Abstract:
Humidity sensors may exhibit a relatively small amount of hysteresis and/or a faster response time. In some cases, a humidity sensor may include a polymeric humidity sensing layer disposed over an electrode layer. The polymeric humidity sensing layer may include a halogenated layer disposed over the polymeric humidity sensing layer. The polymeric humidity sensing layer may, for example, include a polyimide and the halogenated layer may include a monolayer or less than a monolayer of a halogenated material such as a fluorinated material.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 62/117,208 filed Feb. 17, 2015 entitled SURFACE CHEMISTRY MODIFICATION OF HUMIDITY SENSING FILMS, which application is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    Capacitive and resistive type humidity sensors rely on the ability of the sensing material to quickly absorb and desorb water molecules. The absorbed moisture changes the physical properties of the sensing material either by changing its resistance, permittivity, or stress, which can each directly affect the electrical response of the sensor. Bulk polyimide films are often used as the humidity sensing material in many capacitive and resistive humidity sensors. However, humidity sensors using bulk polyimide films typically exhibit an inherent hysteresis due to the adsorption, absorption, diffusion, and desorption properties of the films. Also, such sensors often have a relatively large response time. What would be desirable is a humidity sensor that exhibits a relatively small amount of hysteresis (e.g. 2 Sigma hysteresis of 1% or less) and/or a faster response time. 
       SUMMARY 
       [0003]    The disclosure generally relates to humidity sensors that exhibit a relatively small amount of hysteresis and/or a faster response time. In some cases, a humidity sensor may include a polymeric humidity sensing layer disposed over an electrode layer. The polymeric humidity sensing layer may include a halogenated layer disposed over the polymeric humidity sensing layer. In some instances, the polymeric humidity sensing layer may, for example, include a polyimide and the halogenated layer may include a monolayer or less than a monolayer of a halogenated material such as a fluorinated material. 
         [0004]    In a particular example of the disclosure, a humidity sensor may include a substrate and an electrode layer that is supported by the substrate. In some cases, the electrode layer may define a first capacitive plate and a second capacitive plate. A non-halogenated humidity sensing layer may be disposed adjacent the electrode layer and may include a number of openings to increase an effective surface area of the non-halogenated humidity sensing layer. A halogenated layer may adjoin the non-halogenated humidity sensing layer. 
         [0005]    In another example of the disclosure, a humidity sensor may include a substrate and an electrode layer that is supported by the substrate. In some cases, the electrode layer may define a first capacitive plate and a second capacitive plate. A polyimide humidity sensing layer may be disposed adjacent the electrode layer. In some instances, the polyimide humidity sensing layer includes a sensing surface and a thickness, and includes a number of openings formed within the sensing surface to increase an effective surface area of the sensing surface. In some cases, the sensing surface of the polyimide humidity sensing layer may be fluorinated to a depth that is less than the thickness of the polyimide humidity sensing layer, resulting in a fluorinated sensing surface. 
         [0006]    In another example of the disclosure, a method of manufacturing a humidity sensor may include forming an electrode layer over a substrate. A non-halogenated polymeric material may be disposed over the electrode layer as a humidity sensing material. With the non-halogenated polymeric material disposed over the electrode layer, the non-halogenated polymeric material may be hydrogenated to increase the non-halogenated polymeric material&#39;s resistance to humidity sensing hysteresis. 
         [0007]    The above summary of some embodiments is not intended to describe each disclosed embodiment or every implementation of the present disclosure. The Figures, and Detailed Description, which follow, more particularly exemplify some of these embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0008]    The disclosure may be more completely understood in consideration of the following detailed description in connection with the accompanying drawings, in which: 
           [0009]      FIG. 1  is a schematic cross-sectional side view of an illustrative humidity sensor in accordance with an embodiment of the disclosure; 
           [0010]      FIG. 2  is a cross-sectional side view of an illustrative humidity sensor in accordance with an embodiment of the disclosure; 
           [0011]      FIG. 3  is a top view of a plate-type humidity sensor, with the humidity sensing material not shown to reveal the electrodes; 
           [0012]      FIG. 4  is a schematic cross-sectional side view of the humidity sensor of  FIG. 3  taken along line A-A′, showing the humidity sensing material; 
           [0013]      FIG. 5  is a schematic cross-sectional side view of the humidity sensor of  FIG. 3  taken along line A-A′, after additional processing of the humidity sensing material; 
           [0014]      FIG. 6  is a top view of the humidity sensor of  FIG. 5 ; 
           [0015]      FIG. 7  is an alternate top view of the humidity sensor of  FIG. 5 ; 
           [0016]      FIG. 8  is a top view of an interdigitated humidity sensor; 
           [0017]      FIG. 9  is a schematic cross-sectional side view of the humidity sensor of  FIG. 8  take along line B-B′; 
           [0018]      FIG. 10  is an enlarged cross-sectional side view of a portion of  FIG. 9  after additional processing of the humidity sensing material; and 
           [0019]      FIG. 11  is a flow diagram illustrating an illustrative method for making a humidity sensor. 
