Patent Publication Number: US-10766255-B2

Title: Element substrate

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an element substrate equipped with a heat generating resistor that generates thermal energy for ejecting a liquid. 
     Description of the Related Art 
     Some liquid ejection heads used in a liquid ejection apparatus such as ink jet printer are equipped with a heat generating resistor that generates thermal energy as an energy generating element that generates energy for ejecting a liquid. Since repeated driving of the heat generating resistor causes variations in its resistance value, a reduction in drive-induced variations in resistance value is required from the standpoint of prolonging its life. 
     To satisfy the above-described requirement, Japanese Patent No. 3554148 discloses the specific resistance value of a heat generating resistor and the composition of materials used therefor capable of suppressing variations in resistance value thereof. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is an element substrate equipped with a heat generating resistor that generates thermal energy for ejecting a liquid. In the element substrate, the heat generating resistor is a stacked structure having stacked a plurality of resistor layers including a first resistor layer and a second resistor layer each containing a metal silicon nitride and the first resistor layer and the second resistor layer are different in at least one of a silicon content in the metal silicon nitride and a metal element contained in the metal silicon nitride 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view schematically showing an element substrate according to one embodiment of the invention. 
         FIGS. 2A and 2B  are a plan view and a cross-sectional view schematically showing an element substrate according to one embodiment of the invention. 
         FIGS. 3A and 3B  are a plan view and a cross-sectional view schematically showing an element substrate according to one embodiment of the invention, respectively. 
         FIG. 4  is a graph showing variations in the resistance value of Comparative Example 1-1. 
         FIGS. 5A and 5B  are graphs showing variations in the resistance value of Comparative Example 1-2. 
         FIG. 6  is a graph showing variations in the resistance value of Example 1. 
         FIG. 7  is a graph showing variations in the resistance value of Comparative Example 2-1. 
         FIG. 8  is a graph showing variations in the resistance value of Example 2. 
         FIG. 9  is a graph showing variations in the resistance value of Example 3. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     For increasing the speed of a liquid ejection head, a heat generating resistor is required to have an increased resistance, but the higher the resistance of the heat resisting resistor, the larger variations in resistance value become. There is therefore a demand for suppressing variations in the resistance value of a heat generating resistor in particular when the heat generating resistor has a high resistance. A heat generating resistor is specified to have a specific resistance value of 4000 μΩ·cm or less according to the technology described in Japanese Patent No. 3554148 so that an increase in specific resistance in order to have a heat generating resistor having a high resistance makes it difficult to suppress variations in resistance value. 
     With a view to overcoming the above-described problem, an object of the invention is to provide an element substrate capable of suppressing variations in resistance value even if it has a high resistance. 
     According to the invention, a heat generating resistor is a stacked structure having stacked a plurality of resistor layers made of respectively different metal silicon nitrides so that resistance value variation characteristics can be made different among these resistor layers. Variations in the resistance of the whole heat generating resistor can therefore be suppressed by making use of the resistance value variation characteristics of each resistor layer even without providing an upper limit in the specific resistance value of the heat generating resistor. This makes it possible to suppress variations in resistance value even if it has a high resistance. 
     Embodiments and Examples of the invention will hereinafter be described referring to drawings. In each drawing, members having the same function will be identified by the same reference numerals and a description on them may be omitted. 
     EMBODIMENT 
     &lt;Element Substrate&gt; 
       FIG. 1  and  FIGS. 2A and 2B  show an element substrate according to one embodiment of the invention. More specifically,  FIG. 1  is a perspective view schematically showing the element substrate,  FIG. 2A  is an enlarged plan view of a region A of  FIG. 1  and  FIG. 2B  is a cross-sectional view taken along a line B-B of  FIG. 2A . 
     As shown in  FIG. 1 , an element substrate  10  is equipped with ejection orifices  11  for ejecting a liquid. In the example shown in this drawing, two rows of a plurality of ejection orifices  11  are arranged, but the number or arrangement of the ejection orifices  11  is not limited to this example. 
