# Patent

## Title
Method of manufacturing semiconductor chip and semiconductor module

## Abstract

An FOM (figure of merit) enabling evaluation from a cost aspect, as well as evaluation of electrical performance, is newly proposed to provide a method of manufacturing based on the FOM a semiconductor chip intended for a lower cost production in addition to satisfying electrical performance. An FOMC of a semiconductor chip is defined as the product of a term represented by electrical performance of a substrate S and a term represented by a semiconductor chip cost CC; the FOMC of each of the semiconductor chips on substrates SS, SC of different type is determined by calculation of the product thereof. Based on the magnitudes of the calculation results, a desired substrate is selected from the substrates SS, SC and then a semiconductor chip is fabricated by forming a semiconductor element on the desired substrate selected.

## Background
FIELD OF THE INVENTION
The present invention relates to methods of manufacturing semiconductor chips and semiconductor modules.
BACKGROUND OF THE INVENTION
A performance index called an FOM (figure of merit) is employed to determine the performance of a semiconductor chip or a semiconductor module having a plurality of semiconductor chips therein.
The FOM is represented by an equation incorporating electrical characteristics of on-resistance, charge density, and the like in order to determine the electrical performance of a semiconductor chip or a semiconductor module. By using a value of an FOM gained by substituting a specific value of the electrical characteristic into the FOM equation, individual performance of the semiconductor chip or the semiconductor module is determined, or performance of one chip/module is compared with that of another semiconductor chip/module.
For instance, Japanese Unexamined Patent Publication No. 2000-156503 (paragraph 0006) discloses that an FOM of an MOSFET (metal oxide semiconductor field effect transistor) is defined as the product of a charge QGD and an on-resistance RDSON.
Further, Japanese Unexamined Patent Publication (Translation of PCT Application) No. 2004-537162 (paragraph 0005) discloses that an FOM of an MOSFET is defined with a specific on-resistance (Ron, sp) associated with a blocking voltage.
Still further, Japanese Unexamined Patent Publication No. 2007-27266 (paragraph 0013) discloses that an FOM of an MOSFET is defined as the product of an on-resistance Ron and a gate-drain capacitance Qgd.
Yet still further, Japanese Unexamined Patent Publication (Translation of PCT Application) No. 2007-512701 (paragraph 0003) discloses that an FOM of an trench MOSFET is defined with an on-resistance (Rds, on) associated with a gate-drain charge density (Qgd).
In this way, the FOM is used as an index for determining the electrical performance of a semiconductor chip or a semiconductor module. However, even though the semiconductor chip or semiconductor module that satisfies desired performance is designed and manufactured based upon the FOM, such production process does not necessarily result in commercially successful products. For instance, even if desired performance is satisfied, a manufacturing cost of semiconductor chip/module may in fact rise. In this way, the FOM can be used to determine performance of the semiconductor chip or the semiconductor module, but cannot be used as commercial criteria.
SUMMARY OF THE INVENTION
The present invention is directed to overcome foregoing problems, and an object thereof is to newly propose an FOM that allows determination from a cost aspect in addition to determination of electrical performance and to provide a method of manufacturing, based on the FOM, a semiconductor chip and a semiconductor module at lower cost in addition to satisfying electrical performance.
A method of manufacturing a semiconductor chip according to the present invention comprises a comparison step that calculates, based on an equation of an FOM (figure of merit) of a semiconductor chip defined as the product of a term represented by electrical performance of a substrate and a term represented by a manufacturing cost of a semiconductor chip, FOMs of semiconductor chips on substrates of different type, to thereby compare magnitudes of calculation based on the calculation results through the comparison step; and an element formation step that forms a semiconductor element on the desired substrate selected in the selection step.
Furthermore, a method of manufacturing a semiconductor module according to the present invention comprises a comparison step that calculates, based on an equation of an FOM (figure of merit) of a semiconductor chip defined as the product of a term represented by electrical performance of a substrate and a term represented by a manufacturing cost of a semiconductor module, FOMs of semiconductor modules on substrates of different type, to thereby compare magnitudes of calculation results; a selection step that selects a desired substrate based on the magnitudes of the calculation results through the comparison step; an element formation step that forms a semiconductor element on the desired substrate selected in the selection step; and a packaging step that interconnects a plurality of semiconductor chips fabricated in the element formation step, to house the plurality thereof in a package.
According to the present invention, there is provided a method of manufacturing a semiconductor chip intended for a lower cost production, in addition to satisfying electrical performance.
The present invention further provides, in addition to satisfying electrical performance, a method of manufacturing a semiconductor module intended for a lower cost production, based on the foregoing FOM of the semiconductor chip,



BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a table for illustrating an example of results of applying to an inverter FOMs of a semiconductor chip and a semiconductor module according to an embodiment of the present invention.



