Patent Application
20060286487
The present invention relates generally to patterning features on a substrate and in particular, but not exclusively, to applying multiple layers of photoresist to pattern three-dimensional features on a substrate.
Photoresist is commonly used to pattern features on substrates. Much like plywood forms are used in building concrete structures, photoresist layers are usually temporary layers that, when applied and patterned, create forms that can then be filled, coated, etc, with other materials to create structures on a substrate. Once the other materials placed on or in the photoresist are themselves patterned and etched, the photoresist is stripped away, leaving behind a structure on the substrate. Examples of devices in which photoresist is used include microelectronics, microelectromechanical (MEMS) devices, and the like.
In their un-cured form, photoresists are viscous liquids that are applied to the substrate in liquid form and must then be cured before they can be patterned and etched to form features on the substrate. Solvents comprise forty to sixty percent of a typical photoresist, with the remainder being made up of other compounds dissolved in a solvent. Curing involves heating the photoresist so that the solvents evaporate, leaving behind a solid layer.
In some applications it is desirable to use multiple layers of photoresist to create three-dimensional features on a substrate. Certain characteristics of photoresist, however, make this difficult. Cured photoresist is very absorbent of liquids. As a result, if a second layer of photoresist is applied to an existing first layer of cured photoresist, the first layer will absorb solvents from the second layer as soon as the second layer is deposited on the first layer. As it absorbs solvents from the second layer the first layer swells, causing any forms already patterned and etched to deform. The end result is that the first and second layers end up oozing together. In addition to being very absorbent, the viscosity of photoresist can cause gas bubbles to be trapped within the photoresist. As the photoresist is cured, these gas bubbles tend to expand and the photoresist surrounding the gas bubbles hardens, leaving behind voids in the cured second layer. These voids are serious defects that can make it difficult or impossible to properly pattern forms in the cured second layer of photoresist.
Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.
Embodiments of a process and for applying multiple layers of photoresist on a substrate are described herein. In the following description, numerous specific details are described to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail but are nonetheless encompassed within the scope of the invention.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in this specification do not necessarily all refer to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
The first photoresist layer 104 is deposited on the surface of the substrate 102 and cured. Once cured, the first photoresist layer is patterned and developed (e.g., etched) to created the form 106 therein. In one embodiment, the first photoresist layer is patterned using standard photolithographic techniques in which a mask is used to selectively expose certain areas of the photoresist to radiation. In other embodiments, however, other techniques such as electron beam lithography can be used to pattern the first photoresist layer. Once patterned, the photoresist is developed (e.g., etched) to create the form 106. The form 106 has a height δ and in the embodiment shown it is a rounded form. In other embodiments, the form 106 could be any shape that can be patterned and etched in the first photoresist layer; examples include square, triangular, and other compound and complex shapes.
The material from which the sealing layer 202 is made depends on the type of feature is being built on the substrate and what subsequent processes will be used to build that feature. In an embodiment where the feature being built is a three-dimensional metal feature formed using a plating process, the sealing layer 202 can be an electrically conductive layer that both seals the first photoresist layer 104 and provides a seed layer for the subsequent plating process. The conductor can be a metal such as copper (Cu), gold (Au), platinum (Pt), silver (Ag) and the like, or can be a conductive non-metal. The method chosen for depositing the sealing layer will depend in part on what material is used. In the embodiment shown, the sealing layer 202 is a metal seed layer deposited by sputtering, which is an example of a physical vapor deposition (PVD) method. In other embodiments the sealing layer can be deposited on the substrate using one or more methods known in the art including, for example, chemical vapor deposition (CVD).
The second photoresist layer 302 is deposited with thickness Δ; the value of Δ will depend on the height δ of the form 106 in the first photoresist layer, as well as on the desired dimensions of features that will later be formed using the second photoresist layer 302. In one embodiment, δ has a value of 45 μm and the second photoresist layer 302 has a thickness Δ of 70 μm. In another embodiment, δ can have a value of 70 μm and Δ a value of 100 μm. In still other embodiments, δ and Δ each can have values that are different than those in the listed examples and, moreover, δ and Δ need not have the same ratio as those in the listed examples. The second photoresist layer 302 can be deposited on the sealing layer 202 using one or more techniques known in the art, such as spin-coating.
The temperature increase from T0 to TC takes place over a set period of time Δt. The value of Δt can be empirically determined, and will usually depend on the thickness Δ of the second photoresist layer 302, as well as on the particular type of photoresist used. In one embodiment, for example, the initial temperature T0 is an ambient temperature of about 34° C., the cure temperature TC of the photoresist is about 100° C., and the time Δt is about 650 seconds. In the embodiment shown the increase in temperature of the second photoresist layer 302 between the initial temperature T0 and the cure temperature TC is linear and monotonic, but in other embodiments the temperature increase need not be linear or monotonic. Ramp-baking the second photoresist layer as shown prevents the formation of voids due to outgassing.
Once the temperature of the second photoresist layer 302 reaches the cure temperature TC, the layer is maintained at that temperature for a period of time necessary for the photoresist to finish curing. Once the photoresist is fully cured, the substrate build-up, and thus the second photoresist layer, is allowed to cool such that it returns roughly to ambient temperature, at which time the second photoresist layer 302 is fully cured and the substrate build-up is ready for subsequent patterning and development (e.g., etching). In other embodiments, however, it may be desirable not to allow the temperature of the second photoresist layer to return to ambient temperature before subsequent processing.
As with the temperature profile shown in
After the first photoresist layer 104 is removed, a free-standing three-dimensional feature 504 is left behind. The feature 504 includes the plating layer 502 and a portion of the sealing layer 202. Of course, the free-standing feature 504 is merely an example of the type of feature that can be built with the illustrated process embodiment. By constructing the proper free-standing features, or by combining various free-standing features, the process can be used to build devices such as switches, relays, capacitors, inductors, microelectromechanical (MEMS) devices, and the like.
The above description of illustrated embodiments of the invention, including what is described in the abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. These modifications can be made to the invention in light of the above detailed description.
The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.