     As shown in  FIGS. 2A and 2B , the element substrate  10  has a substrate  100  equipped with a heat generating resistor  110  that generates thermal energy for ejecting a liquid and an ejection orifice forming member  200  provided on the substrate  100 . 
     The substrate  100  has a semiconductor substrate  101 . The semiconductor substrate  101  is, for example, a silicon substrate made of single crystal silicon. The semiconductor substrate  101  has thereon a heat storage layer  102  that stores therein thermal energy generated by the heat generating resistor  110 . The heat storage layer  102  is made of, for example, silicon oxide and has electrical insulation property and adequate thermal conductivity. The heat storage layer  102  has a thickness of, for example, from 1.0 μm to 3.0 μm. 
     The heat storage layer  102  has thereon the heat generating resistor  110 . The heat generating resistor  110  is made of a resistor material such as a metal silicon nitride which is a ternary compound containing a metal element, Si (silicon) and N (nitrogen). Examples of the metal silicon nitride include WSiN (tungsten silicon nitride) and TaSiN (tantalum silicon nitride). In the present embodiment, the heat generating resistor  110  is made of a metal silicon nitride. More specifically, the heat generating resistor  110  is a stacked structure having stacked a plurality of resistor layers made of respectively different metal silicon nitrides. Each of the resistor layers of the heat generating resistor  110  has a specific resistance value of, for example, from 3000 μΩ·cm to 6000 μΩ·cm and each resistor layer has a film thickness of, for example, from 10 nm to 25 nm. To the heat generating resistor  110 , a wiring (not shown) for supplying the heat generating resistor  110  with electric power is connected. The wiring is made of, for example, Al (aluminum) or Cu (copper). The heat generating resistor  110  may be directly connected to the wiring or may be connected to the wiring via a plug made of W (tungsten) or the like. 
     The heat storage layer  102  has thereon a protecting layer  103  for protecting the heat generating resistor  110  and the wiring from static electricity or the like and the protecting layer covers the heat generating resistor  110  therewith. The protecting layer  103  is made of, for example, silicon nitride. The protecting layer  103  has electrical insulation property. The protecting layer  103  has a thickness of, for example, from 150 nm to 300 nm. 
     The protecting layer  103  has thereon an anti-cavitation layer  104  for protecting the heat generating resistor  110  from impact caused by cavitation at the time when air bubbles are generated or disappear in a pressure chamber  12  which will be described later. The anti-cavitation layer  104  is made of, for example, Ta (tantalum) or Ir (iridium). The anti-cavitation layer  104  has a thickness of, for example, from 150 nm to 300 nm. 
     The anti-cavitation layer  104  has thereon the ejection orifice forming member  200  having therein an ejection orifice  11 . By the substrate  100  and the ejection orifice forming member  200 , the pressure chamber  12  having therein a liquid to be ejected from the ejection orifice  11  and a flow path  13  communicated with the pressure chamber  12  and guiding the liquid to the pressure chamber  12  are defined. The pressure chamber  12  is provided above the heat generating resistor  110  and the ejection orifice  11  is provided at a position facing the heat generating resistor  110  with the pressure chamber  12  therebetween. 
     In the element substrate  10  thus described above, supply of electric power to the heat generating resistor  110  through the wiring generates thermal energy at the heat generating resistor  110  and air bubbles are produced in the pressure chamber  12  by the resulting thermal energy. Due to the air bubbles, the pressure in the pressure chamber  12  increases and the liquid in the pressure chamber  12  is ejected from the ejection orifice  11 . 