DESCRIPTION OF THE PREFERRED EMBODIMENT
To start with, an FOM according to the present invention is shown as Equation 1.

FOM=Jc/(VF×err×C)   Equation 1

where JC represents a rated current density (A/cm2); VF, an on-state power loss (DC power loss); and err, a switching loss, with each symbol showing an electrical characteristic and C also denoting a manufacturing cost of semiconductor chip CC or the manufacturing cost of a semiconductor module CM. As is evident from Equation 1, the FOM is defined as the product of a term JC/(VF×err×C) represented by electrical performance and a term 1/C represented by either of the costs.

FIG. 1 shows an example of a result of applying to an inverter the FOB shown in Equation 1.
Referring to FIG. 1 (table), a column labeled App shows an application, i.e., an objective product, where is assumed an inverter; a type of a substrate labeled S is shown which is applied to the applicable product, where a silicone (Si) substrate SS and a silicon carbide (SiC) substrate SC are applied. The silicon substrate used is one that is commercially widely available, whereas the silicon carbide substrate is one that is expected as the next generation one. The FOM shown in Equation 1 is highly useful in selecting any one of such substrate materials. A column labeled JC represents a rated current density (A/cm2) of one of the electrical characteristics. A column labeled VF represents an on-state power loss of one of the electrical characteristics, which is a DC loss. A column labeled err represents a switching loss of one of the electrical characteristics. A column labeled CC represents a cost of manufacturing a semiconductor chip C (hereinafter called semiconductor chip cost CC). A column labeled FOMC represents an FOM of a semiconductor chip. Likewise, a column labeled CM represents a cost of manufacturing the semiconductor-chips-mounting semiconductor module (hereinafter called semiconductor module cost CM). Further, a column labeled FOMM shows an FOM of the semiconductor module.
The rated current density JC is shown in an absolute value. Typically, the rated current density JC of the silicon substrate SS is 200 to 250 A/cm2, while the density JC of the silicon carbide substrate SC is 300 to 1,000 A/cm2. In FIG. 1, the density JC of the silicon substrate SS is set to 200 A/cm2, while the density JC of the silicon carbide substrate SC is set to 1,000 A/cm2.
The on-state power loss VF is shown in a relative value, and here the on-state power loss VF of the silicon substrate SS is designated as one (1). At that time, the loss VF of the silicon carbide substrate SC is between 0.6 and 0.8. In FIG. 1, the loss VF of the substrate SC is set to 0.8.
The switching loss err is shown in a relative value and here, the loss err of the silicon substrate SS is designated as one (1). At that time, the loss err of the silicon carbide substrate SC is between 0.1 and 0.3. In FIG. 1, the loss err of the substrate SC is set to 0.1.
The semiconductor chip cost CC is shown in a relative value and here, the semiconductor chip cost CC of the silicon substrate SS is designated as one (1). At that time, the chip cost CC of the substrate SC is between 100 and 1. In FIG. 1, the chip cost CC of the substrate SC is shown as 100, 10, 5 and 1, which are, as a matter of fact, consecutively varying values. Here, a value of 100, which is the chip cost CC of the substrate SC shows that the semiconductor chip cost is 100 times higher than the chip cost CC of the substrate SS. And a value of one (1), which is the semiconductor chip cost CC of the substrate SC, shows that the semiconductor chip cost is equal to the chip cost CC of the substrate SS. Reduction in the chip cost CC of the substrate SC can be achieved by, for instance, cost reduction in material procurement and/or cost reduction in manufacture owing to progress in semiconductor manufacturing technology.
The FOMC of the semiconductor chip is a value calculated by substituting to Equation 1 values of the rated current density JC, the on-state power loss VF, the switching loss err, and the semiconductor chip cost CC, in the silicon substrate SS and the silicon carbide substrate SS.
The semiconductor module cost CM is a value of adding two (2) to the semiconductor chip cost CC. Semiconductor modules are typically manufactured by having semiconductor chips mounted on a semiconductor substrate and then packaging the chips. For that reason, the semiconductor module cost CM is such that the proportion of a cost involved in manufacturing the semiconductor module is added to the chip cost CC. Here, the module cost CM of the silicon substrate SS is shown as a value of three, which is obtained by adding a value of two to the chip cost CC; the module cost CM of the silicon carbide substrate SC, which is obtained by adding a value of 2 to the chip cost CC, falls within a range of 102 to 3. FIG. 1 shows the semiconductor module cost CM of the substrate SC as values of 102, 12, 7 and 3; however, actually, such values are consecutively varying ones. Reduction in the module cost CM thereof can be achieved by, for instance, cost reduction in material procurement and/or cost reduction in manufacture owing to progress in manufacturing technology.
The FOMM of the semiconductor module is a value calculated by substituting into Equation 1 values of the rated current density JC, the on-state power loss VF, the switching loss err, and the semiconductor module cost CM, in the silicon substrate SS and the silicon carbide substrate SC.
The FOMC of the semiconductor chip and the FOMM of the semiconductor module, shown in FIG.1, will be used as described below.
First, the FOMC of the semiconductor chip will be described. The FOMC of the semiconductor chip whose value is greater is advantageous, that is, the greater value of the FOMC thereof, as well as satisfying the performance, allows lower cost products to be placed on the market, thus leading to improvement of their price competitiveness, which provides a commercial advantage.
Referring to FIG. 1, the FOMC of the semiconductor chip of using the silicon substrate SS is 2,000, whereas the FOMC of using the silicon carbide substrate SC, first when the semiconductor chip cost CC is 100, amounts to 125, which is smaller than a value of 200, the FOM of the chip of using the substrate SS. Thus, in this case, the use of the silicon substrate SS as the substrate S allows semiconductor chips to be manufactured at lower cost. Next, when the semiconductor chip cost CC of the silicon carbide substrate SS is a value of 10, the FOMC of the semiconductor chip is a value of 1250, which is larger than a value of 200, the FOMC of the semiconductor chip when using the silicon substrate SS; thus, in this case, the use of the silicon carbide substrate SC as the substrate S allows lower-cost manufacture of semiconductor chips. Likewise, when the manufacture costs of the semiconductor chip CC of the silicon carbide substrate SS are values of 5 and 1 as well, the use of the silicon carbide substrate SC permits the semiconductor chip to be manufactured at lower cost. Here, when the semiconductor chip cost CC is a value of 62.5 in using the silicon carbide substrate SC, the FOMC of the semiconductor chip is a value of 200, which is the same as the FOMC of the semiconductor chip in using the silicon substrate SS. Consequently, the value of 62.5, which is the semiconductor chip cost CC of using the silicon carbide substrate SC, is a criterion value for choosing the desired substrate when using the silicon substrate SS and the silicon carbide substrate SC.
