Semiconductor device manufacturing: process – Formation of electrically isolated lateral semiconductive... – Having substrate registration feature
Reexamination Certificate
1999-10-07
2001-07-17
Smith, Matthew S (Department: 2825)
Semiconductor device manufacturing: process
Formation of electrically isolated lateral semiconductive...
Having substrate registration feature
C438S462000, C438S975000, C438S113000, C438S114000, C438S459000, C438S464000
Reexamination Certificate
active
06261919
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a mechanism of how to create a mark such as a letter, numeric character, or symbol, to be inscribed to a semiconductor device and a method of inscribing the mark. More particularly, the present invention relates to a method of applying a laser mark onto an exposed back surface of a silicon chip, which has conductive balls serving as external contact terminals attached on a front surface, and which is mounted on a wiring substrate in a flip-chip manner.
A semiconductor device is usually produced through a designing step, a mask-formation step, a wafer manufacturing step for forming a wafer from an ingot, a wafer processing step for forming an inner circuit such as a semiconductor element or an integrated circuit on the wafer, an assembly step and an inspection step.
A mark such as a letter, numeric character, or symbol indicating a product name or a pin number is conventionally inscribed on the margin on a surface of a semiconductor chip. The mark is integrally formed as a part of the margin of an inner circuit pattern of a chip while the chip maintains a wafer form before scribing. Thus, generally, a back surface of the chip has not been used for marking.
However, there is another type of chip having conductive balls (serving as external contact terminals) fitted to a front surface which has an inner circuit, such as an integrated circuit or a semiconductor element, formed thereon. In this case, the chip is mounted on a wiring pattern of a circuit board with the conductive balls interposed therebetween. The chip of this type has a following problem. Although an exposed wiring portion is protected, the chip is not sealed with a resin. As a natural consequence, the back surface of the semiconductor substrate is not protected. In such a circuit device having a chip mounted on a circuit board in such a way that a front surface of the chip faces the circuit board, it is impossible to identify a product name of a chip mounted on the circuit board during inspection even if a malfunction of the circuit device takes place. To overcome this, a mark such as a letter, numeric character, or symbol comes to be inscribed on a back surface of the chip, namely, a back surface of a wafer (Japanese Patent Application KOKAI Publication Nos. 4-106960, and 8-191038).
The mark such as a letter, numeric character, or symbol formed on the wafer or the chip is conventionally created by continuously arranging a plurality of dots formed by a laser.
The conventionally-employed laser mark is inscribed by a laser such as a YAG laser on a surface such as a back surface of a silicon wafer or its front surface having an element formed therein, so that the mark can be visually recognized. When the mark is inscribed on a silicon chip, the laser is applied in such a way that dots are not linked with each other in order to reduce damage.
FIGS. 1A
to
1
C are cross-sectional views of a silicon wafer sequentially showing conventional marking processes. A silicon wafer
1
, which is sliced from a rod, is subjected to a lapping operation, and then, sent to a wafer processing step. In this operation, an inner circuit such as a semiconductor element or an integrated circuit, is formed. Accordingly, the silicon wafer
1
has a back surface
3
rendered uneven to a predetermined level by the lapping operation, and a front surface
2
having a plurality of chip formation regions in each of which a semiconductor element and an integrated circuit are formed. The mark such as a letter, numeric character, or symbol is inscribed on, for example, the back surface
3
of the wafer
1
by using a YAG laser. Alternatively, the mark may be inscribed on a back surface of the chip after the wafer
1
is diced into chips. The mark is formed of combination of a plurality of dots
5
(a depth of about 5 &mgr;m), which are formed by applying a laser beam
4
(a beam diameter of about 120 &mgr;m) onto the back surface
3
of the wafer.
However, even if care is taken to avoid linkage of dots in order to reduce damage, the application of a laser beam during the mark inscription produces cracks in the wafer having a thickness of about 500 &mgr;m at most. As a result, a mechanical strength of the wafer is inevitably reduced. When the marking is performed in this method, it is not possible to create a fine-logo mark. Therefore, the fine-logo mark must be omitted in some cases. Although it is possible to create dots continuously to create fine marks, the mechanical strength of a wafer significantly reduces.
BRIEF SUMMARY OF THE INVENTION
An object of the present invention is to provide a semiconductor device having a highly visible mark which is formed without digging a semiconductor substrate deeply and having an excellent die strength, and to provide a method of manufacturing the semiconductor device.
To attain the aforementioned object, the semiconductor device according to a first aspect of the present invention comprises
a semiconductor substrate having a front surface with an inner circuit formed therein and a ground back surface; and
a mark formed of a molten trace on the ground back surface.
The semiconductor device may further comprise
a plurality of contact electrodes formed on the front surface of the semiconductor substrate;
a plurality of contact terminals each being formed of a conductive ball and electrically connected to the inner circuit via a corresponding one of the contact electrodes.
A semiconductor device according to a second aspect of the present invention comprises
a circuit board having a plurality of electrode pads in a front surface thereof;
a plurality of semiconductor chips mounted on the circuit board, each of the semiconductor chips having a front surface with an inner circuit formed therein and a ground back surface;
a plurality of contact electrodes formed on the front surface;
a plurality of contact terminals each formed of a conductive ball and electrically connected to the inner circuit via a corresponding one of the contact electrodes and each being connected to a corresponding one of the electrode pads of the circuit board; and
a mark formed of a molten trace on the ground back surface.
In the semiconductor device according to the first and second aspects, the ground back surface of the semiconductor substrate has an uneven surface and the mark is formed by melting and planarizing the uneven surface.
According to a third aspect of the present invention, there is provided a method of manufacturing a semiconductor device comprising the steps of:
forming an inner circuit in each of a plurality of chip formation regions in a front surface of a semiconductor wafer;
forming an uneven surface by grinding a back surface of the semiconductor wafer; and
forming a mark of a molten trace by a laser in regions of the ground back surface corresponding to the chip formation regions.
The method of manufacturing a semiconductor device may further comprise the steps of:
dicing the semiconductor wafer to separate the chip formation regions; and
forming a plurality of semiconductor chips.
The method of manufacturing a semiconductor device may further comprise steps of
forming a contact electrode on a front surface of each of the semiconductor chips, and
attaching a contact terminal formed of a conductive ball to the contact electrode.
In the step of forming the mark includes the step of melting projections of the uneven surface of a back surface of the semiconductor wafer by a laser to partially form a molten and planarized portion.
It is desirable that the method of the present invention further comprise the steps of:
measuring a difference in height between the molten and planarized portion and the projections;
detecting an intensity of the laser based on measurement results; and
controlling output of the laser based on detection results.
It is desirable that the laser be a YAG laser modulated by an SHG device.
It is desirable that an energy of the laser be 1.0×10
7
W/cm
2
or less.
It is desirable that the step of forming the mark by the laser be performe
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Kabushiki Kaisha Toshiba
Smith Matthew S
Yevsikov Victor V.
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