Abstract:
The invention relates to a sensor structure and a method. The sensor structure includes a first sensor having a sensing element sensitive to humidity of the environment. In_accordance with the invention the sensor structure also includes s second sensor having a sensing element sensitive to humidity, the second sensor comprising a catalytic permeable layer positioned on the second sensor such that it is between the sensing element of the second sensor and the environment.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a capacitive sensor structure and a method for measuring content of a water soluble chemical in gas in gaseous environment. 
         [0002]    Especially, the invention is related to the capacitive sensor structure according to the preamble of claim  1 . 
       BACKGROUND OF THE INVENTION 
       [0003]    In the prior art humidity as such has been measured by capacitive humidity sensors, where the dielectric of the humidity sensor has been sensitive to humidity. By heating also other substances like ammonia have been measured. 
         [0004]    Contents of substances like H 2 O 2  (Hydrogen peroxide), ETO (Ethylene Oxide), and O 3  (Ozone) have been measured by electrochemical cells or by IR-optical devices. These devices are very complicated or short-lived and thus also expensive. 
         [0005]    Thus, there exists a need for improved sensor and method for measuring content of catalytically degradable substances. 
       SUMMARY OF THE INVENTION 
       [0006]    The method is based on finding that the oxidative gas concentration has influence on observed saturation partial pressure of water vapor. So we can assume that with same partial water pressures we get different RH-values depending on exposure to oxidative gas concentrations. Actually chemical potential is the executive force which causes this phenomenon. 
         [0007]    In one embodiment of the invention the sensor structure includes a first sensor having a sensing element sensitive to humidity of the environment, and additionally includes a second sensor having a sensing element sensitive to humidity, the second sensor comprising a catalytic permeable layer positioned on the second sensor such that it is between the sensing element of the second sensor and the environment. 
         [0008]    The practical implementation of the invention is based using two humidity sensors: One with catalytically active layer and another without it. 
         [0009]    In one embodiment of the invention the two sensors are integrated on the same substrate. 
         [0010]    In another implementation the sensor elements are separate units. 
         [0011]    In one preferred embodiment one of the sensors is heatable. In an advantageous embodiment the catalytic sensor is heatable. 
         [0012]    More specifically, the invention is defined in the independent claims. 
         [0013]    The invention provides considerable advantages. 
         [0014]    With help of the invention decomposable chemicals like H 2 O 2  (Hydrogen peroxide), ETO (Ethylene Oxide) or O 3  (Ozone) may be detected and the content measured with a simple and inexpensive sensor structure instead of the complicated and expensive prior art solutions like electrochemical cells or by IR-optical devices. 
         [0015]    The sensor enables use of novel algorithm based on the determination of the activity of an oxidative gas. 
         [0016]    Further, the cost of instrumentation is very low. 
         [0017]    According to one embodiment, the sensitivity may be increased by heating the catalytic sensor element. 
         [0018]    According to one embodiment, sensor unit cost may be decreased in mass production by integrating the two sensors on the same substrate. 
         [0019]    Next, embodiments and advantages of the invention are described in more detail with reference to the attached drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  shows graphically measurement diagrams in accordance with the invention. 
           [0021]      FIG. 2   a  shows a sectional view of a sensor structure in accordance with the invention. 
           [0022]      FIG. 2   b  shows a top view of the sensor structure of  FIG. 2   a.    
           [0023]      FIG. 3   a  shows a photograph of one sensor structure in accordance with the invention. 
           [0024]      FIG. 3   b  shows a top detail of the sensor structure of  FIG. 3   a.    
           [0025]      FIG. 4  shows as a schematical side view the principle of the sensor structure of  FIGS. 3   a  and  3   b.    
           [0026]      FIG. 5  shows as a block diagram steps of the measurement methods. 
           [0027]      FIG. 6  shows as a graph a possible calibration method in accordance with the invention. 
           [0028]      FIG. 7  as a sectional side view a general idea of a sensor structure in accordance with the invention. 
           [0029]      FIG. 8  shows one alternative embodiment of the sensor structure in accordance with the invention. 
           [0030]      FIG. 9  shows alternative embodiments of the sensor structure in accordance with the invention. 
           [0031]      FIG. 10  shows one alternative embodiment of the sensor structure including a heating element in accordance with the invention. 
           [0032]      FIG. 11  shows one alternative embodiment of the catalytic sensor structure in accordance with the invention. 
