Patent Application
20070063297
1. Field of the Invention
The present invention relates to a small-sized and highly sensitive displacement detection device, applicable to an acceleration sensor, a pressure sensor, a gyro-sensor and the like, that is made by a semiconductor production technology, particularly to a displacement detection device comprising a displacement detection element with a flexibly deformable portion and an IC chip.
2. Description of the Related Art
The MEMS (Micro-Electro-Mechanical-System) technology, which manufactures solid fine structures having a mechanical function and an electric function on a semi-conductive substrate by combining a semiconductor production process technology with a machining technology and/or or a material technology, has been applied to widely various fields. Particularly, it has been applied to small-sized and highly sensitive sensor fields, such as an acceleration sensor, a pressure sensor, a gyro-sensor. These sensors, whose flexibly deformable portions are made by the MEMS technology, transform displacements of the flexibly deformable portions into electro-signals by a piezo resistor etc. to detect the displacements. Structures of the flexibly deformable portions may be different depending on applications of the displacement detection devices, but they all transform displacements of the flexibly deformable portions into electro signals and are called, for example, an acceleration sensor or a pressure sensor according to causes of displacements of the flexibly deformable portions.
Transformation devices for deforming flexibly deformable portions into electro-signals include piezo resistors, electro-static capacitors, strain gauges etc. Whatever used, their output voltages are as small as some mV to some tens mV. By the reason, it is necessary that a circuit for amplifying their output voltages is assembled with them in a wide variety of applications, such as automotive, aircrafts, mobile terminals and toys. An acceleration sensor apparatus disclosed in Japanese Laid-open Patent 2003-28891 is shown in an appearance perspective view of
Sensitivity and impact resistance may be changed by a shape of a flexibly deformable portion formed on a silicon substrate by the MEMS technology. Generally speaking, the impact resistance is enhanced when the flexibly deformable portion is made thicker, but the sensitivity is reduced. By contrast, when the flexibly deformable portion is made thinner to obtain a desirable sensitivity, the sensitivity is enhanced but the impact resistance lowers. It is difficult to satisfy both the good sensitivity and the good impact resistance. An acceleration sensor, which has a regulation plate to restrict a movement of a flexibly deformable portion within a predetermined extent even when an excessive external force is applied to the flexibly deformable portion, is described in Japanese Laid-open Patents Hei 4-274005 and Hei 8-233851.
An acceleration sensor having such a regulation plate is shown in
An acceleration sensor example more miniaturized by using an IC chip for a regulation plate instead of a silicon plate and a soda-lime glass is described in Japanese Laid-open Patent 2005-169541. The Japanese Patent Document discloses a structure, in which an IC chip is used for a regulation plate facing a top surface of an acceleration sensor element. The regulation plate 14 of
And, Japanese Laid-open Patent 2005-169541 describes a pressure sensor using an IC chip for a regulation plate. Its schematic structure is shown in a cross-sectional view of
Impact resistance in a displacement detection device has been enhanced by making a regulation plate restrict deformation of a flexibly deformable portion, and a small-sized and highly sensitive displacement detection device has been realized by using an IC chip for the regulation plate. However, among a number of displacement detection devices manufactured, displacement detection devices that have an operation problem due to silicon broken pieces caused from IC chips have occurred, though it was a very low happening frequency. By disassembling and investigating the displacement detection devices that had the operation accident, it has been revealed that silicon broken pieces are trapped into gaps between the flexibly deformable portions and the IC chips to obstruct movements of the flexibly deformable portions. It was confirmed by comparing the silicon broken pieces trapped into the gaps with cracks on side ridges of the IC chips that the silicon broken pieces were caused from the IC chips.
In a displacement detection device, a displacement detection element and an IC chip bonded to the displacement detection element are housed in a protection case. Silicon broken pieces caused from the IC chip are confined in the protection case, move around in the protection case, and are trapped into gaps between the flexibly deformable portion of the displacement detection element and the IC chip, and the trapped silicon broken pieces obstruct the movement of the flexibly deformable portions.
