Patent Application
20100254052
The present invention relates to an electrostatic discharge (ESD) protector for absorbing static electricity.
In recent years, in order to satisfy the demands for electronic devices, such as mobile phones, having a small size and a more sophisticated function, IC has been further miniaturized and highly integrated, and had its withstanding voltage reduced accordingly. Thus, the IC is broken or operates abnormally even due to a surge, such as a static electricity discharge surge, having small energy that is generated when, e.g. a human body contacts a terminal of an electronic device.
As a countermeasure, an electrostatic discharge (ESD) protector is connected between a ground and a wiring allowing static electricity entering thereto so as to prevent a high voltage to be applied to the IC. The ESD protector has a characteristic to have a large resistance value preventing electricity from flowing through the ESD protector in a normal status, and to have a small resistance value allowing electricity to flow through the ESD protector when a high voltage, such as static electricity, is applied to the ESD protector. A zener diode, a multilayer chip varistor, and a gap surge absorber are known as the ESD protector having the above characteristic.
Patent Documents 1 and 2 disclose a conventional ESD protector, a gap surge absorber. The gap surge absorber includes a ceramic body having a cavity, a pair of discharge electrodes embedded in the ceramic body, and terminal electrodes connected to the discharge electrodes. The discharge electrodes surface each other across the cavity. The discharge electrodes open between the electrodes. When a high voltage, such as static electricity, is applied to the ESD protector, the voltage causes a discharging in the cavity between the discharge electrodes, thereby causing a current to flow.
The gap surge absorber generally has a smaller parasitic capacitance value than other ESD protectors, such as the zener diode and the multilayer chip varistor. Upon having a large capacitance value, an ESD protector connected to a signal line deteriorates a quality of a signal having a high frequency, thus preferably having a small parasitic capacitance value. Thus, the gap surge absorber can be connected to such a signal line. The cavity having the discharging occurring therein contain nothing but air, and hence, does not cause the surge absorber to break even when static electricity of a high voltage is applied, thus being advantageous against the other ESD protectors.
The pair of discharge electrodes is exposed to the cavity with a predetermined interval between the electrodes. A temperature in the cavity may reach a high temperature, e.g. higher than 2500° C., due to the discharging of static electricity. The static electricity repetitively applied to the surge absorber may melt the discharge electrodes to cause a short-circuiting.
Patent Document 1: JP1-102884A
Patent Document 2: JP11-265808A
An electrostatic discharge (ESD) protector includes a ceramic body having a cavity provided therein, and two discharge electrodes facing each other across the cavity. The discharge electrodes are made of metal containing more than 80 wt. % of tungsten. The discharge electrodes contain not more than 2.0 atomic % of tungsten bonded to oxygen to a total amount of tungsten contained in the discharge electrodes.
This ESD protector does not cause a short-circuiting even upon having high-voltage static electricity applied to the discharge electrodes repetitively, thus having high reliability.
Ceramic body 101 is preferably made of ceramic insulating material mainly containing at least one ceramic composition selected from alumina, forsterite, steatite, mullite, and cordierite. These insulating materials have a low relative permittivity not larger than 15 and can reduce a parasitic capacitance value between discharge electrodes 103 and 104.
Discharge electrodes 103 and 104 are made of metal containing not less than 80% by weight of tungsten. Not larger than 1.8 atomic % of the total amount of the tungsten is bonded to oxygen. The amount of tungsten bonded to oxygen is preferably 0 atomic % of the total amount of tungsten, but actually, can be larger higher than 0 atomic %. Each of portions 103A and 104A of discharge electrodes 103 and 104 facing each other has an area ranging from 0.01 mm2 to 1.0 mm2. Distance D101 between portion 103A and 104A of discharge electrodes 103 and 104 range from 5 μm to 16 μm.
A method for manufacturing ESD protector 111 will be described below.