           [0033]      FIGS. 12   a  and  12   b  shows alternative measurement systems in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0034]    The following lists shows the reference numerals used with the terms of the specification: 
         [0035]      1  first capacitive humidity sensor 
         [0036]      2  second, catalytic capacitive humidity sensor 
         [0037]      11  substrate 
         [0038]      12  bottom electrode, first electrode 
         [0039]      13  polymer dielectric, sensitive dielectic 
         [0040]      14  top electrode, second electrode 
         [0041]      15  protective polymer 
         [0042]      16  porous catalytic metal layer, decomposition layer 
         [0043]      17  contact pad 
         [0044]      18  heating element 
         [0045]      23  general humidity sensitive material 
         [0046]    Ox substance to be measured 
         [0047]    A catalyzer 
         [0048]    B pump 
         [0049]    C, D enclosure 
         [0050]    E valve, typically magnetic valve 
         [0051]    The principle of the invention: 
         [0052]    Basically the capacitive humidity measurement is simply an impedance measurement of a capacitive humidity sensor. The principle is described e.g. in GB-patent 2011093 of the applicant of the present application. 
         [0053]    Referring to  FIGS. 2   a  and  2   b , based on the basic principle we can formulate equations in case of two sensor measurements, where one is a normal capacitive humidity sensor  1  and the other capacitive humidity sensor  2  with porous catalytic metal layer (decomposition layer)  16 : 
         [0054]    Definitions 
         [0055]    Relative humidity is at all temperatures and pressures defined as the ratio of the water vapour pressure to the saturation water vapour pressure (over water) at the gas temperature: 
         [0000]      RH= P   w   /P   ws ·100%   (1)
 
         [0056]    The total pressure does not enter the definition. Above 100° C. the same definition is valid. But as the saturation vapour pressure P ws  is greater than 1 013 hPa (normal ambient pressure) the RH cannot reach 100% in an unpressurised system. Below 0° C. the definition is also valid. Here 100%RH is also impossible because condensation will occur at a lower humidity than 100% (when the vapour is saturated over ice). 
         [0057]    In connection with the present invention: 
         [0058]    Ox: catalytically degradable substance to be measured for example H 2 O 2  (Hydrogen peroxide), ETO (Ethylene Oxide), or O 3  (Ozone). 
         [0059]    RHmix: RH (=Relative Humidity) reading of standard capacitive humidity sensor 
         [0060]    RHcat: RH reading of Pt covered capacitive humidity sensor 
         [0061]    RHcat=(P w +P w (Ox))/P ws ; P ws  independent of oxidative gas concentration, P w (Ox) is vapour pressure of Ox. 
         [0062]    And RHmix=P w /P ws mix 
         [0063]    As the activity of Ox is a measure for chemical potential and a good approximation for activity is: 
         [0000]        a ( Ox )=[ Ox]/[Ox ]sat; and [ Ox ]sat= f ( T ) 
         [0000]      RHcat/RHmix= f ( a ( Ox )) and further  a ( Ox )= f (RHcat/RHmix) 
         [0064]    Combining equations it is possible to calculate the hydrogen peroxide concentration if RHcat, RHmix and T are known: 
         [0000]      [ Ox]=f (RHcat/RHmix)*[ Ox ]sat 
         [0065]    The measurement is possible to do with discrete sensor elements or integrated elements on one chip. The permeable catalytic layer  16  can be deposited by glancing angle evaporation technics on protective polymer layer  15 . Suitable materials are Pt, Rh, silver, MnO 2  etc. The oxidative molecules such as H 2 O 2  (Hydrogen peroxide), ETO (Ethylene Oxide), and O 3  (Ozone) decompose over catalytics even without elevated temperature. But it is also possible to enhance decomposition by integrating micro heater e.g. Pt-resistor on the sensor chip. 
         [0066]      FIG. 1  shows graphically content of H 2 O 2  as a function of relation of catalytic humidity measurement (RHcat) and mixed humidity measurement in two different temperatures. The upper curve is performed at 40° C. and the lower at 24° C. Hence, it can be seen that the rise in temperature enhances sensitivity. In other words the graphic is obtained by measuring relative humidity by a normal humidity sensor  1  together with a humidity sensor  2  with a permeable catalytic layer  16 . 
         [0067]    One embodiment of the sensor is described in  FIGS. 2   a  and  2   b , where  2   a  shows as a sectioned side view the sensor structure, where first sensor  1  is a normal capacitive humidity sensor  1  and the second sensor  2  a catalytic humidity sensor. The structure is formed on a substrate  11 , typically a silicon substrate. On the substrate  11  are formed bottom electrodes  12  with contact areas for both sensors  1  and  2 . Above the bottom electrodes  12  is formed the sensitive, permeable dielectric layer  13 , typically of a suitable polymer. The dielectric properties of the layer  13  are dependent on humidity, in other words H 2 O content. Above the dielectric layer  13  are formed top electrodes  14  for both sensors  1  and  2 . The water content in the dielectric layer may then be determined by measuring capacitance between the electrodes  12  and  13 . A permeable protective layer  15  is formed above the top electrodes  14  and the second sensor is additionally covered with a porous catalytic metal layer, decomposition layer  16  formed e.g. in a process described later. 
