Semiconductor device and its manufacturing method

TECHNICAL FIELD

0001The present invention relates to a wireless IC chip for recognition via radio waves.

BACKGROUND ART

0002Documents referred to in this specification are as follows. Each Document is referred to by using its Document No. <ul id="ul0001" list-style="none" compact="compact"><li>[Document 1] <patcit id="pcit0001" dnum="JP7030323A"><text>JP-A-7-30323</text></patcit></li><li>[Document 2] <patcit id="pcit0002" dnum="JP2000163544A"><text>JP-A-2000-163544</text></patcit></li><li>[Document 3] <patcit id="pcit0003" dnum="JP2002083894A"><text>JP-A-2002-83894</text></patcit></li><li>[Document 4] <patcit id="pcit0004" dnum="JP2002269520A"><text>JP-A-2002-269520</text></patcit></li></ul>

0003[Document 1] discloses that a patch antenna upper electrode is formed on the front surface of a semi-insulating compound semiconductor substrate, a first ground metal is formed on the back surface, the first ground metal is partially removed in an area under the upper electrode of the patch antenna, and an external ground metal constituting a second ground metal is disposed in parallel with the semi-insulating compound semiconductor substrate, being spaced by a constant distance from the back surface of the substrate, to thereby allow radio waves having a frequency of 60 GHz or lower to be irradiated even if the compound semiconductor substrate has a thickness of about 150 µm.

0004[Document 2] discloses that a processor circuit is formed on the front side of a chip and a coil having a magnetic layer is formed on the back side to thereby prevent crosstalk.

0005[Document 3] discloses that an antennal coil is disposed not to be formed on an analog circuit to thereby prevent deterioration of the communication performance of a semiconductor chip.

0006It also discloses that in order to thin a non-contact type semiconductor device as a final product, a thickness of a bare chip is set to 300 µm or thinner, and about 50 µm to 150 µm for a thin type card.

0007[Document 4] discloses that there is a proportional relation between a thickness of a wireless chip connecting an antenna on the front and back of a substrate and a ground serial resistance of a transponder circuit of the wireless chip, and if the ground serial resistance is small, a communication distance becomes long, whereas if the ground serial resistance is large, the communication distance becomes short because a loss resistance of the circuit becomes large.

0008However, as illustratively shown in [Document 1] to [Document 4], there is no conventional technical document which studies an extension of a communication distance by thinning a substrate.

0009An IC chip device with the features of the preamble of claim 1 is disclosed in <patcit id="pcit0005" dnum="EP1320122A"><text>EP 1 320 122 A</text></patcit>. Further related IC structures are discussed <patcit id="pcit0006" dnum="US2002130402A"><text>US 2002/130402</text></patcit> in <patcit id="pcit0007" dnum="EP1160841A"><text>EP 1 160 841 A</text></patcit>, <patcit id="pcit0008" dnum="EP1202296A"><text>EP 1 202 296 A</text></patcit>, and <patcit id="pcit0009" dnum="EP0977145A"><text>EP 0 977 145 A</text></patcit>.

DISCLOSURE OF THE INVENTION

0010The present invention is defined in claim 1. Further advantageous features are set out in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

