Patent Application
20070124926
1. Field of the Invention
The present invention relates to circuit boards for the mounting of electronic components such as semiconductor devices for example, and to methods for manufacturing such circuit boards.
2. Description of the Related Art
Along with the miniaturization and increasingly high functionality of electronic equipment in recent years, there have been advances in increasing the number of pins in the semiconductor devices that constitute such electronic equipment and in miniaturization of such components, and the number and density of the wires of the printed boards on which these are mounted are increasing dramatically. In particular, the increasing minuteness of printed boards (circuit boards) is advancing due to the rapid increase in the number of leads drawn from these semiconductor devices and the number of terminals.
Today's most leading edge fine pitch level is a pitch in the range of 40 to 50 μm in the case of mounting semiconductor devices to FC (flip chips), and the dimensions of the vias and lands used as interlayer connection techniques at this time are respectively 50 μm and 100 μm. Furthermore, it is anticipated that a pitch of 20 μm in FC mounting and a pitch of 36 μm in BGA (ball grid array) mounting will be achieved by the year 2010.
In achieving wiring miniaturization techniques, it has been common to aim for thinner copper foils in the case of using etching and to use a semi-additive method or a full additive method in the case of using plating. Although there are issues such as high costs involved with wiring miniaturization techniques, technological issues basically have been overcome. Patent documents 1 to 7 listed below can be considered.
Although the technologies disclosed in patent documents 1 to 4 do not use a phase change material that is capable of changing alternately between an electrically insulating state and an electrically conductive state, a conductor portion of a wiring pattern is formed by changing a resistance value using laser irradiation. However, these technologies have the following problems.
Patent document 1 discloses a technique in which a predetermined area on an AlN (aluminum nitride) substrate surface is irradiated with a laser, thus reducing the resistivity in the laser irradiated portions of the AlN and thereby forming a conductor portion of a wiring pattern. With this technique, although it is indeed possible to form a wiring pattern with a laser, in addition to a high energy output laser (a highest output of 100 W with a Nd: YAG laser) being necessary, the material costs become higher than the costs of materials normally used for printed boards, and therefore manufacturing costs including installation costs and material costs become higher, and it would be extremely difficult to manufacture a product that could replace the printed boards being used today. Furthermore, this technique is one in which a wiring pattern is formed on a substrate surface and there is no mention in patent document 1 of forming vias with a laser. Further still, there also is the problem that the wiring here has a comparatively large resistance value. This is because, in wiring with a width of 100 μm and a length of 10 mm, the resistance value across the wiring would be approximately 1 ohm.
The below-mentioned patent documents 2 to 4 disclose techniques in which wiring is formed by irradiating an energy beam onto a diamond coat printed circuit board covered with a diamond thin film. In patent document 4, a technique also is shown in which vias made of graphite are formed by inducing phase change in diamond. However, compared to the printed boards that are widespread today, diamond coat printed circuit boards require a special manufacturing process to form the diamond thin film, which presents considerable problems in terms of manufacturing costs. Furthermore, since irradiation is carried out with an argon laser when carrying out laser irradiation, compared to laser irradiation with semiconductor lasers that have considerably smaller output than argon lasers, the laser irradiation process is a comparatively large-scale process. In addition to this, the resistance value of the (graphite) portions of formed wiring patterns that have changed to blackish due to laser irradiation is approximately 3 ohm/cm, so there is also the problem of a comparatively large resistance value. It should be noted that, as is well known, the phase change from diamond to graphite is irreversible and phase change from graphite to diamond does not occur.
Furthermore, patent documents 4 and 5 disclose a technique in which a conductor pattern is formed by carrying out ion irradiation on an electrically insulating surface. However, with this technique, obtaining electrical conductivity that can be used for wiring is difficult or complicated. This technique is one in which a conductor pattern is formed on a substrate surface and there is no mention in patent document 5 of forming vias with this technique. Further still, patent documents 6 and 7 disclose a molded or film formed composite material in which polymerization is caused by light irradiation and only the irradiated portions are changed to be conductive. However, it is difficult to obtain an electrical conductivity that can be used as wiring for a printed board with a material made of such a conductive polymer.