     &lt;Heat Generating Resistor&gt; 
     In the heat generating resistor  110  made of a metal silicon nitride or the like, repeated driving of the heat generating resistor  110  causes oxidation or crystallization, resulting in variations in the resistance value of the heat generating resistor  110 . The oxidation of the heat generating resistor  110  contributes to an increase in resistance value, while crystallization of the heat generating resistor  110  contributes to a reduction in resistance value. This means that the behavior of the variations in the resistance value of the heat generating resistor  110  changes with oxidation characteristics and crystallization characteristics of the heat generating resistor  110  and the oxidation characteristics and the crystallization characteristics of the heat generating resistor  110  change with a material of the heat generating resistor  110 . The behavior of the variations in the resistance value of the heat generating resistor  110  therefore changes depending on the material of the heat generating resistor  110 . 
     For example, a heat generating resistor  110  made of an easily crystallizable material or a hardly oxidizable material has a reduced resistance value due to the influence of crystallization and a heat generating resistor  110  made of a hardly crystallizable material or an easily oxidizable material has an increased resistance value due to the influence of oxidation. 
     By forming a heat generating resistor  110  which is a stacked structure having stacked a plurality of resistor layers made of respectively different materials, it is possible to change the resistance value variation characteristics of each resistor layer and thereby reduce variations in the resistance value of the whole heat generating resistor  110 . 
     In the present embodiment, the heat generating resistor  110  has a stacked structure having stacked two resistor layers (a first heat generating resistor  111  and a second heat generating resistor  112 ) made of materials having respectively different crystallization characteristics. The first heat generating resistor  111  is on the lower layer side than the second heat generating resistor  112 . This means that the heat storage layer  102  has thereon the first heat generating resistor  111  and the second heat generating resistor  112  stacked in order of mention. 
     When the first heat generating resistor  111  and the second heat generating resistor  112  are each made of a metal silicon nitride, their crystallization characteristics differ depending on a Si content (silicon content) which is a percentage of silicon. The first heat generating resistor  111  and the second heat generating resistor  112  are therefore made of metal silicon nitrides having respectively different Si contents. For example, when TaSiN, WSiN or the like is used as the metal silicon nitride, the larger the Si content, the more easily crystallization occurs because it reduces a crystallization temperature. Therefore, the larger the Si content is, the lower the resistance value becomes due to the influence of crystallization, while the smaller the Si content is, the larger the resistance value becomes due to the influence of oxidation. One of the first heat generating resistor  111  and the second heat generating resistor  112  is made of TaSiN (or WSiN) having a large Si content to reduce the resistance value and the other one is made of TaSiN (or WSiN) having a small Si content to elevate the resistance value. For example, the Si content of one of the first heat generating resistor  111  and the second heat generating resistor  112  is set at from about 35 at % to 45 at % and the Si content of the other one is set at from about 15 at % to 25 at %. 
     Alternatively, the first heat generating resistor  111  and the second heat generating resistor  112  may be made of materials having respectively different oxidation characteristics. The first heat generating resistor  111  and the second heat generating resistor  112  made of a metal silicon nitride each have different oxidation characteristics, depending on the work function of a metal element constituting the metal silicon nitride. More specifically, a metal element constituting the metal silicon nitride becomes more hardly oxidizable as it has a larger work function. The first heat generating resistor  111  and the second heat generating resistor  112  are therefore made of metal silicon nitrides different in constituent metal element, respectively. 
     For example, WSiN is more hardly oxidizable than TaSiN because the work function of W is larger than that of Ta. One of the first heat generating resistor  111  and the second heat generating resistor  112  is made of WSiN to reduce the resistance value and the other one is made of TaSiN to elevate the resistance value. 
     At this time, the first heat generating resistor  111  and the second heat generating resistor  112  may have the same Si content or respectively different Si contents. When the first heat generating resistor  111  and the second heat generating resistor  112  are different in Si content, it is preferred that one of the first heat generating resistor  111  and the second heat generating resistor  112  made of WSiN has a larger Si content and the other one made of TaSiN has a smaller Si content. In this case, one of the first heat generating resistor  111  and the second heat generating resistor  112  can be made of a hardly crystallizable and easily oxidizable material and the other one can be made of an easily crystallizable and hardly oxidizable material. In this case, variations in the resistance value of the heat generating resistor  110  can be suppressed more. 