       
    
    
       [0020]    While the disclosure is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the disclosure to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure. 
       DESCRIPTION 
       [0021]    For the following defined terms, these definitions shall be applied, unless a different definition is given in the claims or elsewhere in this specification. 
         [0022]    All numeric values are herein assumed to be modified by the term “about,” whether or not explicitly indicated. The term “about” generally refers to a range of numbers that one of skill in the art would consider equivalent to the recited value (i.e., having the same function or result). In many instances, the terms “about” may include numbers that are rounded to the nearest significant figure. 
         [0023]    The recitation of numerical ranges by endpoints includes all numbers within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5). 
         [0024]    As used in this specification and the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the content clearly dictates otherwise. As used in this specification and the appended claims, the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise. 
         [0025]    It is noted that references in the specification to “an embodiment”, “some embodiments”, “other embodiments”, etc., indicate that the embodiment described may include one or more particular features, structures, and/or characteristics. However, such recitations do not necessarily mean that all embodiments include the particular features, structures, and/or characteristics. Additionally, when particular features, structures, and/or characteristics are described in connection with one embodiment, it should be understood that such features, structures, and/or characteristics may also be used connection with other embodiments whether or not explicitly described unless clearly stated to the contrary. 
         [0026]    The following detailed description should be read with reference to the drawings in which similar structures in different drawings are numbered the same. The drawings, which are not necessarily to scale, depict illustrative embodiments and are not intended to limit the scope of the disclosure.  FIG. 1  is a schematic cross-sectional side view of an illustrative humidity sensor  10 , showing particular features of the humidity sensor  10 . The illustrative humidity sensor  10  includes a substrate  12 , which in some instances may be a silicon substrate, although other substrates may be used. The illustrative humidity sensor  10  includes an electrode layer  14 . As drawn, the humidity sensor  10  is a plate-style humidity sensor in which the electrode layer  14  includes a pair of capacitive plates  14 a and  14 b. In some cases, however, the humidity sensor  10  may be an interdigitated style humidity sensor in which the electrode layer includes a pair of interdigitated electrodes, or any other suitable humidity sensor configuration as desired. 
         [0027]    The illustrative humidity sensor  10  further includes a humidity sensing material  16  that in some cases may include a first layer  18  and a second layer  20 . In some cases, the first layer  18  may be a polymer such as but not limited to a polyimide, a parylene, a benzocyclobutene, and divinylsiloxane bis(benzocyclobutene). In some cases, the first layer  18  may be a polyimide and the second layer  20  may be a halogenated layer that is deposited onto the first layer  18 . In some cases, the second layer  20  may be a fluorinated layer, although other halogens such as chlorine and bromine may also be used. 
         [0028]    In some cases, the second layer  20  may not be a separate layer, but may instead represent a fluorination of at least part of an outer surface of the first layer  18 . In some instances, for example, the humidity sensing layer  16  may include a sensing surface  22  and an overall thickness that is indicated on  FIG. 1  as a dimension D 1 . The second layer  20  may represent a halogenated portion of the first layer  18 , and may have a thickness (or perhaps thought of as a halogenation depth) that is indicated as a dimension D 2 . In some cases, the second layer  20  may represent a portion of the first layer  18  that has been fluorinated to a depth D 2  that is less than the thickness D 1  of the first layer  18 . 
         [0029]    In some cases, the second layer  20 , representing a halogenation such as fluorination of the sensing surface, may be considered as forming a mono-layer or in some cases a partial mono-layer of the halogen such as fluorine. In some cases, the sensing surface  22  may, for example, be fluorinated by plasma deposition of C x H y F z  onto the polyimide humidity sensing layer  16 , where x is an integer ranging from 1 to 4, y is an integer ranging from 0 to 2 and z is an integer ranging from 2 to 8. In some instances, the sensing surface  22  may be fluorinated via plasma deposition of one or more of CF 4 , C 2 F 6 , C 4 F 8 , CHF 3  and CH 2 F 2 . In some cases, there may be some displacement of hydrogen within the sensing surface  22 . In some cases, particularly during plasma deposition of C x H y F z , there may be deposition of a heterogeneous mixture of fluorocarbons and hydrofluorocarbons on the sensing surface  22 . 