0011<ul id="ul0002" list-style="none" compact="compact"><li><figref idref="f0001">Fig. 1</figref> shows a wireless IC chip mounting an antenna and cross sectional views of the IC chip.</li><li><figref idref="f0002">Fig. 2</figref> is a diagram illustrating the advantages obtained by thinning the substrate of a wireless IC chip.</li><li><figref idref="f0003">Fig. 3</figref> is a diagram showing a circuit structure of a wireless IC chip.</li><li><figref idref="f0004">Fig. 4</figref> is a diagram showing the structure that an antenna is made not to be broken by giving a taper to an insulating layer of a wireless IC chip.</li><li><figref idref="f0005">Fig. 5</figref> is a diagram showing the shape of an antenna mounted on a wireless IC chip.</li><li><figref idref="f0006">Fig. 6</figref> is a diagram showing the structure that an antenna mounted on a wireless IC chip is protected.</li><li><figref idref="f0007">Fig. 7</figref> is a diagram showing that a terminal of an antenna mounted on a wireless IC chip is shared with an external antenna terminal.</li><li><figref idref="f0008">Fig. 8</figref> shows diagrams illustrating a process of thinning a substrate of a wireless IC chip.</li><li><figref idref="f0009">Fig. 9</figref> shows diagrams illustrating the structure that a wireless IC chip mounting an antenna is buried in a paper medium.</li><li><figref idref="f0010">Fig. 10</figref> is a cross sectional view showing a wireless IC chip mounting an antenna bonded to a stapler.</li><li><figref idref="f0011">Fig. 11</figref> is a graph showing measured data representative of a relation between a width of an antenna of a wireless IC chip and a communication distance.</li></ul>

BEST MODE FOR CARRYING OUT THE INVENTION

0012An issue of reducing a product manufacture unit cost exists in wireless IC chips to be used for RFID tags or the like which chips are required to be disposable because wireless IC chips circulate in a massive scale and require a very high collection cost. The issue of reducing a manufacture unit cost can be solved by improving mass productivity by reducing a chip size and increasing the number of chips (RFID tags) capable of being cut from a wafer.

0013An antenna on chip can be formed by semiconductor processes of manufacturing a wireless IC chip. As compared with a wireless IC chip having an external antenna, the advantages are considerable in terms of reduction in a manufacture unit cost because it is unnecessary to use components for connecting an antenna to terminals and an antenna connection process.

0014If an antenna is simply formed on a chip and if only conventionally known design parameters such as an antenna size, an antenna width, an antenna resistance value and an output power of a reader apparatus are changed, a practically usable communication distance, which is an important design object, cannot be obtained depending upon application conditions to the used and a read precision of the reader apparatus. The reason that the communication distance cannot be elongated resides in that if the antenna width is made wide, a parasitic capacitance relative to a semiconductor element becomes large and an energy loss appears, the material capable of lowering the antenna resistance value is economically limited and there is an economic lower limit value, and the like. If the size of an antenna mounted on a wireless IC chip is made large, the communication distance can be elongated. However, the large antenna cannot be mounted on a fine chip having a small energy capturing area.

0015Since a semiconductor substrate is conductive, if an a.c. magnetic field is externally applied to an on-chip antenna, an eddy current is generated principally. Basing upon the finding that a substrate thickness can be used as a design parameter, the present invention aims to thin a substrate thickness in order to eliminate a loss or reduction in an energy to be caused by eddy current. By utilizing the energy of an external a.c. magnetic field for semiconductor circuit operations, absorption of an energy is prevented, reduction in an amount of current flowing through an antenna is prevented, and the communication distance is elongated.

0016<figref idref="f0001">Fig. 1</figref> shows an embodiment of the present invention. <figref idref="f0001">Fig. 1(a)</figref> is a plan view of the present invention. An on-chip antenna 102 exists on a wireless IC chip 101. <figref idref="f0001">Fig. 1(b)</figref> is a cross sectional view along A-A' in <figref idref="f0001">Fig. 1(a)</figref>. A resin layer made of polyimide resin or the like is formed on a device layer 104 constituting circuits of the wireless IC chip, and the on-chip antenna made of gold is formed on the resin layer. As electromagnetic waves reach from a reader, eddy current is generated in a semiconductor substrate 105. A spatial energy of radio waves from the reader is dispersed on the substrate side and on the on-chip antenna side to reduce an amount of current flowing through the on-chip antenna 102. Therefore, since the current amount becomes lower than that necessary for operating the wireless IC chip, resulting in shortening a maximum communication distance. It is therefore effective to thin a thickness of the semiconductor substrate. The present inventor has found that in order to reduce eddy current, it is effective to set a thickness of the semiconductor substrate not over 200 µm. An efficiency can be improved if an insulating substrate is used excepting the device layer, because an eddy current can be prevented from being generated.

