JP2018176175A - Bonding method of semiconductor element and substrate using Pb free Zn-Al based solder alloy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a joining method of a semiconductor element and a substrate using a Pb-free Zn-Al-based solder alloy where jointing can be easily performed in a general decompressed reflow device without applying physical external force such as scrubbing, loading and the like in a solder joint step and favorable jointing strength can be ensured.SOLUTION: In a joining method of a semiconductor element and a substrate whose surface is at least Ni using a Pb-free Zn-Al-based solder alloy whose main component is Zn, the Zn-Al-based solder alloy and the semiconductor element are placed on the substrate in a state of being superposed in this order; temperature rising is performed in a first inert gas atmosphere, an inert gas is newly supplied in a prescribed flow rate during the temperature rising when temperature reaches first temperature of the liquid phase temperature of the Zn-Al-based solder alloy or higher and a second inert gas atmosphere is prepared; and the new inert gas is continuously supplied, second temperature is kept for a prescribed time when temperature reaches second temperature and the semiconductor element and the substrate are joined.SELECTED DRAWING: None

Description

  The present invention relates to a bonding method of bonding a semiconductor element and a substrate with a Pb-free Zn-Al based solder alloy.

In the assembly process of various electronic parts including die bonding of elements for power transistors, high temperature soldering is performed, and a solder alloy having a relatively high melting point of about 300 ° C. (hereinafter referred to as “solder for high temperature Alloy) is used. As such a high temperature solder alloy, a Pb-based solder alloy represented by a Pb-5 mass% Sn alloy has been mainly used conventionally.
However, in recent years, there is a strong tendency to limit the use of Pb in consideration of environmental pollution, and, for example, Pb is a substance to be regulated under the RoHS Directive (a directive to restrict the use of harmful substances). In response to these trends, provision of Pb-free (lead-free) solder alloys, that is, Pb-free solder alloys is also required in the field of assembly of electronic parts and the like.

An Au-Sn alloy may be used for Pb-free solder alloys for high temperature, but Au-Sn alloys occupy 80% of the whole and are very expensive and limited to special applications. Therefore, in order to realize a Pb-free solder alloy at low cost, a solder alloy mainly composed of Bi or Zn has been developed by various organizations.
In particular, as a Pb-free solder alloy for high temperature containing Zn as a main component, Patent Document 1 proposes a Zn-Al solder alloy in which Al is added to lower the melting point to Zn. However, although a solder alloy containing Zn as a main component has a high melting point and is excellent in heat resistance, it has a property that the surface is easily oxidized naturally and the oxide film thereof is not easily reduced. Therefore, it is difficult to remove all the oxide film on the surface even if the solder alloy mainly composed of Zn is heated to near the melting temperature in a reducing atmosphere. Therefore, conventionally, in order to remove the oxide film on the surface, a physical external force such as scrub is applied at the time of melting of the solder alloy in the solder bonding process to break the oxide film on the surface, and the molten solder alloy is made to flow out. It is made to join a to-be-joined surface and a solder alloy.
Moreover, in Zn-Al solder alloy, it is known that variation occurs in the bonding strength at the time of bonding with the surface to be bonded. However, Patent Document 2 proposes a bonding method of bonding while applying a load to a Zn-Al solder alloy in a solder bonding process in order to suppress variations in bonding strength and improve product yield.

Unexamined-Japanese-Patent No. 2009-125753 JP, 2013-030607, A

  However, applying a physical external force such as scrubbing or load to the solder alloy in the process of joining the surface to be joined and the solder alloy complicates the operation and also applies a load to the solder alloy. In some cases, it also causes the dispersion of the joint strength between the solder alloy and the surface to be joined.

  Therefore, the present invention is a general pressure reduction reflow apparatus without applying physical external force such as scrubbing or applying a load in a solder bonding process with a bonding surface using a Pb-free Zn-Al alloy. It is an object of the present invention to provide a method of bonding a semiconductor element and a substrate using a Pb-free Zn-Al based solder alloy which can be easily bonded and can secure a good bonding strength.

