrative embodiments, an etch-stop layer and methods of use are described. One of ordinary skill in the art appreciates that many variations that are in accordance with the present teachings are possible and remain within the scope of the appended claims. These and other variations would become clear to one of ordinary skill in the art after inspection of the specification, drawings and claims herein. The invention therefore is not to be restricted except within the spirit and scope of the appended claims.
The present application is related to U.S. patent application Ser. No. ______ (Avago Docket Number 10060280-1) filed concurrently herewith and entitled “Electrically Addressable Liquid Dispenser” to Lamers, et al. The disclosure of this application is specifically incorporated herein by reference.
