JP2007067893A - Acoustical sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an acoustical sensor capable of improving noise tolerance for an acoustic signal. <P>SOLUTION: An acoustical sensor 100 comprises a vibrating electrode 4 formed on a silicon substrate 1 and a fixed electrode 6 faced at the vibrating electrode 4 and arranged at a predetermined interval. The vibrating electrode 4 and the fixed electrode 6 constitute a capacitor. A modified SiOC layer 8a is formed of boron (B) introduced by ion implantation into a SiOC film [composition: SiO<SB>x</SB>(CH<SB>3</SB>)<SB>y</SB>], which is a low dielectric constant insulating film. The modified SiOC layer 8a is used as a film for fixing the fixed electrode 6 to the silicon substrate 1. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an acoustic sensor, and more particularly to an acoustic sensor formed on a semiconductor substrate.

  Conventionally, a capacitor-type silicon microphone has been proposed as a semiconductor sensor for detecting acoustic vibration (see, for example, Patent Document 1 and Patent Document 2). This microphone includes a vibrating electrode (diaphragm electrode) and a fixed electrode (back plate electrode) that is opposed to the vibrating electrode and arranged at a predetermined interval, and a capacitor is formed by the vibrating electrode and the fixed electrode. The

When the sound pressure is applied to the microphone, the vibration electrode vibrates, and the capacitance changes due to a change in the distance between the vibration electrode and the fixed electrode due to the vibration. Furthermore, an acoustic signal is obtained by electrically reading a change in voltage between the two electrodes accompanying a change in capacitance.
JP-T 60-500841 JP 2002-328117 A

  In the microphone described above, noise countermeasures are important in order to measure changes in capacitance due to vibration. In particular, when the fixed electrode is displaced due to vibration or the like, there is a problem that an accurate capacitance change cannot be measured, and noise is added to the obtained acoustic signal.

  In view of the above problems, an object of the present invention is to provide an acoustic sensor with improved noise resistance against acoustic signals.

  In order to achieve the above object, an acoustic sensor according to an aspect of the present invention has a first electrode provided on a semiconductor substrate, a first electrode and a first electrode so as to constitute a capacitor, and a predetermined interval. And a first insulating film provided on the second electrode and fixing the second electrode to the semiconductor substrate. The first insulating film is formed into an insulating film by ion implantation. On the other hand, it is a film into which impurities are introduced.

  In the reforming method using ion implantation, the temperature of the film is substantially increased to about 800 ° C. while the impurity is implanted, and the film changes to a dense state. At this time, the film is densified, but the film stress is relaxed because the film is densified while the bonds in the film are broken by the implantation of impurities. After the implantation of impurities, when the film returns from about 800 ° C. to an equilibrium state at room temperature, a force that causes the film to expand is generated, and a compressive stress (stress acting in the direction of expanding with respect to the base) is generated as the film stress.

  According to this aspect, since the compressive stress (stress acting in the direction of expansion with respect to the base) is caused by ion implantation in the first insulating film, the second electrode is in the outer direction (direction of expansion with respect to the base). Fixed in the pulled state. For this reason, when sound pressure is applied to the second electrode, vibration (displacement) of the second electrode is suppressed. As a result, the noise added to the acoustic signal is reduced as compared with the case where an insulating film that is not ion-implanted is used, so that a low-noise acoustic sensor capable of measuring an accurate capacitance change can be provided.

  In the acoustic sensor according to another aspect of the present invention, the first insulating film is a film in which impurities are introduced into an insulating film containing Si, O, and C by ion implantation.

  According to this aspect, since the first insulating film is a film in which impurities are introduced into the insulating film containing Si, O, and C, the dielectric constant is lower than that of a conventional insulating film such as a silicon oxide film or a silicon nitride film. Rate. As a result, the parasitic capacitance added to the electrostatic capacitance between the first electrode and the second electrode, specifically, the parasitic capacitance caused by the first insulating film fixing the second electrode (parasitic capacitance≈dielectric constant of the material) (Rate × area / thickness) can be reduced, so that the sensitivity of the acoustic sensor (sensitivity≈bias voltage × capacitance change due to vibration / capacitance) can be improved.