First, as shown in
Next, as shown in
Next, as shown in
Next, as shown in
Next, unsintered layered structure 308 is cut and divided into plural chips. The chips of unsintered layered structure 308 are sintered in mixture atmosphere made of nitrogen and not less than 0.8 vol. % of hydrogen. While unsintered layered structure 308 is sintered, hydrogen reduces oxide on surfaces of metal layers 302 and 306. This sintering, as shown in
Finally, as shown in
In the above manufacturing method, the ceramic paste for forming green sheets 301 and 307 is prepared by mixing powders of the above-described ceramic composition, binder resin, and plasticizer with solvent. The resin paste for forming resin layer 303 is prepared by kneading solid resin bead 303C and resin paste 303D. Resin bead 303C is an acrylic bead. Resin paste 303D is acrylic resin. Acrylic resin is decomposed at a low temperature more easily than other resins, and prevents ceramic body 101 from having defects around cavity 102. Instead of the acrylic resin, the resin paste may be made of another resin that is easily decomposed at a low temperature.
The conductive paste forming metal layers 302 and 306 is made of metal containing more than 80 wt. % of tungsten.
The ceramic paste for forming green sheets 304 and 305 is prepared by mixing ceramic composition powder, binder resin, and plasticizer with solvent, similarly to the ceramic paste forming green sheets 301 and 307. However, the ceramic paste for forming green sheets 304 and 305 contains the binder resin at a more content rate than that of the ceramic paste for forming green sheets 301 and 307. This arrangement prevents green sheets 301, 304, 305, and 307 constituting ceramic body 1 from delaminating. Resin layer 303 and green sheets 304 and 305 may be formed in any order to provide the same effects.
A multilayered structure without green sheets 304 and 305 may cause undulation in resin layer 303 to prevent the conductive paste for forming metal layer 306 from being precisely applied by the screen printing, and produce a defect in ceramic body 1 after the sintering. Green sheets 304 and 305 can eliminate the undulation, hence allowing the conductive paste to be applied precisely, hence to form metal layer 306.
Next, samples of ESD protector 111 were prepared. These samples were subjected to a static electricity discharge test based on the IEC-6100-4-2 standard (4 to 20 kV-150 pF-330Q).
In order to evaluate the status of the surfaces of discharge electrodes 103 and 104, the upper surface of ceramic body 101 was polished so as to expose surfaces of discharge electrodes 103 and 104. The exposed surfaces of discharge electrodes 103 and 104 were measured by an X-ray photoelectron spectroscopy (XPS) analysis with an X-ray source of Al-Kα, photoelectron extraction angle of 45 degrees, an analysis area of 100 μmΦ, and a voltage of 25.9 W. The amount of tungsten bonded to oxygen and the amount of tungsten not bonded to oxygen on the surfaces of discharge electrodes 103 and 104 were detected. The amount of oxide of tungsten was calculated based on the detected amounts.
The above-described static electricity discharge test was also performed under conditions of 8 kV-150 pF-330Ω. A static electricity pulse was repetitively applied until the number of times the pulse was applied reached 1000. Then, a change in the insulation resistance of ESD protector 111 was measured.
Samples 1 to 4 are different from one another only in the baking atmosphere. In sample 1 sintered in baking atmosphere ATM101 containing 100% of nitrogen and 0% of hydrogen, although an electrostatic discharge occurred between electrodes 103 and 104, the surfaces of electrodes 103 and 104 had 6 atomic % of tungsten bonded to oxygen to the total amount of tungsten. The oxide existing on the surfaces of electrodes 103 and 104 increases a resistance on the surfaces, accordingly suppress the electrostatic discharge. Thus, sample 1 exhibited a high discharge-starting voltage of 8 kV and a very high suppressed peak voltage due to the static electricity of a high voltage applied. In samples 3 and 4 sintered in the baking atmosphere containing more than 0.2 vol. % of hydrogen, the amount of tungsten bonded to oxygen at the surfaces of discharge electrodes 103 and 104 was lower than 2 atomic %, and the discharge-starting voltage and the suppressed peak voltage were low, thus providing superior characteristics. The baking atmosphere contains a lot of hydrogen reduces the composition pf the ceramic body during the sintering, and loses the insulation property, thus changing into semiconductor. Depending on the type of the ceramic composition and the sintering temperature, the upper limit of the concentration of hydrogen in the baking atmosphere changes. Thus, the upper limit may be determined appropriately.