     Of the first heat generating resistor  111  and the second heat generating resistor  112 , that having a decreased resistance value (that made of an easily crystallizable material or a hardly oxidizable material) will hereinafter be called “resistance decreasing layer” and that having an increased resistance value (that made of a hardly crystallizable material or an easily oxidizable material) will hereinafter be called “resistance increasing layer”. Variation characteristics of the resistance of the heat generating resistor  110  slightly strongly reflect the influence of the first heat generating resistor  111  on the lower layer side than the second heat generating resistor  112  on the upper layer side. In order to suppress a reduction in the resistance value of the heat generating resistor  110 , it is therefore preferred to form the heat generating resistor  110  so as to have the first heat generating resistor  111  as a resistance increasing layer and the second heat generating resistor  112  as a resistance decreasing layer. This makes it possible to suppress application of an excessive burden to the heat generating resistor  110 . 
     In order to suppress an increase in the resistance value of the heat generating resistor  110 , on the other hand, it is preferred to form the heat generating resistor  110  so as to have the first heat generating resistor  111  as a resistance decreasing layer and the second heat generating resistor  112  as a resistance increasing layer. This makes it possible to suppress an ejection failure such as non-ejection caused by reduction in thermal energy generated in the heat generating resistor  110 . 
     The first heat generating resistor  111  and the second heat generating resistor  112  are preferably formed continuously under reduced pressure to prevent formation of a natural oxide film on the interface between the first heat generating resistor  111  and the second heat generating resistor  112 . In addition, a ratio of the specific resistance value of the second heat generating resistor  112  to that of the first heat generating resistor  111  is preferably 2 or less. 
     The heat generating resistor  110  shown in the example of  FIGS. 2A and 2B  is formed of two layers, that is, the first heat generating resistor  111  and the second heat generating resistor  112 , but it may be formed of three or more layers.  FIGS. 3A and 3B  show an element substrate  10  having a heat generating resistor  110  formed of three layers, that is, the first heat generating resistor  111 , the second heat generating resistor  112  and a third heat generating resistor  113 . In this case, the first heat generating resistor  111 , the second heat generating resistor  112  and the third heat generating resistor  113  are formed so that the resistance increasing layer and the resistance decreasing layer are stacked alternately. 
     According to the present embodiment, as described above, the heat generating resistor  110  is a stacked structure having stacked a plurality of resistor layers made of respectively different metal silicon nitrides so that variation characteristics of the resistance value of the resistor layers can be made different from one another. Without providing an upper limit for the specific resistance value of the heat generating resistor  110 , variations in the resistance value of the whole heat generating resistor  110  can be suppressed by making use of the variation characteristics of the resistance value of each resistor layer, making it possible to suppress variations in resistance value even if the heat generating resistor has a high resistance. 
     In the present embodiment, the metal silicon nitrides constituting the resistor layers of the heat generating resistor  110 , respectively, are different from one another in at least one of the silicon content and the metal element and this makes it possible to appropriately suppress variations in the resistance of the whole heat generating resistor  110 . 
     In the present embodiment, the resistor layer include a layer made of WSiN and a layer made of TaSiN and the layer made of WSiN has a larger Si content than the layer made of TaSiN. This makes it possible to suppress variations in the resistance of the whole heat generating resistor  110 . 