  Further, since impurities are introduced into the insulating film containing Si, O, and C, the densification of the film by ion implantation and the subsequent expansion change more greatly, and therefore higher compression compared to the conventional insulating film. Stress (stress acting in the direction of expansion with respect to the substrate) is caused. For this reason, the second electrode is fixed in a state of being pulled outward by the first insulating film (the direction in which the second electrode expands with respect to the base), and when sound pressure is applied to the second electrode, the vibration of the second electrode ( (Displacement) is suppressed. As a result, the noise added to the acoustic signal is reduced as compared with the case where the conventional insulating film is used, so that it is possible to provide a low noise acoustic sensor capable of measuring an accurate capacitance change.

  According to another aspect, the first insulating film includes a first region into which impurities are introduced and a second region into which impurities are not introduced. According to this aspect, since the first insulating film for fixing the second electrode includes the first region into which the impurity is introduced, the second electrode can be fixed in a state of being pulled outward, and propagation of sound waves or the like can be performed. The vibration (displacement) of the second electrode due to can be suppressed. In addition, since the insulating film into which the impurity is introduced is densified by the ion implantation method, the dielectric constant is higher than that of the insulating film before the ion implantation. Therefore, the first insulating film introduces the impurity. By including the second region which is not present, the relative dielectric constant of the first insulating film can be reduced as compared with the case where the first insulating film is configured only by the first region. As a result, the parasitic capacitance added to the electrostatic capacitance between the first electrode and the second electrode, specifically, the parasitic capacitance due to the first insulating film fixing the second electrode can be reduced. The sensitivity of the acoustic sensor can be improved.

  According to still another aspect, the impurity is introduced into the second electrode through the first insulating film. By doing so, the adhesion between the second electrode and the first insulating film is improved by the mixing action at the interface between the second electrode and the first insulating film, and the second electrode is more firmly bonded to the first insulating film. Therefore, it is possible to further improve noise resistance against an acoustic signal.

  ADVANTAGE OF THE INVENTION According to this invention, the acoustic sensor which improves the noise tolerance with respect to an acoustic signal can be provided.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, embodiments of the invention will be described with reference to the drawings. In all the drawings, the same reference numerals are given to the same components, and the description will be omitted as appropriate. However, it should be noted that the drawings are schematic and ratios of dimensions and the like are different from actual ones. Therefore, specific dimensions and the like should be determined in consideration of the following description. Moreover, it is a matter of course that portions having different dimensional relationships and ratios are included between the drawings.

(First embodiment)
1 is a top view showing the configuration of the acoustic sensor 100 according to the first embodiment of the present invention, FIG. 2 is a cross-sectional view taken along the line AA ′ of the acoustic sensor 100 of FIG. 1, and FIG. It is BB 'sectional drawing of the 1 acoustic sensor 100. FIG.

  As shown in FIGS. 1 to 3, the acoustic sensor 100 according to the first embodiment of the present invention is opposed to the vibrating electrode 4 formed on the silicon substrate 1 and the vibrating electrode 4 with a predetermined interval therebetween. The fixed electrode 6 is provided, and the vibrating electrode 4 and the fixed electrode 6 constitute a capacitor.

  The acoustic sensor 100 includes an air gap layer 11, a modified SiOC layer 8a, a sacrificial layer 5, a fixed electrode 6, a vibrating electrode 4, a substrate opening 10a, an acoustic hole 7a, a vibrating electrode pad electrode 9a, and a fixed electrode pad electrode 9b. , Etch stopper 2a, silicon substrate 1 and the like. As is clear from FIGS. 2 and 3, in FIG. 1, the air gap layer 11 and the like cannot be directly seen from the upper surface. However, in order to facilitate the understanding of the structure, portions that are not directly visible are shown so that they can be seen as appropriate.

  The silicon substrate 1 is a substrate for the acoustic sensor 100. As shown in FIGS. 2 and 3, sound holes 10 are formed in the silicon substrate 1 from the upper side to the lower side of the silicon substrate 1. As shown in FIG. 1, the substrate opening 10a of the sound hole 10 on the upper surface of the silicon substrate 1 has a quadrangular shape. The upper surface of the silicon substrate 1 is provided with an etch stopper 2a.