0017<figref idref="f0001">Fig. 1(c)</figref> shows an embodiment in which the semiconductor substrate including the substrate bottom surface is completely removed. This ultimate structure is effective for not generating a wasteful eddy current. A substrate principal surface is a surface on which elements constituting circuits are formed, whereas the substrate bottom surface is a surface opposite to the substrate principal surface. The device layer is a layer formed on the substrate principal surface side and made of elements and wirings constituting circuits.

0018A substrate thickness is intended to mean a thickness from the substrate bottom surface to the device layer excluding the wiring layer. Namely, the substrate thickness is a thickness from the substrate bottom surface to the layer in which the elements constituting the circuits are formed. A device layer thickness corresponds to the layer in which elements and wirings constituting circuits are formed.

0019<figref idref="f0002">Fig. 2</figref> is a drawing illustrating the advantages of the present invention. Communication distances were measured by irradiating micro waves of 300 mW from a reader to wireless IC chips. The wireless IC chips were formed by depositing polyimide resin having excellent heat resistance and a thickness of 4 µm on the wireless IC chips and by further depositing an on-chip antenna which is formed by gold plating and has a thickness of 10 µm. The on-chip antenna widths were 5 µm and 20 µm. A specific resistance of the silicon substrate was 10 Qcm.

0020It can be seen from this measurement data that a communication distance elongates abruptly in the range where the semiconductor substrate thickness does not exceed 200 µm. A communication distance of at least 600 µm can be retained.

0021At the antenna width of 20 µm, a parasitic capacitance increases between the antenna and semiconductor substrate. An increase in the parasitic capacitance increases a leak of a.c. waves, and the energy is leaked. Even if the thickness of the semiconductor substrate is thinned at the antenna width of 20 µm, the performance changes not so much.

0022Even in this case, although the effects of elongating the communication distance by thinning the semiconductor substrate are cancelled by the parasitic capacitance, the effects of elongating the communication distance still exist by setting the semiconductor substrate thickness to 200 µm or thinner.

0023As shown in the graph, under the experiment conditions, a slope of the graph indicating the communication distance relative to the semiconductor substrate thickness becomes steep in the range not over 100 µm, and the communication distance is elongated and can take at least 700 µm.

0024In the range not exceeding 50 µm in thickness of the semiconductor substrate, a communication performance continues to be improved ultimately, and a communication distance of at least 1000 µm can be retained. This is because the eddy current reduces further.

0025In the above-described experiments, although the thickness of polyimide resin is set to 4 µm, the polyimide resin may be thickened so that a parasitic capacitance between the antenna and semiconductor substrate can be reduced. It is therefore sufficient if polyimide resin has a thickness of at least 4 µm.

0026In the above-described experiments, although the reader irradiates micro waves of 300 mW, generally, the micro waves from a reader change in a range of 300 mW ± 150 mW. It is anticipated that micro waves from a reader have 200 mW.

0027The structure that the semiconductor substrate thickness is 200 µm or thinner is effective for improving the communication performance because of a small eddy current. At the semiconductor substrate thickness of 200 µm or thicker, the eddy current flows almost steadily, and the relation between the thickness and performance is not so remarkable.

0028<figref idref="f0003">Fig. 3</figref> shows the structure of the inside of a wireless IC chip according to the present invention. An antenna 102 corresponds to the antenna 102 shown in <figref idref="f0001">Fig. 1</figref>, and a rectifier circuit 302, a clock extracting circuit 303, a load switch 304 and a counter/memory circuit 305 are formed in the wireless IC chip 101 shown in <figref idref="f0001">Fig. 1</figref>. The antenna 102 is connected to the rectifier circuit 302. The clock extracting circuit 303 extracts a low frequency clock pulse from a high frequency carrier, and inputs the low frequency clock pulse to the counter/memory circuit 305. An memory output is transmitted to the reader, by changing the impedance relative to the antenna with the load switch 304. The clock extracting circuit extracts a clock width and interval. Since the antenna obtains energy from induction electromagnetic wavers, its shape is often a coil shape. However, the shape is not limited only thereto. In order to improve a Q value of the antenna, it is necessary to reduce inductive coupling between the antenna and device layer and lower the antenna resistance value. To this end, the resin layer is inserted between the antenna and device layer.