Patent Document 1: JP H1-173505 A
Patent Document 2: JP H3-268477 A
Patent Document 3: JP H5-175359 A
Patent Document 4: JP H5-36847 A
Patent Document 5: JP H2-184095 A
Patent Document 6: JP H3-297191 A
Patent Document 7: JP H7-188399 A
On the other hand, in interlayer connection technologies that use vias, there are not only merely cost-related issues in forming miniaturized vias above the current level, but also technological difficulties. This is because there is a limit to the miniaturization of via formation related to physical hole processing since either a technique is used in which vias are formed by plating after a drilling process or a technique is used in which vias are formed by plating after laser processing.
In other words, with hole processing based on drilling, at best only via holes of a size around 100 μm can be formed, and with hole processing that uses a CO2 laser, it is possible to manage to form via holes of a size in the range of 30 to 50 μm. Although there are also techniques in which excimer lasers or the like are used, these are unlikely to be used in practice due to considerations of high cost. Furthermore, there is a limit due to aspect ratio when forming via holes using a photolithographic process, and when the aspect ration must be made, for example, two or lower, there will be a limit of 15 μm to the diameter of the via hole when the thickness of the electrically insulating layer is 30 μm.
Furthermore, if the holes (via holes) are to be made small, there is the problem that the smaller the holes become, the more difficult it is to fill the holes with a conductive material. When filling the holes with a conductive material using plating, considering the permeation conditions of the plating liquid, it is difficult to fill the holes well unless the aspect ratio is 2 or lower. Even when filling the holes with a conductive material using a conductive paste, holes with a diameter of at best 50 μm can be filled, and it is extremely difficult technologically to fill holes of 30 μm or smaller with the conductive material.
If reducing the surface area of circuit boards is desired as part of miniaturizing electronic devices, the limit dimension of via diameters becomes a bottleneck to design when the diameter of vias cannot be made more minute than current levels. In other words, being unable to form extremely minute vias easily has become an obstacle to miniaturizing circuit boards.
In order to solve the above-described conventional problems, the present invention provides a circuit board in which very small vias and conductors can be formed and a method for manufacturing such a circuit board.
A circuit board according to the present invention is a circuit board, including:
a substrate, and
a phase change layer on the substrate, the phase change layer including a phase change material that includes a chalcogenide semiconductor and changes between an electrically insulating state in an amorphous phase and an electrically conductive state in a crystalline phase by irradiation of laser light,
wherein a conductive path is defined in the phase change layer by the phase change material in the electrically conductive state.
Note here that the phrase “on the substrate” as used herein should be interpreted broadly, and encompasses formation directly on a substrate or the presence of intervening materials.
A method for manufacturing a circuit board according to the present invention is a method for manufacturing a circuit board wherein, on a substrate, in at least a portion of a phase change layer including a phase change material that is capable of changing alternately between an electrically insulating state and an electrically conductive state, a conductive path is formed that has been put into an electrically conductive state by a phase change in the phase change layer,
wherein the phase change material includes a chalcogenide semiconductor, changes between the electrically insulating state and the electrically conductive state by irradiation of laser light, goes into the electrically conductive state in a crystalline phase, and goes into the electrically insulating state in an amorphous phase, the method including the steps of
(a) forming a phase change layer by depositing a phase change material that is capable of changing alternately between an electrically insulating state and an electrically conductive state; and
(b) forming in the phase change layer a conductive path including the phase change material by irradiating laser light on the phase change layer.
With the present invention, a conductive path is formed by irradiating laser light onto a phase change layer using phase change in a phase change layer formed from a phase change material that is capable of changing alternately between an electrically insulating state and an electrically conductive state, and therefore very small-dimension minute vias and conductors can be formed. Furthermore, the phase change of a chalcogenide semiconductor according to the present invention is reversible, and therefore subsequent repair, rework, or trimming also is easy. As a result, production yields can be improved.
A circuit board according to the present invention can be used singly by itself and also can be used formed on a substrate. When used singly by itself, it is first constructed on a temporary substrate then transferred to a final substrate.