  As shown in FIGS. 2 and 3, the vibrating electrode 4 is formed so as to cover the sound hole 10 in the cross section of the silicon substrate 1. The sound pressure is transmitted from the lower side of the sound hole 10, and the vibrating electrode 4 can be vibrated by the sound pressure. Specifically, the vibration electrode 4 is formed in a floating state in the region covering the sound hole 10, and is fixed on the silicon substrate 1 (etch stopper 2 a) in the outer peripheral region of the sound hole 10.

  As shown in FIGS. 2 and 3, the fixed electrode 6 is provided above the vibration electrode 4 and forms a capacitor together with the vibration electrode 4. The capacitor has a characteristic that the capacitance value changes when the vibrating electrode 4 vibrates due to sound pressure. The fixed electrode 6 is designed to have a size that occupies at least a part of the substrate opening 10 a, that is, the sound hole 10.

As shown in FIGS. 2 and 3, the modified SiOC layer 8 a is formed so as to cover the fixed electrode 6, and the fixed electrode 6 is fixed to the silicon substrate 1 at the outer peripheral portion of the fixed electrode 6. Here, the modified SiOC layer 8a is formed by applying impurities such as boron (B) to the SiOC film [composition: SiO _x (CH ₃ ) _y ], which is one of low dielectric constant insulating films, using an ion implantation method. It is an introduced film.

  As shown in FIGS. 2 and 3, the sacrificial layer 5 is formed so as to insulate the vibrating electrode 4 and the fixed electrode 6. Here, the space formed between the modified SiOC layer 8 a and the fixed electrode 6 and the vibration electrode 4 is referred to as an air gap layer 11. The modified SiOC layer 8a and the fixed electrode 14 form a plurality of acoustic holes 7a.

  The vibration electrode pad electrode 9a and the fixed electrode pad electrode 9b are connected to the vibration electrode 4 and the fixed electrode 6, respectively, and apply a predetermined voltage. Further, if the capacitance of the capacitor by the vibrating electrode 4 and the fixed electrode 6 changes, the potential difference between the vibrating electrode pad electrode 9a and the fixed electrode pad electrode 9b also changes. Output as. That is, the vibration electrode pad electrode 9a and the fixed electrode pad electrode 9b indirectly detect a change in capacitance of the capacitor. For example, the output audio signal is output by a speaker or converted into a digital signal and stored.

  The silicon substrate 1 is the “semiconductor substrate” of the present invention, the vibrating electrode 4 is the “first electrode” of the present invention, the fixed electrode 6 is the “second electrode” of the present invention, and the modified SiOC layer 8 a is the present invention. This is an example of “first insulating film: a film in which impurities are introduced into an insulating film containing Si, O, and C”.

  Table 1 summarizes the film characteristics (residual stress, BHF etching rate, relative dielectric constant) of the modified SiOC layer 8a. Residual stress (internal stress) includes stress acting in the direction of contraction with respect to the underlying film (tensile stress) and stress acting in the direction of expansion (compressive stress). A negative value indicates a tensile stress.

固定電極６を固定する絶縁膜が圧縮応力を有することによって、絶縁膜が固定電極６を外周方向（下地に対して膨張する方向）に引っ張るように作用するため、大きな圧縮応力を有する絶縁膜であればあるほど固定電極６をより強固に引っ張った状態に固定することができる。

Since the insulating film for fixing the fixed electrode 6 has a compressive stress, the insulating film acts to pull the fixed electrode 6 in the outer peripheral direction (the direction in which it expands with respect to the base). The more there is, the more the fixed electrode 6 can be fixed in a pulled state.

As is apparent from Table 1, the modified SiOC film has a compressive stress opposite to the tensile stress of the unmodified SiOC film, and further, a silicon nitride film (SiN) or a silicon oxide film (SiO _2). ) Has a compressive stress that is nearly twice as large as that of. This is because a modified SiOC film (a SiOC film into which impurities are introduced by ion implantation) is compressed by compressing stress (expanded with respect to the base) due to densification of the film by ion implantation and subsequent expansion. This is because the stress acting in the direction of

  In the first embodiment, since the modified SiOC layer 8a made of the modified SiOC film having a high compressive stress fixes the fixed electrode 6, the fixed electrode 6 is pulled outward (in the direction of expansion with respect to the base). Fixed in a fixed state. For this reason, when sound pressure is applied to the fixed electrode 6, vibration (displacement) of the fixed electrode 6 is suppressed. As a result, the noise added to the acoustic signal is reduced as compared with the case where the SiOC film into which no impurity is introduced is used, so that the low-noise acoustic sensor 100 capable of measuring an accurate capacitance change can be provided.