         [0030]      FIG. 2  is a side view of an illustrative humidity sensor  50 . As illustrated, the humidity sensor  50  is a plate-style humidity sensor. The illustrative humidity sensor  50  includes a substrate  52  and a dielectric layer  54  disposed on the substrate  52 . In some cases, the substrate  52  may be silicon and the dielectric layer  54  may, for example, be SiO 2 . A porous platinum layer  56  forms a common plate that is shared by two series capacitors CX 1  and CX 2 . Layer  58  is a patterned layer that forms the other plates of capacitors CX 1  and CX 2 . For example, layer  58  may be patterned as illustrated in  FIG. 3  to form a capacitor plate  58 A for capacitor CX 1  and a separate capacitor plate  58 B for capacitor CX 2 . Capacitor plate  58 A may be electrically connected to gold pad  64 A and capacitor plate  58 B may be electrically connected to gold pad  64 B. In some cases, the layer  58  may be a titanium tungsten alloy. A passivation layer  60  may be formed over the porous platinum layer  56 , and may be a polyimide, for example. A dielectric layer  62  is provided between the porous platinum layer  56  and the titanium tungsten alloy layer  58  and in some cases functions as the humidity sensing layer. The dielectric layer  62  may be halogenated or include a halogenated layer such as a fluorinated layer. Gold pads  64 A and  64 B permit electrical connections to be made to the humidity sensor  50 . 
         [0031]      FIGS. 3 through 10  provide additional examples of humidity sensors.  FIG. 3  is a top view of a plate-type humidity sensor  30 , with the humidity sensing material not shown to reveal the electrode layer  14 , while  FIG. 4  is a schematic cross-sectional side view of the humidity sensor of  FIG. 3  taken along line A-A′, illustrating a sequence of the substrate  12 , the electrode layer  14  and the humidity sensing material  16 .  FIG. 5  is a schematic cross-sectional side view of the humidity sensor of  FIG. 3  taken along line A-A′, after additional processing of the humidity sensing material to create pores. In  FIG. 5 , openings  32  have been formed in the humidity sensing material  16  to increase the porosity and thus the effective surface area of the humidity sensing material  16 . This may increase the sensitivity of the humidity sensor.  FIG. 6  is a top view of the humidity sensor  30  of  FIG. 5 , illustrating that the openings  32  of  FIG. 5  may include a plurality of pores  34  formed within the humidity sensing material  16 .  FIG. 7  is an alternate top view of the humidity sensor  30  of  FIG. 5 , illustrating that the openings  32  of  FIG. 5  may include one or more elongated channels  36  formed within the humidity sensing material  16 . 
         [0032]      FIG. 8  is a top view of an interdigitated humidity sensor  40 , while  FIG. 9  shows a schematic cross-sectional side view of the interdigitated humidity sensor  40  of  FIG. 8  taken along line B-B′ illustrating the sequence of a substrate  12 , an electrode layer  14  and a humidity sensing material  16 . As can be seen, in this example, the electrode layer includes two interdigitated electrodes.  FIG. 10  is an enlarged cross-sectional side view of a portion of the interdigitated humidity sensor  40  of  FIG. 9  after additional processing of the humidity sensing material  16 . In  FIG. 10 , openings  42  have been formed in the humidity sensing material  16  to increase the porosity and thus the effective surface area of the humidity sensing material  16 . This may increase the sensitivity of the interdigitated humidity sensor. 
         [0033]      FIG. 11  is a flow diagram showing an illustrative method of manufacturing a humidity sensor such as the humidity sensor  10  ( FIG. 1 ), the humidity sensor  30  ( FIG. 3 ), the humidity sensor  40  ( FIG. 8 ) or the humidity sensor  50  ( FIG. 2 ). As generally indicated at block  70 , an electrode layer may be formed over a substrate. In some cases, the electrode layer may be formed directly on the substrate while in other cases there may be one or more intervening layers. A non-halogenated polymeric layer may be disposed over the electrode layer, as generally seen at block  72 . In some cases, the non-halogenated polymeric layer may be a non-halogenated polyimide layer. In some cases, as optionally shown at block  74 , a plurality of openings, such as channels, pores or other void spaces, may be formed within the non-halogenated polymeric layer. 
         [0034]    Next, as generally indicated at block  76 , the non-halogenated polymeric layer may be halogenated. In some cases, halogenating the non-halogenated polymeric layer may include a chemical reaction and/or a thermal reaction, such as in a plasma environment. In some cases, halogenating the non-halogenated polymeric material may include an ion implantation process. In some cases, halogenating the non-halogenated polymeric material may include fluorinating the non-halogenated polymeric material, and may for example result in an organofluorine thin film disposed on the non-halogenated polymeric material. 