In a preferred embodiment of the present invention, the conductive path is at least one of a via and a conductor, and the phase change material is a material that changes between an electrically insulating state and a electrically conductive state due to irradiation of laser light, and the conductive path is made from the phase change material in the electrically conductive state.
The phase change material is a material that undergoes phase transition between a crystalline phase and an amorphous phase. The phase change material is a chalcogenide semiconductor.
In a preferred embodiment of the present invention, a via is formed as the conductive path in the phase change layer, and a conductor made of metal is connected to the via at a surface of the phase change layer.
In a preferred embodiment of the present invention, a via is formed as the conductive path in the phase change layer, and a conductor made of the phase change material also is formed at the phase change layer.
In a preferred embodiment of the present invention, the conductor is connected to the via.
In a preferred embodiment of the present invention, a plurality of vias are formed as the conductive path, and at least one of the plurality of vias is formed at an inclination from a normal line direction of the circuit board.
In a preferred embodiment of the present invention, a base substrate on which a conductor layer is formed on at least its surface further is provided as a base substrate of the phase change layer.
In a preferred embodiment of the present invention, a further phase change layer is formed on the phase change layer, and a conductive path is formed also in the further phase change layer and is made of the phase change material.
In a method for manufacturing a circuit board of the present invention, the laser light in the step (b) preferably is irradiated with a semiconductor laser.
The laser light in the step (b) preferably is irradiated in a state in which the phase change layer is rotatable.
In a preferred embodiment, in the step (b), a conductor is formed as the conductive path in a surface of the phase change layer and a via is formed as the conductive path extending from a portion of the conductor.
A method for manufacturing the circuit board of the present invention further may include, a step of forming a third phase change layer made of the phase change material on the second phase change layer, a step of forming a via made of the phase change material in the third phase change layer by irradiating laser light from a semiconductor laser to the third phase change layer, a step of forming a fourth phase change layer made of the phase change material on the third phase change layer, and a step of forming a conductor made of the phase change material in the fourth phase change material by irradiating laser light from the semiconductor laser to the fourth phase change layer.
In the method for manufacturing the circuit board of the present invention, the step of forming the second phase change layer may include a step of depositing a phase change material onto the first phase change layer so as to cover the conductor made of metal and a step of flattening the deposited phase change material.
The present invention provides a circuit board on which very small dimension vias can be formed. As mentioned above, although it is possible with conventional techniques to form a via with a diameter of approximately 100 μm using a drill, or a via at best in an approximate range of 30 to 50 μm using a C02 laser, it is quite difficult technologically to form vias with smaller dimensions than that.
In these circumstances, the inventors examined the forming of vias by a different approach than conventional techniques, thus leading to the present invention. This is not a technique of forming a hole on a board and then filling that hole with a conductive material, but rather a technique in which, without forming a hole, a via is formed by irradiating with a semiconductor laser a phase change layer made of a phase change material that is capable of changing alternately between an electrically insulating state and an electrically conductive state. By using such a method, it became evident that it is possible for form minute vias with a diameter of approximately 1 μm for example.
The phase change material in which the vias are formed is made from a material in which a resistance value is changed by a phase transition (a phase transition between a crystalline phase and an amorphous phase), and it is possible to cause a change from the electrically insulating state (amorphous phase) to the electrically conductive state (crystalline phase) by the irradiation of laser light. Vias are configured by the phase change material in the electrically conductive state. For the phase change material, it is possible to use a chalcogenide semiconductor that is capable of phase transition between a crystalline phase and an amorphous phase.
The electrical conductivity of a chalcogenide semiconductor can be made to have an approximately four or five orders of magnitude of difference between the crystalline phase and the amorphous phase at room temperature and, moreover, since it is possible to maintain the crystalline phase and the amorphous phase stably with a chalcogenide semiconductor at room temperature, it is possible to form electrically insulating portions and electrically conductive portions (vias and the like) by phase change between the electrically insulating state and the electrically conductive state.