  Next, a method for manufacturing the acoustic sensor 100 according to the first embodiment of the present invention will be described with reference to FIGS. 4 to 7 correspond to the A-A ′ cross section in the acoustic sensor 100 of FIG. 1, similarly to FIG. 2.

  First, in step 1 shown in FIG. 4 (A), etch stoppers 2a and 2b are formed to a thickness of about 200 nm on the double-side polished silicon substrate 1 using a low pressure CVD method. The etch stopper 2b serves as a mask when the silicon substrate 1 is etched from the back surface, and the etch stopper 2a serves as a stopper when the silicon substrate 1 is later etched from the back surface. A silicon nitride film is generally used for the etch stoppers 2a and 2b. Gases used for forming the silicon nitride film are monosilane and ammonia, dichlorosilane and ammonia, and the film formation temperature is 300 to 600 ° C.

  In step 2 shown in FIG. 4B, a sacrificial layer 3 having a thickness of about 500 nm is formed on the entire surface of the etch stopper 2a by using a plasma CVD method or an atmospheric pressure CVD method. The sacrificial layer 3 is generally a silicon oxide film containing phosphorus (P), but any film may be used as long as it is soluble in hydrofluoric acid (HF). The sacrificial layer 3 is a film that does not remain in the final structure because it is removed later by etching with HF. Thereafter, unnecessary portions of the sacrificial layer 3 are removed using a normal photolithography technique and an etching technique.

  In Step 3 shown in FIG. 4C, the vibrating electrode 4 is formed to a thickness of about 1 μm on the entire surface of the etch stopper 2a and the sacrificial layer 3. Polysilicon is generally used for the vibrating electrode 4, but other conductive materials may be used. Then, unnecessary portions of the vibrating electrode 4 are removed using a normal photolithography technique and an etching technique.

  In step 4 shown in FIG. 5D, a sacrificial layer 5 of about 3 μm is formed on the vibrating electrode 4. As the sacrificial layer 5, a silicon oxide film containing phosphorus (P) is generally used as in the sacrificial layer 3, but any film may be used as long as it is soluble in hydrofluoric acid (HF). A portion of the sacrificial layer 5 that becomes the air gap layer 11 (see FIG. 2) in a later process is not left in the final structure because it is later removed by etching with HF. The remaining sacrificial layer 5 in the final structure functions as an insulating layer that insulates the vibrating electrode 4 and the fixed electrode 6 from each other. Then, the periphery of the sacrificial layer 5 is removed using a normal photolithography technique and an etching technique to form the sacrificial layer 5 having an opening. Here, since the film thickness of the sacrificial layer 5 becomes the final air gap distance between the electrodes, capacitance (C = e * S / t, e: dielectric constant, S: electrode area, t: air gap distance) ), That is, it is reflected in the sensitivity, and has a great influence on the robustness of the structure of the acoustic sensor 100. For example, if the air gap layer 11 is too narrow, the vibrating electrode 4 and the fixed electrode 6 come into contact with each other, and sensing cannot be performed.

  In step 5 shown in FIG. 5E, a conductive film for forming the fixed electrode 6 is formed over the sacrificial layer 5. From the viewpoint of mechanical strength, the conductive film is preferably polysilicon. Then, unnecessary portions are removed using a normal photolithography technique and an etching technique. At the time of this patterning, the acoustic hole 7 for the air in the air gap layer to move according to the displacement of the vibrating electrode 4 is patterned at the same time.

In step 6 shown in FIG. 5F, an SiOC film 8 that is an insulating film containing Si, O, and C is formed on the fixed electrode 6 and the sacrificial layer 5 by using a plasma CVD method. Specifically, a mixed gas composed of trimethylsilane and oxygen is used under the conditions of a film forming temperature of about 350 ° C., a film forming pressure of about 4.0 Torr, and a high frequency power of about 600 W. Thereby, the low dielectric constant SiOC film 8 having a composition of SiO _x (CH ₃ ) _y is formed.