IC chips are obtained by dicing an IC wafer. Chippings caused on side ridges of the IC chips were not minded as long as they did not affect the IC circuits. Also, if silicon broken pieces are completely removed from the IC chips, the silicon broken pieces are not trapped in the gaps between the flexibly deformable portions and the IC chips nor disturb movement of the flexibly deformable portions. Chippings at issue are loose chippings, which remain at defected portions, that is, silicon broken pieces partially held by the IC substrates. During assembling and/or using the displacement detection devices, silicon broken pieces drop from loose chippings and affect properties of the displacement detection devices.
The present invention has been made to solve the above-mentioned problem and has an object to provide an IC chip, used for a regulation plate for restricting deformation of a flexibly deformable portion of a displacement detection device, which does not have loose chippings.
A displacement detection device according to the present invention comprises:
Grinding direction on the rear surface of the IC chip is the same as that of grinding traces left on the IC chip. The direction is defined by an angle of the grinding traces with a vertical line on a side ridge of the IC chip. The angle of 45 degrees means the direction at an angle of +/−45 degrees with the vertical line. In other words, the grinding traces are at an angle of 45 degrees or 135 degrees with the vertical line. The grinding direction (grinding traces) at an angle of 0 to 45 degrees with a vertical line with the side ridges minimizes chipping appearance ratio including loose chippings on the side ridges.
Silicon wafers, for manufacturing IC's, having 525 μm thick and 625 μm thick are mainly used, and their front surfaces are mirror-polished, while their rear surfaces are polished to such an extent that the surfaces can be suctioned by vacuum. IC wafers are ground on their rear surfaces to reduce their thickness to 100 to 300 μm after producing IC circuits on them, since the IC wafers thicker than 500 μm are too thick for a regulation plate for restricting deformation of flexibly deformable portions of displacement detection devices. A waterproof sheet is bonded on a surface with IC circuit to protect the IC circuit and the rear surface of the IC wafer is ground by a diamond grinding wheel, while pouring a grinding coolant. The ground surface roughness is about 0.1 μm to 0.2 μm in Rmax. It is known from experience that there is a relationship between the surface roughness on the rear surface of the IC wafer and chipping appearance frequency on side ridges after dicing into IC chips, and the coarser the surface roughness, the more the chippings caused on the side ridges, If the rear surface is polished to reduce chippings, the polishing is machinng-hour consuming but also increases the temperature of the IC wafer to hurt the IC circuit function. By the reason, not polishing but grinding is adopted. Also, because a thinned IC wafer is rarely handled with suctioning, a polished surface is not necessary for handling but a ground surface is enough.
In the displacement detection device according to the present invention, it is preferable that the grinding traces on the rear surface of the IC chip are at an angle of 10 to 45 degrees with the vertical line on the side ridges of the IC chip.
Forming grinding traces at an angle of 10 degrees to 45 degrees with a vertical line on side ridges of the IC chip can drastically reduce the number of chippings including loose chippings caused on the side ridges. Chipping appearance frequency for grinding traces at an angle less than 10 degrees with a vertical line is reduced to about 1/20 in comparison with the cases more than 45 degrees. When the rear surface roughness is improved to that of a polished surface of less than 0.01 μm in Rmax, the chipping appearance frequency even for a grinding trace angle more than 45 degrees reduces, but it is almost constant in a rear surface roughness range of Rmax 0.05 to 0.2 μm. By the reason, chippings including loose chippings caused on side ridges can be reduced by controlling the angle of grinding traces with the dicing direction on dicing IC wafers having a ground surface on the rear surface.
In a displacement detection device using a rectangular IC chip, it is preferable that the grinding traces of the rear surface of the IC chip are at an angle of 0 to 45 degrees with a vertical line on one of the side ridges of the rectangular IC chip, and that side ridges perpendicular to the one of the side ridges are bonded to the displacement detection element by adhesive.
When grinding traces of a rear surface of a rectangular IC chip are at an angle of 0 degrees to 45 degrees with a vertical line on one of side ridges of the IC chip, loose chipping appearance ratio can be reduced for the one of the side ridges. The rectangular IC chip has other side ridges at an angle of 90 degrees with the one of the side ridges. Then, the other side ridges of the rectangular IC chip might have an increased appearance ratio of loose chippings. Bonding the other side ridges by adhesive can prevent silicon broken pieces from removing from loose chippings caused on the other rear ridges. In the case, the chipping appearance ratio can be reduced to almost zero, when the grinding traces on the rear surface of the IC chip are at an angle of 20 degrees to 45 degrees with a vertical line on the one of the side ridges.