A method for forming conductive paths (vias, conductors, and the like) according to the present invention is effective, since it is possible to form conductive paths by carrying out irradiation with a semiconductor laser on a phase change material that is capable of changing alternately between an electrically insulating state and an electrically conductive state. It also is possible to form minute vias precisely with good efficiency, because it is not necessary to form holes by drill processing or laser processing and then to fill these holes with a conductive material.
It is preferable that the thickness of the phase change material layer is in the range of 0.5 to 20 μm and more preferably in the range of 1 to 10 μm. The phase change material layer can be formed by a method such as spin coating, vacuum deposition, and sputtering. Furthermore, a protective layer may be provided on the phase change material. A dielectric material of a thickness in the range of 10 to 100 nm can be used for the protective layer. For example, ZnS—SiO2 can be used for the dielectric material.
The present invention can be applied to various uses such as flexible printed boards, double-sided substrates, and multilayer boards.
The following are descriptions of embodiments of the present invention with reference to the accompanying drawings. The present invention is not limited to the following embodiments. In order to simplify description, in the following drawings, structural elements having a substantially same function are indicated with the same reference numerals.
The following is a description of a circuit board according to an embodiment of the present invention with reference to
The circuit board 100 shown in
The phase change material that constitutes the phase change layer 10 and the conductive path 20 (via 21) is a material that changes between an electrically conductive state and an electrically conductive state at least by irradiation of laser light. In the example shown in
The value of electrical resistance of the conductive path preferably is in the range of 101 to 104 S/cm.
In the present embodiment, the conductive path 20 (via 21) is formed, as shown in
In the present embodiment, a chalcogenide semiconductor is used for the phase change material. The chalcogenide semiconductor is an alloy that includes at least one chalcogen element (namely a group six element) as an essential element. It is possible to use a chalcogenide semiconductor of different characteristics depending on the mixing proportions or the constitute elements therein. The chalcogenide semiconductor of the present embodiment includes a chalcogen element (S, Se, Te) that is a principal constituent, and a pnicogen element (such as As and Sb) that is a secondary constituent. A preferable abundance ratio of the principal constituent to the secondary constituent is one in which the element ratio of chalcogen element to pnicogen element is in the range of 1:0.1 to 1:1.
The principal constituent of the former is two-coordinated and the secondary constituent of the latter is three-coordinated, forming a two-dimensional covalent network structure. In this way it is possible to induce phase change easily. That is, with chalcogenide semiconductors, the bonding has a chain structure, and therefore the structure becomes flexible and rearrangement of the structure occurs easily. As a result both the crystalline state and the amorphous state can be maintained. It should be noted that the networks are bonded with Van der Waals force, and are therefore structurally flexible compared to an amorphous semiconductor constituted entirely by covalent bonds. Amorphous chalcogenide is also called chalcogenide glass (chalcogen compound glass).
With Te—Ge—Sn—Au based and Sn—Te—Se based chalcogenide semiconductors, it is possible to induce amorphous-crystalline phase changes using light irradiation. Furthermore, with Te—As—Ge—Si based semiconductors, it is possible to induce an amorphous-crystalline phase change using heat generated by an electric current for example. Other examples include Ce—Sb—Te based semiconductors and Te based semiconductors with As and Sb added. Other additional examples of alloys in which phase change can be induced include: GaSb, InSb, InSe, Sb2Te3, and GeTe as two-component based alloys, Ge2Sb2Te5, InSbTe, GaSeTe, SnSb2Te4, and InSbGe as three-component based alloys, and AgInSbTe, (GeSn)SbTe, GeSb(SeTe), and Te81Ge15Sb2S2 as four-component based alloys,
The phase change layer 10 made of a phase change material is formed on a base substrate 30. The base substrate 30 is a substrate on which a conductor layer is formed on at least the surface. In the present embodiment, a portion (for example, a land) of the conductor layer of the base substrate 30 is positioned at an area of a bottom surface of the via 21. The base substrate 30 may be, for example, a rigid board (a typical printed board) and a single sided or a double sided rigid board may be used in the present embodiment. It should be noted that in
With the circuit board 100 of the present embodiment, the via 21 is formed by irradiating laser light 52 on the phase change layer 10, and therefore the dimension of the via 21 (its diameter for example) can be made small. That is, it is possible to achieve a circuit board having via dimensions that have been difficult to achieve when forming vias by forming holes with drill processing or laser processing and then filling these holes with a conductive material.