Samples 5 to 8 are different from one another only in the materials of discharge electrodes 103 and 104. Tungsten mixed with copper forms alloy having a low melting point and providing electrodes 103 and 104 with high conductivity. In the case that samples 5 and 6 for which the metal constituting discharge electrodes 103 and 104 contained not more than 80 wt. % of tungsten, no short-circuiting occurred between electrodes 103 and 104 even when ESD was executed 1000 times. In sample 7 including electrodes 103 and 104 containing 70 wt. % of tungsten, short-circuiting occurred when the ESD was executed 500 times. In sample 8 including electrodes 103 and 104 made of platinum, a short-circuiting occurred when the ESD occurred only 50 times. The ESD raises a temperature of cavity 102 and electrodes 103 and 104 to 2500 to 3000° C. Upon having a melting point equal to or higher than this temperature, electrodes 103 and 104 are prevented from short-circuiting even when the ESD is executed repetitively.
Samples 9 to 12 are different from one another only in facing area S101 at which discharge electrodes 103 and 104 face each other. Samples having large facing area S101 causes the ESD executed repetitively, having a small insulation resistance. Sample having small facing area S101 have a suppressed peak voltage and a high discharge-starting voltage.
Sample 11 having facing area S101 larger than 1.0 mm2 had a low insulation resistance after 1000 times of the ESD on the order of 106Ω but had no short-circuiting. Thus, facing area S101 is preferably not larger than 1.0 mm2. Sample 12 having facing area S101 smaller than 0.01 mm2 had no ESD by static electricity of 4 kV. Thus, facing area S101 is preferably not smaller than 0.01 mm2.
Samples 13 to 20 are different from each other in the resin paste for forming cavity 102. If the diameter of the resin beads contained in the resin paste and the content rate of the resin beads are different, the height of cavity 102, i.e., distance D101 between electrodes 103 and 104, changes. Smaller distance D101 reduces the insulation resistance due to the ESD executed repetitively. Samples 13 and 14 having distance D101 shorter than 5 μm had a low insulation resistance ranging from 1×105Ω to 1×108Ω although no short-circuiting occurred between electrodes 103 and 104. On the other hand, larger distance D101 suppresses the ESD and provides a higher suppressed peak voltage. Samples 19 and 20 having distance D101 more than 20 μm had a high suppressed peak voltage higher than 900V for the static electricity of 6 kV. The height of cavity 102, i.e., distance D101 between electrodes 103 and 104, ranges preferably from 5 to 20 μm. Sample 18 having distance D101 more than 16 μm had no ESD at static electricity of 4 kV although having a low suppressed peak voltage. Thus, distance D101 between electrodes 103 and 104 ranges preferably from 5 to 16 μm.
Ceramic body 101 can have another circuit as to further lower the suppressed peak voltage. For example, ceramic body 101 can have a fine line patterned to form an inductor. Alternatively, the surface of ceramic body 101 can be coated or printed with resistance paste to form a resistance.
Hydrogen contained in the baking atmosphere for sintering unsintered layered structure 308 reduces the oxide on the surface of discharge electrodes 103 and 104. Instead of hydrogen, the baking atmosphere can contain other reducible gas, such as carbon monoxide or sulfurous gas, for reducing the oxide on the surface of discharge electrodes 103 and 104 (metal layers 302 and 306).