         [0068]    The metal film above the second sensor  2  is advantageously formed by a method described in the EP-patent 665303 of the applicant of this patent application. In this method the microporous metal film is attained by adjusting an angle alpha between the surface to be metallized and the source evaporating the metal to a value in the range 5-30 degrees. Here the surface to be metalized is the layer  15  or an adhesion layer e.g. of Cr above it. By altering the angle, the porosity and pore size of the metal film can be modified so that a small value of the angle alpha gives an extremely porous layer of large pore size, while a larger value of the angle alpha results in a less permeable layer of smaller pores. 
         [0069]    Good adherence is attained by first vacuum evaporating a layer of a slightly self-oxidizing metal (such as Cr, Ni or Ti) to a thickness of 10-300 nm. The plugging of pores through oxidation is prevented by subsequently vacuum evaporating from the same angle a precious metal layer (of Au, Pt or Pd) to a thickness of 10-300 nm. Typically, the total thickness of these layers is in the range 30-400 nm. 
         [0070]    Advantageously, the pore size (minimum diameter of the pores) is smaller than 30 nm, whereby a filtering effect against high-molecular-weight molecules is achieved. 
         [0071]    In  FIGS. 3   a  and  3   b  is presented one embodiment of the sensor structure in accordance with the invention, where the normal capacitive sensor  1  and the catalytic capacitive humidity sensor  2  are independent separate units. 
         [0072]    This arrangement is presented in more detail in  FIG. 4  where also the chemical mechanism of the sensor is more clearly indicated. The catalytic process decomposes H 2 O 2  into O 2  and H 2 O and hence changes the reading of sensor  2 . 
         [0073]      FIG. 5  shows the flow chart of the process and indicates how the concentration of substances that can be catalytically decomposed may be indicated. 
         [0074]      FIG. 6  shows a graph about a method how the sensor in accordance with the invention may be calibrated. 
         [0075]    In  FIG. 7  is presented a general structure for the second sensor in accordance with the invention. In principle the general idea of the invention is to produce a sensor with any kind of humidity sensitive material  26  covered with a decomposition layer  16  and compare the reading of this sensor structure with a reading of corresponding sensor structure without the decomposition layer  26 . 
         [0076]      FIG. 8  presents a sensor structure, where the sensors  1  and  2  are on the same substrate and the structure includes a common heating element  18  beneath the substrate in order to heat both of the elements  1  and  2 . 
         [0077]      FIG. 9  shows three alternative layouts for sensors  1  and  2 . 
         [0078]      FIG. 10  shows one layout for positioning the heating element in relation to the sensors  1  and  2 . 
         [0079]      FIG. 11  shows one embodiment of the catalytic sensor  2 , where the sensing layer is formed above finger-electrodes  12  and  14  and the decomposing layer is positioned above the sensing layer. In the complete sensor structure there is a similar second sensor  1  (not shown) without the decomposing layer  16 . 
         [0080]    In accordance with  FIGS. 12   a  and  12   b  the measurement system in accordance with one embodiment of the invention can be realized at least in two ways: 
         [0081]    According to  FIG. 12   a  using a catalyzer (A) like structure  16  above, a pump (B) and measuring enclosure (C) including a humidity sensor  1  where the enclosure (C) rapidly equilibrates with the ambient gas concentration; 
         [0082]    or according to  FIG. 12   b  by using two catalyzers (A) like structure  16  above, a pump (B), a closed measuring enclosure (D) with a humidity sensor  1  and a magnetic valve (E). In this system the enclosure receives in turns gas to the enclosure D either through catalyzer (A) or directly from the target to be measured. 
         [0083]    In connection with capacitive humidity sensors the relative humidity may be calculated by a known formula (VIZ) using the temperature information of the ambient air in accordance with the following known formula, when the sensor is heated above the temperature of the ambient air. This principle may be used if heating is used in connection with the present invention. 
         [0000]    
       
         
           
             
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         [0084]    where 
         [0085]    RH a =true relative humidity 
         [0086]    RH s =relative humidity of a mixture contiguous with 
         [0087]    a humidity sensitive film on a substrate 
         [0088]    eW s =the saturation vapor pressure at the substrate 
         [0089]    temperature measured by temperature sensor 
         [0090]    eWa=saturation vapor pressure of the surrounding mixture at temperature T a    
         [0091]    T s =substrate temperature measured by temperature sensor 
         [0092]    T a =ambient temperature measured by independent sensor 
         [0093]    Specifications for the Catalytic Sensor  2  in Accordance with  FIGS. 3   a - 3   b:    
         [0094]    2 capacitance measurements for elements  1  and  2 . 