There is no particular problem in making the dimension (diameter) of the via 21 in the range of 100 μm to 30 μm or larger than that for example, but the comparatively smaller size has a large technological significance. When the shape of the via 21 is substantially round, the diameter of the via 21 may be at most 10 μm for example, and when a small via diameter is desired, the diameter may be at most 1 μm for example (in the range of 0.1 to 0.5 μm as one example).
Being able to make a small via dimension allows leeway in the precision of uniformity of the lands that connect to the vias. As a result, limitations in circuit board design are eased and it is also possible to achieve the effect of making it easier to estimate error (tolerance) at the manufacturing stage. That is to say, when the precision of uniformity between the vias and lands is ±25 μm, a land dimension (the diameter for example) is required to be at least 100 μm if attempting to form vias with a via diameter of 50 μm with typical manufacturing methods. On the other hand, when forming vias with a via diameter of 10 μm using a structure of the present embodiment, it is sufficient for the land dimension (the diameter for example) to be 60 μm, and therefore the level of design freedom can be increased by the proportion by which the lands can be made small. Furthermore, this proportion offers leeway in the precision of uniformity when keeping the land dimension as it is at 100 μm, such that it becomes easier to execute manufacturing processes and it becomes possible to improve yields.
Further still, it is possible to use the semiconductor laser 50 as the irradiation source of the laser light 52, and therefore it is also possible to achieve the effect of being able to keep equipment costs down. Compared to using a high output laser device (a CO2 laser for example) as the irradiation source of the laser light 52, it is possible to reduce the involved equipment costs to one-hundredth or less when using the semiconductor laser 50. Furthermore, the semiconductor laser 50 also is easy to handle and in that sense too it has great merit in manufacturing processes.
Additionally, since the conductive path 20 is made of a phase change material that is capable of changing alternately between the electrically insulating state and the electrically conductive state, even after being formed once, it has the special property of being able to be erased again through a specific process. That is, even after being phase changed from the amorphous state (reference numeral “51” in
When manufacturing the circuit board 100 of the present embodiment, first the phase change layer 10 made of a phase change material in an amorphous state may be formed by depositing the phase change material on the base substrate 30, after which the via 21 (conductive path 20) made of the phase change material in a crystalline state may be formed by irradiating the laser light 52 on the phase change layer 10. The dimension (diameter) of the via 21 can be regulated by the beam diameter of the laser. There is no particular limitation to the thickness of the phase change layer 10, but it was set in the range of 5 to 30 μm in the present embodiment. When an upper surface and a lower surface of the via 21 are respectively exposed at an upper surface and a lower surface of the phase change layer 10, the thickness of the phase change layer 10 is the height of the via 21.
In the structure shown in
Further still, in the phase change layer 10, it is also possible to form the via 21 and a conductor that extends in continuity with the via 21. This will be described with reference to
Firstly, as shown in FIG, 3A, a conductor 22 (the conductive path 20) is formed on the surface of the phase change layer 10 by moving the laser light 52 on the phase change layer 10 on the base substrate 30. The conductor 22 is made of phase change material. The width of the conductor 22 can be regulated by the beam diameter of the laser light 52, and the path of the conductor 22 can be regulated by controlling the movement of the laser light 52.
Next, as shown in
With this structure, there is no joint between the conductor 22 and the via 21, and therefore there is the advantage of superior connection reliability between the conductor 22 and the via 21. That is, in the case of an ordinary via connection, the via portion (conductive paste or plating) and the land portion that is formed on the via portion are connected by being brought into contact, and therefore reliability decreases when the substrate thermally expands in the thickness direction. On the other hand, with the structure shown in
Furthermore, since the conductor 22 and the via 21 are formed from a phase change material from the start, similarly to the phase change layer 10, the coefficient of thermal expansion of the conductor 22 and the via 21 is the same or extremely close to the coefficient of thermal expansion of the phase change layer 10. Accordingly, there is superior reliability in that sense too. Additionally, since the conductor 22 and the via 21 are formed in continuity with the same material with no joint, it is also possible to avoid the problem of mismatched impedance between the vias and the lines.