In step 7 shown in FIG. 6G, ion implantation is performed to modify the SiOC film 8. Specifically, boron ions (B ⁺ ) are implanted under conditions of an implantation energy of about 140 keV and an implantation amount of about 2 × 10 ¹⁵ cm ⁻² . Thereby, the modified SiOC layer 8a in which boron is introduced into the film is formed.

  In the reforming method using ion implantation, the temperature of the film is substantially increased to about 800 ° C. while the impurity is implanted, and the film changes to a dense state. At this time, the film is densified, but the film stress is relaxed because the film is densified while the bonds in the film are broken by the implantation of impurities. After the implantation of impurities, when the film returns from about 800 ° C. to an equilibrium state at room temperature, a force that causes the film to expand is generated, and a compressive stress (stress acting in the direction of expanding with respect to the base) is generated as the film stress. As a result, a modified SiOC layer 8a having a high compressive stress is obtained.

  Further, by introducing boron ions into the SiOC film 8 using an ion implantation method, the adhesion between the fixed electrode 6 and the modified SiOC layer 8a is achieved by the mixing action at the interface between the fixed electrode 6 and the SiOC film 8. Since the fixed electrode 6 is fixed in a state in which it is more strongly pulled by the modified SiOC layer 8a, the noise resistance against the acoustic signal can be further improved.

  In step 8 shown in FIG. 6H, unnecessary portions of the modified SiOC layer 8a are removed by using a normal photolithography technique and an etching technique. Here, the unnecessary portion includes not only the peripheral portion but also the pad portion 20 and the acoustic hole 7a. During the patterning of the modified SiOC layer 8a, the acoustic hole 7a having a diameter smaller than that formed by the conductive film is formed simultaneously with the position of the acoustic hole 7 processed previously. Thereby, since the modified SiOC layer 8a is also provided on the side wall of the fixed electrode 6, the modified SiOC layer 8a and the fixed electrode 6 are more firmly fixed.

  In step 9 shown in FIG. 6 (I), pad electrodes such as a vibration electrode pad electrode 9 a and a fixed electrode pad electrode 9 b (not shown) are formed on the pad portion 20. A low resistance metal film such as aluminum, copper, or gold is particularly suitable for the pad electrode. As a formation method, there is a method using a normal photolithography technique and an etching technique, but a technique such as a so-called plating resist method or a resist etch-off method may be applied.

  In step 10 shown in FIG. 7J, unnecessary portions of the etch stop 2b on the back surface side of the silicon substrate 1 are removed by using a normal photolithography technique and an etching technique.

  In step 11 shown in FIG. 7K, using the patterned etch stop 2a as a mask, isotropic etching with an alkaline etchant such as an aqueous potassium hydroxide solution (KOH) or an aqueous tetramethylammonium hydroxide solution (TMAH) is performed. I do. This isotropic etching is automatically stopped by the etch stopper 2a formed in step 1. Thereby, a substrate opening 10 is formed in the silicon substrate 1.

  In step 12 shown in FIG. 7 (L), the etch stopper 2a in the opened portion is removed from the back side of the silicon substrate 1 by an etching solution (for example, phosphoric acid) or dry etching. Thereafter, the sacrificial film 3 is removed from the back surface side of the silicon substrate 1 using HF, and the sacrificial film 5 is selectively etched away from the acoustic hole 7a side to finally form the air gap layer 11. .

(Second Embodiment)
FIG. 8 is a cross-sectional view showing an acoustic sensor 100a according to the second embodiment of the present invention. The difference from the first embodiment is that the insulating film for fixing the fixed electrode 6 is a laminated film of the SiOC film 8c and the modified SiOC layer 8b. The rest is the same as in the first embodiment.

  Formation of the laminated film can be easily realized by controlling the modification depth by reducing the energy injected into the SiOC film 8 in step 7 in the first embodiment. In the second embodiment, for example, the implantation energy is set to 100 keV.

  The modified SiOC layer 8b is an example of the “first region” in the present invention, and the SiOC film 8c is an example of the “second region” in the present invention.