         [0095]    2 resistance measurements. 
         [0096]    Heating of sensor element  2  triggered by high RH-value. 
         [0097]    The humidity sensor  2  with an evaporated catalytic layer (Pt) deposited on protective polymer film may be formed in an advantageous solution with the following parameters: 
         [0098]    Pt-layer  16  is evaporated on 14° angle. 
         [0099]    Thickness of Pt layer  16  is typically 1000 nm. 
         [0100]    Adhesion layer Cr (thickness about 50 nm) is formed between polymer  15  and Pt-layer  16 . 
         [0101]    Protection layer is formed on CrNiAu-lead (LIMA: SiAlOx). 
         [0102]    As a conclusion the measurement is based on measurement of two RH-sensors  1  and  2 . One with a catalytic protection layer  2  is used to measure partial water pressure (RHcat) and the other  1  without the catalytic layer is used to indicate mixture of hydrogen peroxide and water (RHmix). 
         [0103]    The catalytic sensor  2  comprises e.g. a Pt layer  16  as catalytic decomposer purpose to prevent H 2 O 2  penetration in sensing polymer. 
         [0104]    Difference between readings of the sensors RHmix (sensor  1 ) and RHcat (sensor  2 ) indicates the vapor concentration of H 2 O 2 . 
         [0105]    In the following equations when a calibration option with sensor heating is used in accordance with  FIG. 6 : 
         [0000]      RHmix= Pw/Pws mix 
         [0000]      RHcat=( Pw+Pw (H 2 O 2 ))/ Pws    
         [0000]      &lt;1 ppm H 2 O 2  then RHmix=RHcat 
         [0000]      or 
         [0000]      &lt;-1 ppm H 2 O 2  then RHmix=RHcat 
         [0106]    This is executed by changing Cdry of RHmix sensor. 
         [0107]    Method works if drift in one sterilization cycle is less than 1 ppm (0.4% RH in 25° C.) 
         [0108]    Alternative Solutions of the Invention: 
         [0109]    Suitable materials for the porous decomposition layer  16  are listed in the following: 
         [0110]    Pt, Rh, Ag, Mn or other transition metal and their compounds. 
         [0111]    Objects to be measured are listed in the following: 
         [0112]    hydrogen peroxide, ozone, peracetic acid or other catalytically degradable substance. 
         [0113]    As humidity sensors may be used any humidity sensor structures that can be measured electrically. 
         [0114]    The measurement may be based e.g. on:
       impedance, like capacitive or resistive or inductive sensors   resonators like BAW, SAW etc.   semiconductors       
 
         [0118]    Essential for the invention is an element, typically a layer  13  sensitive to humidity, especially to relative humidity. The sensitivity may be based on change of permittivity (capacitive measurement), conductivity (measurement of resistivity) or mass (resonators). Materials sensitive for these parameters are polymers, ceramics and composites. 
         [0119]    The material  13  sensitive to the relative humidity may be positioned on the sensor or sensor field or inside the sensor structure, typically between sensor layers. Also cylindrical structures are possible. 
         [0120]    The catalytic layer (decomposition layer)  16  may also act as a surface electrode for the measurement. 
         [0121]    In connection with the invention the catalytic permeable layer  16  encloses the sensing element  13  at least essentially. This means in practice that the catalytic permeable layer  16  has to cover the sensing element  13  so well that the decomposition happens to the substance Ox to be measured in such a way that content of Ox may be calculated. Typically the coverage of the sensing element  13  by the layer  16  is around 70-100%, most preferably around 90-98%. 
         [0122]    In one embodiment of the invention only one sensor may be used but the measurement is made such that the sensor gets sequentially measurement gas in a first phase directly from the space to be measured and in the second phase through a catalytic permeable layer  16  and these results from these two phases will be compared like the results of the two sensors  1  and  2  in the other embodiments of the invention. In this embodiment the permeable catalytic layer  16  may function also as a particle filter for the sensor. 
         [0123]    In accordance with the invention the catalytic permeable layer  16  is only one embodiment of the invention. The catalytic reaction needed for reference measurement may be performed in many ways, for example by a catalytic matrix structure, catalytic particle filter, catalytic particle cloud in a fluidized filter structure etc. 
         [0124]    The reference measurement by one sensor on the other hand may be performed in a sequentially with alternating flows through the sensor either directly from the object to be measured or through or in contact with a material reacting catalytically with the gas to be measured. Then the two measurements will be compared repeatedly with each other in accordance with the two sensor measurement described above. 
         [0125]    In one preferred embodiment of the invention with two sensors at least one reference measurement is made with such a gas that does not include the gas to be measured (Ox) in order to compensate any difference between the two sensor readings. By this procedure drifting or the sensors may be eliminated.