     An example of the first heat generating resistor  111  and the second heat generating resistor  112  each made of a metal silicon nitride was described above but they are not necessarily made of a metal silicon nitride. The heat generating resistor  110  is only required to be a stack of a plurality of resistor layers including a resistor (first resistor layer) showing a resistance value decreasing tendency when it is driven and a resistor (second resistor layer) showing a resistance value increasing tendency when it is driven. This makes it possible to suppress variations in the resistance value of the whole heat generating resistor  110 . The term “resistance value increasing tendency” or “resistance value decreasing tendency” means a resistance value changing tendency after driving is started except variations in resistance value in a markedly short term after driving is started. Further, it is preferred that the resistance of the first resistor layer after it is driven predetermined number of times or more continuously is lower than that before it is driven and the resistance of the second resistor layer after it is driven predetermined number of times or more continuously is higher than that before it is driven. Still further, the resistance of the first resistor layer after it is driven 1×10 8  times or more continuously is preferably lower than that before it is driven. The resistance of the second resistor layer after it is driven 1×10 8  times or more continuously is preferably higher than that before it is driven. At least one of the first heat generating resistor  111  and the second heat generating resistor  112  is preferably a metal silicon nitride. 
     EXAMPLE 1 
     The element substrates shown in each Example and Comparative Example have the same constitution except for a heat generating resistor and are manufactured as described below. 
     First, a semiconductor substrate  101  made of single crystal silicon was provided. Predetermined members such as transistor (not shown) were formed on the semiconductor substrate  101  and then, a heat storage layer  102  having a thickness of 2.0 μm was formed on the semiconductor substrate  101 . A heat generating resistor was formed on the heat storage layer  102  and the heat generating resistor and the wiring were connected to each other via a plug made of W. A protecting layer  103  having a thickness of 300 nm and made of silicon nitride was formed on the heat storage layer  102  to cover the heat generating resistor  110 . An anti-cavitation layer  104  having a thickness of 300 nm and made of Ta was formed on the protecting layer  103 . An ejection orifice forming member  200  was provided on the anti-cavitation layer  104  and an ejection orifice  11 , a pressure chamber  12  and a flow path  13  were formed. 
     A durability test of the element substrate thus manufactured was performed and variations in resistance were checked. In the durability test, unless otherwise particularly specified, pulse power having a pulse width of 1.0 μs, a pulse frequency of 15 kHz and a voltage value of 20.0 V was used as electric power to be supplied to the element substrate and electric power corresponding to 1.0×10 10  pulses was supplied to the element substrate. Variations in resistance were evaluated using the maximum variation width of resistance value (=(maximum resistance value−minimum resistance value)/(initial resistance value)). 
     Comparative Example 1-1 
     A heat generating resistor made of a single layer of TaSiN (Si content: 40 at %) having a specific resistance of 4000 μΩ·cm and a film thickness of 40 nm was formed as Comparative Example 1-1. 
       FIG. 4  shows variations in the resistance of Comparative Example 1-1. In  FIG. 4 , the number of pulses, that is, the number of application times of pulse power to the element substrate is plotted along the abscissa and a variation width of the resistance value (=(resistance value−initial resistance value)/initial resistance value) is plotted along the ordinate. The abscissa has a logarithmic scale. 
     As shown in  FIG. 4 , the resistance of Comparative Example 1-1 continues to decrease with the number of pulses and the maximum variation width of the resistance is about 24%. 
     Comparative Example 1-2 
     A heat generating resistor made of a single layer of TaSiN (Si content: 20 at %) having a specific resistance of 4000 μΩ·cm and a film thickness of 40 nm was formed as Comparative Example 1-2. 
       FIGS. 5A and 5B  show variations in the resistance of Comparative Example 1-2. In  FIGS. 5A and 5B , the number of pulses is plotted along the abscissa and a variation width of the resistance is plotted along the ordinate. In  FIG. 5A , the abscissa has a logarithmic scale, while in  FIG. 5B , the abscissa has a real number scale. 
     As shown in  FIG. 5A , the resistance of Comparative Example 1-2 decreases once, but then it continues to increase. The resistance decreases during a very short period of time after the durability test is started as shown in  FIG. 5B , suggesting that the resistance of Comparative Example 1-2 generally increases with the number of pulses. The maximum variation width of the resistance is about 18%. 