In the actual manufacture, such values can be used for selecting a type of the substrate S to be applied for the manufacture of semiconductor chips. For instance, given that if any of the silicon substrate SS and the silicon carbide substrate SC is used and semiconductor chips satisfying predetermined performance can be provided, then the use of the substrate, whichever is more economical for the manufacture, allows reduction of the semiconductor chip cost. In this case, lower-cost semiconductor chips can be provided if comparison is made between the FOMC of the semiconductor chip of using the silicon substrate SS and that of using the silicon carbide substrate SC, and then, semiconductor chips are manufactured using, as a preferable substrate S, that of the semiconductor chip having a greater FOMC. Consequently, if the semiconductor chips are manufactured by the semiconductor-chip process that includes a comparison step through which comparison is made between the FOMC of the semiconductor chip of using the silicon substrate SS and that of using the silicon carbide substrate SC, a selection step that selects through the comparison step the substrate S having a greater FOMC, as a desired substrate SS, and an element formation step that forms, on the substrate S selected through the selection step, a semiconductor element, for instance, a diode element or a semiconductor element such as diode, MOSFET and IGBT (insulated gate bi-polar transistor), then it is possible to provide lower-cost semiconductor chips in addition to performance satisfaction being achieved, which leads to enhancement of market price competitiveness, resulting in such chips being commercially advantageous.
Next, an FOMM of a semiconductor module will be described. The FOMM in the semiconductor module whose value is greater is advantageous, as with that of the FOMC in the semiconductor chip. That is, a greater value of the FOMM in the semiconductor module allows, as well as satisfying its performance, lower cost products to be introduced on the market, thus leading to enhancement of their market price competitiveness and resulting in such chips being commercially advantageous.
Referring to FIG. 1, the FOMM of the semiconductor module of using the silicon substrate SS is 67, whereas when the semiconductor module cost CM is 102, the FOMM of using the silicon carbide substrate SC amounts to 123. This value is greater than a value of 67, which is the FOMM of the semiconductor module of using the silicon substrate SS. Thus, the use of the silicon substrate SC as the substrate S allows the semiconductor module to be manufactured at lower cost. Likewise, when the semiconductor module cost CM of the silicon carbide substrate SC is values of 12, 7 and 3 as well, the use of the silicon carbide substrate SC permits the semiconductor module to be manufactured at lower cost. Here, when the semiconductor module cost CM in using the silicon carbide substrate SC is a value of 186.57, the FOMC of the semiconductor chip is substantially the same as the FOMC of the semiconductor module in using the silicon substrate SS. Consequently, the module cost CM—the value of 186.57—in using the substrate SC is a criterion value of using the silicon substrate SS or the silicon carbide substrate SC. Here, when the module cost CM of using the substrate SC is a value of 186.57, the chip cost CC is 184.57; thus, when manufacturing semiconductor modules, the chip cost CC is allowed to take a value up until approximately 184.57.
In the actual manufacture, the criterion value can be used for selecting a type of the substrate S to be applied for manufacture of semiconductor modules. For instance, given that if any of the silicon substrate SS and the silicon carbide substrate SC is used and semiconductor modules satisfying predetermined performance can be provided, then the use of the substrate, whichever is more economical for the manufacture, allows the semiconductor module cost to be reduced. In this case, lower-cost semiconductor modules can be provided if comparison is made between the FOMM of the semiconductor module of using the silicon substrate SS and that of using the silicon carbide substrate SC and then, semiconductor modules are manufactured using, as a desired substrate S, that of the semiconductor module with a greater FOMM. Consequently, lower-cost semiconductor modules can be provided in addition to performance satisfaction being achieved, if the semiconductor-module process includes a comparison step through which comparison is made between the FOMM of the semiconductor module of using the silicon substrate SS and that of using the silicon carbide substrate SC, and a selection step that selects through the comparison step the substrate S with a greater FOMM, as a desired substrate S, and if semiconductor chips are manufactured by forming a semiconductor element, such as diode, MOSFET, and IGBT, on the substrate S selected through the selection step, and also if a semiconductor module is manufactured using a packaging step that mounts and interconnects a plurality of semiconductor chips achieved by way of the element formation step, to then house the plurality thereof in a package. Thus, this manufacturing process leads to enhancement of market price competitiveness, resulting in such modules being commercially advantageous.
Note that while the present embodiment is described using the example of the inverter as the objective product, it should be understood of course that the present invention is not limited to the inverter but applicable to other products such as a converter, a PFC (power factor control) circuit, and a brake circuit as well.
Note again that although the present embodiment is described using an example of the silicon substrate SS or the silicon carbide substrate SC, it should be understood of course that the present invention is not limited to such substrate but is applicable to every possible substrate.
Note further that although the present embodiment is described using specific values for the rated current density JC, the on-state power loss VF and the switching loss err, representing electrical characteristics, such values are not fixed values but those having a predetermined range and it should be understood of course that they may be selected as appropriate from within the predetermined range.