  As shown in Table 1, the residual stress of the SiOC film is a tensile stress and cannot be fixed in a state where the fixed electrode 6 is pulled, but the relative dielectric constant of the film is 3.0, and the silicon oxide film (relative dielectric constant) 4.2) and a low dielectric constant as compared with silicon nitride film (relative dielectric constant 7). In the second embodiment, since the insulating film for fixing the fixed electrode 6 includes the modified SiOC layer 8b into which impurities are introduced, the fixed electrode 6 can be fixed in a state of being pulled outward, such as a sound wave. The vibration (displacement) of the fixed electrode 6 due to the propagation of can be suppressed. The modified SiOC layer 8b has a higher dielectric constant than the SiOC film before ion implantation because the film is densified by the ion implantation method, so that the insulating film for fixing the fixed electrode 6 introduces impurities. By including the SiOC film 8c that has not been formed, the relative dielectric constant of the film can be reduced as compared with the case where the insulating film for fixing the fixed electrode 6 is composed of only the modified SiOC layer 8a. As a result, the parasitic capacitance applied to the electrostatic capacitance between the vibrating electrode 4 and the fixed electrode 6, specifically, parasitic caused by the insulating films (modified SiOC layer 8 b and SiOC film 8 c) that fix the fixed electrode 6. Capacitance (parasitic capacitance≈dielectric constant of material × area / thickness) can be reduced, so that the sensitivity (sensitivity≈bias voltage × capacitance change due to vibration / capacitance) of the acoustic sensor 100a is improved. Can do.

  Further, as shown in Table 1, the SiOC film 8c and the modified SiOC layer 8b each have a lower etching rate of BHF (buffered hydrofluoric acid) than the silicon oxide film and the silicon nitride film. For this reason, since the film loss due to the HF treatment when removing the sacrificial layer 3 and the sacrificial layer 5 is suppressed, the same effect can be obtained as when the insulating film for fixing the fixed electrode 6 is substantially thickened. As a result, the fixed electrode 6 can be fixed while being pulled more firmly. Further, since the etching rate of BHF is low, the process margin for removing the sacrificial layer in the actual manufacturing process (step 12) is expanded, and a high-performance and high-yield acoustic sensor can be realized.

  As described above, the present invention naturally includes various embodiments not described herein. Therefore, the technical scope of the present invention is defined only by the invention specifying matters according to the scope of claims reasonable from the above description.

It is a top view which shows the structure of the acoustic sensor which concerns on 1st Embodiment of this invention. It is A-A 'sectional drawing of the acoustic sensor of FIG. FIG. 2 is a B-B ′ cross-sectional view of the acoustic sensor of FIG. 1. It is sectional drawing for demonstrating the manufacturing process of the acoustic sensor which concerns on 1st Embodiment of this invention. It is sectional drawing for demonstrating the manufacturing process of the acoustic sensor which concerns on 1st Embodiment of this invention. It is sectional drawing for demonstrating the manufacturing process of the acoustic sensor which concerns on 1st Embodiment of this invention. It is sectional drawing for demonstrating the manufacturing process of the acoustic sensor which concerns on 1st Embodiment of this invention. It is sectional drawing which shows the acoustic sensor which concerns on 2nd Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Silicon substrate 2a, 2b Etch stop 3 Sacrificial layer 4 Vibrating electrode 5 Sacrificial layer 6 Fixed electrode 7a Acoustic hole 8a Modified SiOC layer 9a Vibrating electrode pad electrode 10a Substrate opening 11 Air gap layer 100 Acoustic sensor

Claims (4)

A first electrode provided on a semiconductor substrate;
A second electrode opposed to the first electrode so as to form a capacitor with the first electrode, and disposed at a predetermined interval;
A first insulating film provided on the second electrode and fixing the second electrode to the semiconductor substrate;
The acoustic sensor according to claim 1, wherein the first insulating film is a film in which impurities are introduced into the insulating film by ion implantation.   The acoustic sensor according to claim 1, wherein the first insulating film is a film in which impurities are introduced into an insulating film containing Si, O, and C by ion implantation.   The acoustic sensor according to claim 2, wherein the first insulating film includes a first region into which the impurity is introduced and a second region into which the impurity is not introduced.   The acoustic sensor according to claim 1, wherein the impurity is introduced into the second electrode through the first insulating film.

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