     EXAMPLE 1 
     Example 1 is an example of a heat generating resistor  110  formed of two layers, that is, a first heat generating resistor  111  and a second heat generating resistor  112 . In this Example, the first heat generating resistor  111  and the second heat generating resistor  112  are made of metal silicon nitrides same in kind but different in Si content. More specifically, the first heat generating resistor  111  was made of TaSiN having a specific resistance of 4000 μΩ·cm, a film thickness of 20 nm and a Si content of 20 at %. The second heat generating resistor  112  was made of TaSiN having a specific resistance of 4000 μΩ·cm, a film thickness of 20 nm and a Si content of 40 at %. 
       FIG. 6  shows variations in the resistance of Example 1. In  FIG. 6 , the number of pulses is plotted along the abscissa and a variation width of the resistance is plotted along the ordinate. The abscissa has a logarithmic scale. 
     In Example 1, as shown in  FIG. 6 , the resistance decreases, but variations in resistance is suppressed compared with those in Comparative Examples 1-1 and 1-2. More specifically, the maximum variation width of the resistance in Example 1 is about 11%, smaller than that in Comparative Examples 1-1 and 1-2. 
     EXAMPLE 2 
     Comparative Example 2-1 
     As Comparative Example 2-1, a heat generating resistor made of a single layer of WSiN (Si content: 40 at %) having a specific resistance of 4000 μΩ·cm and a film thickness of 40 nm was formed. 
       FIG. 7  shows variations in the resistance of Comparative Example 2-1. The number of pulses is plotted along the abscissa and a variation width of the resistance is plotted along the ordinate. The abscissa has a logarithmic scale. In Comparative Example 2-1, as shown in  FIG. 7 , the resistance continues to decrease with the number of pulses and the maximum variation width of the resistance is about 22%. 
     Comparative Example 2-2 
     In Comparative Example 2-2, valuations in resistance are suppressed as in Comparative Example 1-2. 
     EXAMPLE 2 
     Example 2 is an example of a heat generating resistor  110  formed of two layers, that is, a first heat generating resistor  111  and a second heat generating resistor  112 . In this Example, the first heat generating resistor  111  and the second heat generating resistor  112  are made of metal silicon nitrides different in kind. More specifically, the first heat generating resistor  111  was made of TaSiN (Si content: 20 at %) having a specific resistance of 4000 μΩ·cm and a film thickness of 20 nm. The second heat generating resistor  112  was made of WSiN (Si content: 40 at %) having a specific resistance of 4000 μΩ·cm and a film thickness of 20 nm. 
       FIG. 8  shows variations in the resistance of Example 2. In  FIG. 8 , the number of pulses is plotted along the abscissa and a variation width of the resistance is plotted along the ordinate. The abscissa has a logarithmic scale. 
     In Example 2, as shown in  FIG. 8 , the resistance varies, but variations in resistance are suppressed compared with those in Comparative Examples 2-1 and 2-2. More specifically, the maximum variation width of the resistance in Example 2 is about 11%, smaller than that in Comparative Examples 2-1 and 2-2. 
     EXAMPLE 3 
     Example 3 is an example of a heat generating resistor  110  formed of three layers, that is, a first heat generating resistor  111 , a second heat generating resistor  112  and a third heat generating resistor  113 . More specifically, the first heat generating resistor  111  was made of TaSiN (Si content: 20 at %) having a specific resistance of 4000 μΩ·cm and a film thickness of 13.3 nm. The second heat generating resistor  112  was made of WSiN (Si content: 40 at %) having a specific resistance of 4000 μΩ·cm and a film thickness of 13.3 nm. The third heat generating resistor  113  was made of TaSiN (Si content: 20 at %) having a specific resistance of 4000 μΩ·cm and a film thickness of 13.3 nm. In this example, two layers, that is, the first heat generating resistor  111  and the third heat generating resistor  113  are each a resistance increasing layer and one layer, that is, the second heat generating layer  112  is a resistance decreasing layer. 