## Claims

1. A method of manufacturing a semiconductor chip, comprising:
a comparison step that calculates, based on an equation of an FOM (figure of merit) of a semiconductor chip defined as a product of a term represented by electrical performance of a substrate and a term represented by a manufacturing cost of a semiconductor chip, FOMs of semiconductor chips on substrates of different type, to thereby compare magnitudes of calculation results;
a selection step that selects a desired substrate from the substrates of different type, based on the magnitudes of the calculation results through the comparison step; and
an element formation step that forms a semiconductor element on the desired substrate selected in the selection step.

2. The method of manufacturing a semiconductor chip of claim 1, wherein the term represented by electrical performance is designated as Jc/(VF×err) when a rated current density is designated as JC, an on-state power loss as VF, and a switching loss as err, the term represented by the manufacturing cost of semiconductor chip is designated as 1/C when the cost thereof is designated as C, and the desired substrate is a substrate having the greater calculation result of the FOM in the substrates of the different type.

3. The method of manufacturing a semiconductor chip of claim 2, wherein the substrates of the different type are a silicon substrate and a silicon carbide substrate.

4. A method of manufacturing a semiconductor module, comprising:
a comparison step that calculates, based on an equation of an FOM (figure of merit) of a semiconductor module defined as a product of a term represented by electrical performance of a substrate and a term represented by a manufacturing cost of a semiconductor chip, FOMs of semiconductor modules on substrates of different type, to thereby compare magnitudes of calculation results;
a selection step that selects a desired substrate based on the magnitudes of the calculation results through the comparison step;
an element formation step that forms a semiconductor element on the desired substrate selected in the selection step; and
a packaging step that interconnects a plurality of semiconductor chips fabricated in the element formation step, to house the plurality thereof in a package.

5. The method of manufacturing a semiconductor module of claim 4, wherein the term represented by electrical performance is represented as JC/(VF×err) when a rated current density is designated as JC, an on-state power loss as VF, and a switching loss as err, the term represented by the manufacturing cost semiconductor module is represented as 1/C when the cost thereof is designated as C, and the desired substrate is a substrate having the greater calculation result of the FOM in the substrates of the different type.

6. The method of manufacturing a semiconductor module of claim 5, wherein the substrates of the different type are a silicon substrate and a silicon carbide substrate.