The circuit board 100 of the present embodiment also can be a multilayer structure.
Conductive paths 20 (21 and 22) are formed in the first phase change layer 10a and conductive paths 20 (21 and 22) also are formed in the second phase change layer 10b. In the example shown in
Next, a method for manufacturing the circuit board 100 of the present embodiment having a multilayer structure will be described with reference to
Firstly, after preparing the base substrate 30 as shown in
Next, by irradiating laser light 52 from the semiconductor laser 50 as shown in
Next, as shown in
It should be noted that it is possible to produce a circuit board 100 having a three-layer structure or a multilayer structure greater than that by further carrying out the same process.
Further still, the circuit board 100 of the present embodiment also can be produced as shown in
Firstly, after preparing the base substrate 30 as shown in
Next, as shown in
In this way, the circuit board 100 of the present embodiment can be produced. It is possible to produce a circuit board 100 having a multilayer structure of three or more layers by repeating the same process. With the manufacturing method shown in
Furthermore, it is also possible to form only the vias 21 from the phase change material and to form the conductors from a metal material. This manufacturing method will be described with reference to
Firstly, after preparing the base substrate 30 as shown in
Next, as shown in
Next, as shown in
Next, as shown in
With the circuit board 100 obtained by this manufacturing method, the metal conductor (copper conductor for example) 26 is used for the wiring, and therefore can be made to have a lower resistance than the conductor 22 made of phase change material. Furthermore, there is the advantage of superior solderability for connecting components on the conductors 26 with soldering.
It should be noted that, as shown in
Furthermore, the flattening process shown in
At such times, by carrying out a flattening process, the surface of the phase change material 11 can be flattened as shown in
In manufacturing a circuit board 100 of the present embodiment, it is possible to carry out the forming of the conductive paths 20 (21 and 22) using the irradiation of the laser light 52 under a condition in which the phase change layer 10 is rotatable. For example, as shown in
When using the configuration shown in
The conductive paths 20 of the present embodiment are formed with the laser light 52, and therefore, unlike paths obtained by typical formation methods of vias and conductors, there is comparative freedom in selecting paths (for example, three-dimensional inclinations).
For example, as shown in
By using the conductive paths 20 shown in
Further still, the conductive paths 20 of the present embodiment are applicable also to trimming technologies. “Trimming technologies” generally refers to technologies by which circuits or their elements are configured such that the manufacturing deviance of the entire circuitry can be corrected by fine adjustments of a small number of resistors, and then adjusting their resistance values after manufacture to achieve high-precision circuits. Laser trimming methods generally are used for trimming and carried out, for example, by partially removing printed electric components using an expensive high output laser device. On the other hand, by using the configuration of the present embodiment, trimming can be carried out easily and accurately without using a large-scale laser trimming method. This will be described with reference to
Firstly, as shown in
Further still, along with the development in recent years of small-size, high-density mounting technologies of electronic devices represented by mobile telecommunications devices and notebook computers, circuit boards applicable to SMT (surface mount technology) have become increasingly widespread, and there is a tendency for a greater number of complex circuit components to be used in which trimming is essential, such as the VCO (voltage controlled oscillators) and TCXO (temperature compensated crystal oscillators) mounted in these. Therefore the trimming used for the configuration of the present embodiment has a large technological significance.
For comparison, a connection structure using a conventional plating method is shown in
As a different comparison, a connection structure using a conventional conductive paste is shown in
For the structure of
Preferred embodiments of the present invention have been described above, but these descriptions are not limitations and naturally various other modifications are possible.
The invention may be embodied in other forms without departing from the spirit or essential characteristics thereof The embodiments disclosed in this application are to be considered in all respects as illustrative and not limiting. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-067845 | Mar 2004 | JP | national |
This application is a Division of application Ser. No. 11/075,578, filed Mar. 8, 2005, which application is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11075578 | Mar 2005 | US |
| Child | 11674454 | Feb 2007 | US |