         [0035]    In some cases, the humidity sensing material may include, beyond those already discussed, a polymer film (polyimides, benzocyclobutene, nylons, etc.), an organosilicate film (Si—O—C polymers, divinylsiloxane bis (benzocyclobutene), etc.), and/or an inorganic film that has been halogenated, such as with fluorine, chlorine or bromine, and/or coated with a halogenated thin film such as a perfluorocarbon or an organofluorine (CxFy) thin film. By halogenating and/or coating the humidity sensing material, the capacitive and/or resistive hysteresis effect of the humidity sensor may be diminished compared to a humidity sensor that uses an untreated humidity sensing material. In general, halogenating the humidity sensing material may make the humidity sensing material more hydrophobic, and thus less susceptible to hysteresis. Fluorination is a favorable method of halogenation due to the availability of a large number of fluorine sources within a typical microelectronics fabrication facility, although chlorine and bromine are also available for use. In some cases, the humidity sensing material may be first etched or otherwise processed to increase its effective surface area before halogenating and/or coating the humidity sensing material. It is contemplated that the halogenation may extend into etched pores, holes, trenches and/or other etched features. 
         [0036]    As noted above, the humidity sensing material may include, for example, polyimide s, parylene s, benzocyclobutene s, polyethersulfone, celluloseacetatebutyrate, poly(methyl methacrylate), and/or any other suitable humidity sensing material. Such humidity sensing materials may be improved by modifying their surface. In some cases, the humidity sensing material may be improved by either directly halogenating the humidity sensing material or by depositing an organofluorine (CxFy) thin film on the humidity sensing material. The deposited organofluorine thin film may be on the order of a single monolayer to many monolayers, as desired. It is contemplated that other halocarbons such as haloalkanes may be used to form hydrophobic surfaces. This can be accomplished by one or a combination of chemical reaction, ion implantation and deposition. 
         [0037]    The chemical reaction of fluorine with organic and organosilicate humidity sensing materials can be accomplished with a variety of chemicals. A simple way to create reactive fluorine is in a plasma environment using fluorine containing gases such as F 2 , NF 3 , anhydrous HF, SiF 4 , ClF 3 , and XeF 2 . In some cases, these gases may be blended with inert diluents such as Ar, He, and N 2  to improve the production of fluorine radicals. Most plasma etch and plasma chemical vapor deposition systems control the temperature of the processed substrate in order to allow for control of the chemical reaction between the fluorine and the humidity sensing material. In some cases, downstream plasmas may be used to eliminate ion interactions with the substrate of the humidity sensor, thus allowing for purely chemical interactions. It is also possible to fluorinate surfaces of polymer films with XeF 2  vapors. The SPTS Xactix tool, for example, is specifically built for XeF 2  reactions and is used within MEMS manufacturing. 
         [0038]    Ion implantation of fluorine and fluorinated species into the humidity sensing material is also contemplated. For the organic films, low energy ions may be used to keep the ions near the surface of the humidity sensing material. Ion implanters typically use BF 3  to implant boron. In fact, ions of BF 2  are implanted into the silicon. This can be done by plasma immersion ion implantation. One commercially available tool that produces low energy ions is the Varian VIISta PLAD. Higher energy ions are also possible with other implanters that are configured to directly implant F-ions or BF x -ions into the humidity sensing material. Since these tools typically handle a fluorinated species they can also be plumbed with NF 3 , SF 6 , F 2 , HF, and SiF 4 . After implantation, the humidity sensing material can be annealed/heat treated in a conventional furnace to allow additional chemical reactions and diffusion of the fluorine. 
         [0039]    Deposition of C x F y  and C x H y F z  type materials is possible in plasma etch and plasma enhanced CVD tools. Under appropriate conditions, deposition of these species is possible with, for example, one or more of the following gases: CF 4 , C 2 F 6 , C 4 F 8 , CHF 3 , CH 2 F 2 , C 3 F 8 , C 4 F 6 , and C 5 F 8 . Additives and diluents may including O 2 , He, N 2 , Ar, H 2 , NF 3 , SF 6 , XeF 2 , ClF 3 , F 2 , SiF 4 , and/or HF may be added to the fluorocarbon gases in order to modify the chemical species available for deposition. Other halocarbons can be formed by the above processes using other halogenated gases commonly found in a microelectronics or MEMS fabrication facility, such as HCl, Cl 2 , HBr, ClF 3 , HI, BCl 3 , and BBr 3 . In some cases, all three interactions may take place in plasma-based tools. 
         [0040]    In one example of the disclosure, divinylsiloxane bis (benzocyclobutene) (DVS-bis-BCB) is etched in O 2 +F 2  plasma which modifies the surface and bulk material properties of BCB. Optimizing the ratio of O 2  and F 2  may provide greater control of the etch rate of DVS-bis-BCB and the film thickness. DVS-bis-BCB has good adhesion properties and should adhere better than polyimide when exposed to chemical cleaners such as ammonia. 
         [0041]    It should be understood that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of steps without exceeding the scope of the disclosure. This may include, to the extent that it is appropriate, the use of any of the features of one example embodiment being used in other embodiments.