It is preferable that the IC chip is rectangular, and that the grinding traces of the rear surface of the IC chip are at an angle of 0 degrees to 20 degrees with the vertical line on the one of the side ridges of the rectangular IC chip.
When grinding traces of the rear surface of the rectangular IC chip are at an angle of 0 degrees to 20 degrees with a vertical line on the one of the side ridges of the rectangular IC chip, the other side ridges of the IC chip have the grinding traces of the rear surface of the IC chip at an angle of 70 degrees to 90 degrees with a vertical line. Since loose chipping appearance ratio is low for the grinding traces at angles of 0 degrees to 20 degrees or 70 degrees to 90 degrees with a vertical line on a side ridge, silicon broken pieces caused by loose chippings can be avoided.
Since IC circuit patterns cannot be disregarded to cut into IC chips, IC wafers are set on a grinding machine to direct the grinding direction of the IC wafers in a predetermined direction with respect to the dicing direction. When using a rotary grinding machine, the grinding directions change from an outside periphery of the grinding table to an inside periphery, if the IC wafers are set in the same direction. It is desirable to set the IC wafers on the grinding table in different directions depending on radial positions to make the grinding traces in a predetermined direction.
Chippings including loose chippings caused on side ridges of IC chips can be reduced by controlling an angle of grinding traces of rear surfaces of IC chips with an IC chip dicing direction. Using IC chips having loose chippings on side ridges as regulation plates can be avoided, and a highly reliable displacement detection device has been accomplished.
Embodiments of the present invention are described in detail below, referring to the accompanied drawings, in which same numeral references are used for same parts for convenience of understanding.
A relationship between an angle of grinding traces with a dicing direction of IC wafers and chipping appearance frequency will be explained below. First of all, methods of grinding and dicing and conditions for taking data are described. After reducing thickness of IC wafers from 625 μm to 250 μm in silicon thickness by grinding, the wafers were cut into IC chips of 2.6 mm×2.2 mm by a diamond dicing wheel. A waterproof sheet was bonded onto an IC circuit surface to protect the IC circuit from a grinding coolant for grinding. The grinding was done by a rotary grinding machine with a diamond grinding wheel of #2000 at a revolution speed of about 600 rpm. The surface roughness of the ground surface was 0.13 μm in Rmax and 0.014 μm in Ra. Rmax and Ra were measured according to JIS B 0601. Since a thickness of the IC circuit was about 5 μm, the thickness to cut was about 255 μm. The IC wafer was bonded to an adhesive sheet, and the IC wafer was diced until a half of the adhesive sheet was cut. The dicing was done by a diamond dicing wheel of #3000 at a high speed revolution of about 30,000 rpm to 40,000 rpm. A grinding coolant was poured during the dicing.
As shown in
It is understood from the result that the angle of the grinding traces with the vertical line is desirably from 10 degrees to 45 degrees. When the angle is from 0 degrees to 10 degrees, the chipping appearance ratio is about at 5% that is very low comparing to a chipping appearance ratio of the case more than 45 degrees. In comparison to a displacement detection device manufactured regardless of angle of the grinding traces, a very reliable displacement detection device can be obtained when using IC chips having grinding traces of from 0 degrees to 45 degrees.
Chippings at issue for a displacement detection device are loose chippings, in which silicon broken pieces are left at defected portions. During assembling or using displacement detection devices, the silicon broken pieces drop from the loose chippings and affect properties of the displacement detection devices. From the result of this EXAMPLE, it is understood that the total number of the loose chippings becomes to about 1/20 in comparison with loose chipping number when manufactured regardless of grinding trace angle, and the number of the loose chippings is zero in a case of grinding trace angle of 10 degrees to 45 degrees. In other words, a problem ratio becomes to 1/20 when the grinding trace angle is less than 45 degrees, and further the problem ratio reduces to zero by using the grinding trace angle of 10 degrees to 45 degrees.