       FIG. 9  shows variations in the resistance of Example 3. In  FIG. 9 , the number of pulses is plotted along the abscissa and a variation width of the resistance is plotted along the ordinate. The abscissa has a logarithmic scale. 
     In Example 3, as shown in  FIG. 9 , variations in resistance are more suppressed than those in Examples 1 and 2 and the maximum variation width of the resistance is about 9%. 
     EXAMPLE 4 
     Example 4 is an example of a heat generating resistor formed of a first heat generating resistor  111  and a second heat generating resistor  112  having respectively different specific resistances. 
     EXAMPLE 4-1 
     In Example 4-1, the first heat generating resistor  111  was made of TaSiN (Si content: 21 at %) having a specific resistance of 3600 μΩ·cm and a film thickness of 20 nm. The second heat generating resistor  112  was made of WSiN (Si content: 39 at %) having a specific resistance of 4400 μΩ·cm and a film thickness of 20 nm. This means that a ratio of the specific resistance of the second heat generating resistor  112  to the specific resistance of the first heat generating resistor  111  is 1.2. 
     In the durability test, pulse power having a pulse width of 1.0 μs, a pulse frequency of 15 kHz and a voltage of 19.9 V was used as electric power to be supplied to the element substrate. The element substrate was supplied with electric power corresponding to 1.0×10 10  pulses. 
     The maximum variation width of the resistance in Example 4-1 is about 10%, which is similar to that of Example 2. 
     EXAMPLE 4-2 
     In Example 4-2, the first heat generating resistor  111  was made of TaSiN (Si content: 25 at %) having a specific resistance of 2400 μΩ·cm and a film thickness of 20 nm. The second heat generating resistor  112  was formed using WSiN (Si content: 35 at %) having a specific resistance of 5600 μΩ·cm and a film thickness of 20 nm. This means that a ratio of the specific resistance of the second heat generating resistor  112  to the specific resistance of the first heat generating resistor  111  is 2.3. 
     In the durability test, pulse power having a pulse width of 1.0 μs, a pulse frequency of 15 kHz and a voltage of 18.3 V was used as electric power to be supplied to the element substrate and the element substrate was supplied with electric power corresponding to 1.0×10 10  pulses. 
     The maximum variation width of the resistance in Example 4-2 is about 15%. The present example is less effective for suppressing variations in resistance than Example 2, but is more effective for suppressing variations in resistance than Comparative Examples 2-1 and 2-2. 
     EXAMPLE 4-3 
     In Example 4-3, a first heat generating resistor  111  was made of TaSiN (Si content: 27 at %) having a specific resistance of 2000 μΩ·cm and a film thickness of 20 nm. A second heat generating resistor  112  was made of WSiN (Si content: 33 at %) having a specific resistance of 6000 μΩ·cm and a film thickness of 20 nm. This means that a ratio of the specific resistance of the second heat generating resistor  112  to the specific resistance of the first heat generating resistor  111  is 3.0. 
     In the durability test, pulse power having a pulse width of 1.0 μs, a pulse frequency of 15 kHz and a voltage of 17.3 V was used as electric power to be supplied to the element substrate and the element substrate was supplied with electric power corresponding to 1.0×10 10  pulses. 
     The maximum variation width of the resistance in Example 4-3 is about 18%, suggesting that the effect of suppressing variations in the resistance is smaller than that in another Example. 
     When the specific resistance of the first heat generating resistor  111  is not larger than that of the second heat generating resistor  112 , a ratio of the specific resistance of the second heat generating resistor  112  to that of the first heat generating resistor  111  is preferably 1 or more to 2 or less, more preferably closer to 1. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2018-072071, filed Apr. 4, 2018, which is hereby incorporated by reference herein in its entirety.