0029<figref idref="f0004">Fig. 4</figref> is a cross sectional view showing antenna connection electrodes 502 and 504 shown in <figref idref="f0001">Fig. 1</figref>. The cross sectional view of <figref idref="f0004">Fig. 4</figref> also shows electrodes 502 and 504 shown in <figref idref="f0005">Fig. 5</figref> and electrodes 502 and 504 shown in <figref idref="f0007">Fig. 7</figref>. An antenna 102 shows a cross section of the antenna 102 shown in <figref idref="f0001">Fig. 1</figref>. In the cross sectional view of <figref idref="f0004">Fig. 4</figref>, the antenna 102 covers insulating resin 402 and is connected to a connection pad 406 partially covered with an oxide film 403 and formed above semiconductor elements, a wiring layer 103 and a semiconductor substrate 105. As shown in <figref idref="f0004">Fig. 4</figref>, since the insulating resin is as thick as 4 µm or more, it has a tapered cross sectional shape so that the antenna pattern can be vacuum deposited without breakage.

0030<figref idref="f0005">Fig. 5</figref> shows another embodiment of the present invention. The present inventor has found that a width of an antenna 102 formed on a wireless IC chip 101 contributes to the performance. Namely, although the on-chip antenna is connected between electrode terminals 502 and 504 on the wireless IC chip, this structure makes the antenna have a parasitic capacitance relative to the semiconductor substrate, if the insulating film cannot be made thick. Although the parasitic capacitance can be lowered by thickening the insulating film, this results in an increase in a manufacture unit cost. The present inventor has devised the structure of narrowing the width of an on-chip antenna.

0031<figref idref="f0011">Fig. 11</figref> shows measurement data representative of a relation between an antenna width and a communication distance. This measurement data indicates that a communication distance can be retained by setting an antenna width to 10 µm or narrower. An antenna thickness was set to 10 µm. It is sufficient if an antenna thickness is equal to or thicker than that of a high frequency skin depth effect. An antenna thickness is 2.6 µm or thicker assuming that micro waves have a frequency of 2.45 GHz. Namely, if the antenna thickness becomes 2.6 µm or thinner, the communication distance becomes short. Therefore, a communication distance can be retained by setting the antenna width to 2.6 µm or wider and 10 µm or narrower. The antenna width and thickness shown in <figref idref="f0011">Fig. 11</figref> may be reversed. Namely, the abscissa of <figref idref="f0011">Fig. 11</figref> may represent the antenna thickness, and the antenna width is set to 10 µm, with similar results. The antenna material is desired to have a low resistance and is made of gold.

0032<figref idref="f0006">Fig. 6</figref> shows an embodiment in which a wireless IC chip with an on-chip antenna is mounted on a tape in order to make it easy to handle the wireless IC chip with an on-chip antenna shown in <figref idref="f0001">Fig. 1</figref>. An on-chip antenna 102 on the surface of a wireless IC chip 101 is attached in a face-down direction to a film 602 coated with an adhesive layer 605 and wound around a reel 606. Gold is used as the material of an on-chip antenna. If a protective film is not attached to the surface of gold, the gold is likely to be damaged, resulting in a low performance. In this embodiment, the adhesive layer of the tape provides both an adhesion effect and a protective effect of the surface of gold antenna. Wireless IC chips wound by the reel are pulled out and cut out when necessary and adhered to objects.