A side ridge 53 is perpendicular to another side ridge 54 in a rectangular IC chip 24 shown in
Using IC chips having loose chippings, acceleration sensors that are a kind of displacement detection devices were assembled. An exploded perspective view of the assembled acceleration sensor is shown in
Using 300 pieces of IC chips with loose chippings 61, as shown in
The acceleration sensors of Sample A and Sample B were set on a vibrator to apply acceleration of 20 G to them, and output voltages from them were measured. Excluding acceleration sensors that showed extraordinary output, the acceleration sensors were freely fallen five times from a height of 1 meter to a wooden board of 100 mm thick, output from the acceleration sensors were measured during applying acceleration of 20 G, and the number of acceleration sensors showing extraordinary output was counted. When an acceleration sensor is freely fallen from the above-mentioned height, an impact of about 1500 G to 2000 G is applied to the acceleration sensor, and the flexibly deformable portions of the acceleration sensor element collide with a rear surface of an IC chip.
In the measurement of the acceleration sensors before the free fall, three pieces of Sample A and one piece of Sample B showed extraordinary output. By disassembling the acceleration sensors and investigating, it was revealed that silicon broken piece was trapped between a bottom of the weight of the acceleration sensor element and an inner bottom of the protection case in two pieces among the three of Sample A. It was recognized from comparison in shape that the silicon broken piece was fallen from loose chippings of the IC chip. Since an excessive impact was not applied to the acceleration sensors, it is considered that the silicon broken piece dropped during assembling by any reason. It is thought that remaining one piece of Sample A and the one of Sample B were injured on the flexibly deformable portions during assembling, and loose chippings of the IC chips were not a cause.
After 297 pieces of Sample A and 299 pieces of Sample B, excluding the initial-accidental products, were applied to by a falling impact, one piece of Sample A produced an extraordinary output, but none of Sample B did. It was revealed from disassembling to investigate the acceleration sensor showing the extraordinary output that a silicon broken piece dropped from the IC chip and was trapped between a top of flexibly deformable portions and a rear surface of the IC chip. The silicon broken piece trapped was 20 μm×18 μm×170 μm in dimension. The sensor was built to have a gap g1 of 15 μm between the top of the flexibly deformable portions and the rear surface of the IC chip. It is thought that, when an applied impact worked to move the weight down and to expand the gap, the silicon broken piece was trapped in the gap and was left there. An investigation did not show whether the drop of the silicon broken piece was caused by a collision with the flexibly deformable portions at the impact or only by the impact. It was confirmed from the result that loose chippings of the IC chip are one of the causes for problems that happen on using an acceleration sensor. Since one piece among 297 pieces of the acceleration sensors assembled with an IC chip having loose chippings had a problem, the problem appearance ratio was 1/297.
From the results of EXAMPLE 1 and EXAMPLE 2, the number of acceleration sensors, which would have a problem caused by a drop of loose chippings from an IC chip after product delivery, is presumed below. Appearance ratio of chippings on side ridges is 12.4% among 100,000 pieces of IC chips manufactured regardless of grinding trace angle. As explained above, loose chippings to all chippings are 11.1%, and the appearance ratio of problem caused by a drop of loose chippings during the use of acceleration sensors is 1/297=0.33%. Then, 100,000 pieces×12.4%×11.1%×0.33% equals to 4.5 pieces. This means that there is a risk of causing problems of 4.5 pieces among 100,000 pieces of conventional acceleration sensors with IC chips manufactured regardless of grinding trace angle. By contrast, the appearance ratio of problems caused by the loose chippings during the use can be made almost zero by using IC chips according to the present invention regulating an angle of the grinding traces with the dicing direction in the IC chips, and more reliable acceleration sensors can be supplied. In the same manner as the acceleration sensor, pressure sensors and gyro-sensors were studied, and results of them were almost the same as the acceleration sensor.
In
In the acceleration sensor of EXAMPLE 3, a risk of accident appearance can be made smaller than the acceleration sensor of EXAMPLE 2.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-270434 | Sep 2005 | JP | national |