An electrostatic (ESD) protector according to Exemplary Embodiment 2 has the same structure as ESD protector 111 shown in
The ESD protector according to Embodiment 2 can be manufactured by the method shown in
A green sheet is designed to provide ceramic body 101 with an overall size of 2.0 mm by 1.2 mm by 0.8 mm after the sintering.
Next, samples of the ESD protector according to Embodiment 2 were prepared. Similarly to Embodiment 1, these samples were subjected to a static electricity discharge test in the electrostatic test circuit shown in
The above static electricity discharge test was performed under conditions of 8 kV-150 pF-330Ω. A static electricity pulse was repetitively applied until the number of times the pulse was applied reached 1000. Then, a change in the insulation resistance of the ESD protector according to Embodiment 2 was measured.
Samples 21 to 24 are different from one another only in the baking atmosphere. In sample 21 sintered in baking atmosphere ATM101 containing 100% of nitrogen and 0% of hydrogen, although an ESD occurs between electrodes 103 and 104, the surfaces of electrodes 103 and 104 have tungsten bonded to oxygen in an amount as high as 6 atomic % to the total amount of tungsten. An X-ray photoelectron spectroscopy (XPS) analysis merely analyzes a part of the thickness of electrodes 103 and 104 by only a few nanometers from the surfaces of electrodes 103 and 104 and thus has substantially no influence on the resistance of the entire electrode. Oxide existing on the surfaces of electrodes 103 and 104 increases a resistance on the surfaces, thus suppressing the ESD. Thus, sample 21 has a high discharge-starting voltage of 15 kV and a very high suppressed peak voltage to static electricity of a high voltage. In samples 23 and 24 sintered in the baking atmosphere containing not less than 0.2 vol. % of hydrogen, the amount of tungsten bonded to oxygen on the surfaces of discharge electrodes 103 and 104 is lower than 2 atomic %, and the discharge-starting voltage and the suppressed peak voltage are low, thus providing superior characteristic. The upper limit of the concentration of hydrogen in the baking atmosphere can be any value so long as ceramic is not reduced during sintering.
Samples 25 to 28 are identical to samples 21 to 24, respectively, except for that samples 25 to 28 have a large facing area of 1.0 mm2. Similarly to samples 21 to 24, in samples 25 to 28, the amount of tungsten bonded to oxygen on the surfaces of discharge electrodes 103 and 104 is lower than 2 atomic %, and the discharge-starting voltage and the suppressed peak voltage are low, thus providing superior characteristics.
Samples 29 to 32 are different only in the materials of discharge electrodes 103 and 104. Tungsten mixed with copper forms alloy having a low melting point and improve conductivity of electrodes 103 and 104. In samples 29 and 30 for which the metal constituting discharge electrodes 103 and 104 contains not less than 80 wt. % of tungsten, no short-circuiting occurred between electrodes 103 and 104 even when the ESD is repeated 1000 times. In sample 31 including electrodes 103 and 104 containing 70 wt. % of tungsten, short-circuiting occurred when ESD was repeated 500 times. In sample 32 including electrodes 103 and 104 made of platinum, short-circuit occurred when ESD was repeated only 50 times. The ESD increases the temperature of cavity 102 and electrodes 103 and 104 to a high temperature ranging from 2500 to 3000° C. If discharge electrodes 103 and 104 have a melting point not lower than this temperature, electrodes 103 and 104 are free from short circuit even when ESD is repeated.
In Embodiments 1 and 2, the terms, such as “upper surface” and “directly above” indicating directions indicate relative directions depending on a relative position of components, such as the green sheets, the metal layers, and the resin layer, of the ESD protector, and do not indicate absolute directions, such as a vertical direction.
An electrostatic discharge protector according to the present invention does not cause a short-circuiting even upon having high-voltage static electricity applied to the discharge electrodes repetitively, thus having high reliability, and is useful for various devices requiring static electricity countermeasure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-305411 | Nov 2007 | JP | national |
| 2007-316900 | Dec 2007 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2008/003406 | 11/20/2008 | WO | 00 | 3/19/2010 |