0033<figref idref="f0007">Fig. 7</figref> shows an embodiment in which an external antenna of a wireless IC chip is connected by utilizing a chip having the same device layer and electrode portions as those of the wireless IC chip with an on-chip antenna shown in <figref idref="f0001">Fig. 1</figref>. The electrode terminals 502 and 504 shown in <figref idref="f0005">Fig. 5</figref> and described above function also as mount terminals for external radiation antennas 701 and 702. With this arrangement, external antenna terminals and on-chip antenna can be manufactured separately, by using the same processes immediately before the last gold plating process among semiconductor manufacture processes and changing only the mask of the last gold metal plating process.

0034<figref idref="f0008">Fig. 8</figref> shows another embodiment of the present invention. The embodiment in which the semiconductor substrate is completely removed has been described earlier. A method of realizing this will be described. <figref idref="f0008">Fig. 8(a)</figref> is a cross sectional view showing a finished wafer with an on-chip antenna constituted of a substrate 105, an oxide film 802 and a layer 804 including an on-chip antenna and semiconductor elements. This wafer is adhered to a substrate 805 such as a glass substrate with adhesive 803. <figref idref="f0008">Fig. 8(b)</figref> illustrates a next process and is a cross sectional view of the process immediately after the wafer formed in <figref idref="f0008">Fig. 8(a)</figref> is immersed in solution of potassium hydroxide or the like to remove the silicon on the wafer back surface side. The oxide film in the wafer is not etched with potassium hydroxide so that it has a function of an etching stopper layer. <figref idref="f0008">Fig. 8(c)</figref> is a cross sectional view of the process immediately after etching grooves 806 are formed by forming a mask pattern on the back surface by photoresist techniques and etching the oxide film 802 and the layer 804 including the on-chip antenna and semiconductor elements, after the process shown in <figref idref="f0008">Fig. 8(b)</figref>.

0035<figref idref="f0009">Fig. 9</figref> shows the structure that a wireless IC chip with an on-chip antenna is buried in securities such as bills. <figref idref="f0009">Fig. 9(a)</figref> is a plan view of securities 901, and a thread 902 mounting a wireless IC chip 101 is buried in the paper. <figref idref="f0009">Fig. 9(b)</figref> is a cross sectional view along A-A' in <figref idref="f0009">Fig. 9(a)</figref>. The characteristic point in <figref idref="f0009">Fig. 9(b)</figref> resides in that a wireless IC chip 101 with an on-chip antenna 102 is formed very thin by the processes shown in <figref idref="f0008">Fig. 8</figref> or the like, about 10 µm in thickness for example. If such a thin wireless IC chip is buried in a paper medium, it is difficult to mechanically endure during a paper compression process at a high pressure by calendar processing. In order to avoid this, a recess is formed in the thread and projections 902 and 904 are formed at peripheral sides. With the function of these projections, even if a high pressure is applied to the paper, it is possible to mechanically protect the wireless IC chip. The material of the thread and projections is preferably plastics such ad PET, fiber materials such as paper, or the like. The material not deformed by pressure application is preferable.

0036<figref idref="f0010">Fig. 10</figref> is a cross sectional view showing the mount state that a wireless IC chip with an on-chip antenna described above is mounted on or inside a stapler. A wireless IC chip 101 and an antenna 102 are mounted on a staple 1003 of a stapler and inserted into a paper or film 1001. Wireless IC chips are mounted beforehand on a train of staples of the stapler in a proper order. This staple train is mounted on the stapler and each staple is attached to a paper or film. Wireless IC chips can be mounted as desired in a manner similar to a conventional stapler. A conventional stapler itself can be used for mounting a wireless IC chip. The advantages of reducing a mount cost of the wireless IC chip can be obtained. In the case in which the stapler is pushed against a document or the like to bend a staple, and the wireless IC chip 101 receives a stress and is broken, it is effective that a groove is formed in the stapler and the wireless IC chip is buried in the groove.

0037The present invention made by the inventor has been described in connection with the preferred embodiments. But although the present invention is realized by using an SOI substrate, the present invention is not limited to the SOI substrate but a general Si substrate may also be used.

INDUSTRIAL APPLICABILIY

0038The present invention is applicable to the technical field of RFID which is the background of the present application.