  The method of bonding a semiconductor element to a substrate using a Pb-free Zn-Al based solder alloy according to the present invention comprises: using a Pb-free Zn-Al based solder alloy containing Zn as a main component and not containing Pb; In the bonding method of bonding a substrate having at least a main element on the surface of Ni, the Pb-free Zn-Al based solder alloy and the semiconductor element are sequentially placed on the substrate, The temperature is raised under the first inert gas atmosphere, and when the temperature reaches the first temperature higher than the liquidus temperature of the Pb-free Zn-Al based solder alloy during the heating, the inert gas is newly added. After supplying a predetermined flow rate to create a second inert gas atmosphere and continuing to supply the new inert gas at the predetermined flow rate after creating the second inert gas atmosphere, the second temperature When you reach Holding a predetermined time to a second temperature, it is characterized in that to join the substrate and the semiconductor element.

  The method of joining a semiconductor element and a substrate using the Pb-free Zn-Al based solder alloy according to the present invention is a semiconductor using a Pb-free Zn-Al based solder alloy containing Zn as a main component and not containing Pb. In a bonding method for bonding an element and a substrate in which at least the main element on the surface is Ni, the solder alloy and the semiconductor element are sequentially stacked on the substrate, and the first defect The temperature is raised under an active gas atmosphere, and when the temperature reaches the first temperature above the liquidus temperature of the Pb-free Zn-Al based solder alloy during the temperature rise, the pressure reduction is started, and the vacuum state is 200 Pa or less After the pressure reduction is stopped and the pressure reduction is stopped, an inert gas is newly supplied at a predetermined flow rate to create a second inert gas atmosphere, and then the second inert gas atmosphere is formed. Also The supply of the new inert gas at the predetermined flow rate is continued, and when the second temperature is reached, the second temperature is maintained for a predetermined time to bond the semiconductor element and the substrate. And

  Further, in the method of bonding a semiconductor element and a substrate using the Pb-free Zn-Al based solder alloy of the present invention, the temperature is raised under the first inert gas atmosphere having an oxygen concentration of 200 ppm or less When the temperature reaches the first temperature above the liquidus temperature of the Pb-free Zn-Al based solder alloy, the depressurization is started, the depressurization is stopped until the vacuum state becomes 100Pa or less, and the depressurization is stopped. It is preferable to carry out the supply of the new inert gas at the predetermined flow rate to create the second inert gas atmosphere simultaneously with the stop.

  Further, in the method of bonding a semiconductor element and a substrate using the Pb-free Zn-Al based solder alloy of the present invention, the time for performing the pressure reduction is preferably within about 1 minute.

  Further, in the method of joining a semiconductor element and a substrate using the Pb-free Zn-Al based solder alloy of the present invention, the first temperature is 400 ° C. or higher than the liquidus temperature of the Pb-free Zn-Al based solder alloy. It is preferable to set it as the following predetermined temperature.

  In the method of bonding a semiconductor element and a substrate using the Pb-free Zn-Al based solder alloy of the present invention, the amount of the new inert gas for creating the second inert gas atmosphere is 5 ml / min or more. It is preferable to supply at a flow rate of

  According to the present invention, it is possible to use a general pressure reduction reflow apparatus without applying any physical external force such as scrubbing or applying a load in a solder bonding process with a bonding surface using a Pb-free Zn-Al alloy. Thus, it is possible to obtain a method of bonding a semiconductor element and a substrate using a Pb-free Zn-Al based solder alloy, which can be easily bonded and can ensure good bonding strength.

It is sectional drawing which shows an example of the joining aspect of the semiconductor element and board | substrate which used the solder alloy.

  Prior to the description of the embodiment of the present invention, the structure and bonding mode of the semiconductor element and the substrate will be described with reference to the cross-sectional view of FIG. Note that FIG. 1 is a drawing schematically shown for ease of understanding, and there are places where the relationship between the thickness and the planar dimension, the ratio of the thickness of each layer, and the like are exaggerated.

FIG. 1 is a cross-sectional view showing an example of bonding of a semiconductor element and a substrate using a solder alloy.
The semiconductor element 1 is bonded onto the substrate 2 via the solder alloy 3. The semiconductor element 1 is not particularly limited, but other wide band gap semiconductor elements such as a SiC (silicon carbide) element, a GaN (gallium nitride) element, a diamond element, a ZnO (zinc oxide) element and the like can be applied. In addition, below, the example using a SiC element is demonstrated from acquisition being easy. An ohmic contact 11 is formed on the back side of the semiconductor element 1, and a mounting electrode 12 is coated on the surface of the ohmic contact 11 for the purpose of improving the wettability of the solder, preventing the penetration of the solder, and improving the adhesive force. There is. The mounting electrode 12 is formed of, for example, a Ti / Ni / Au laminated vapor deposition film or a Ti / Ni / Ag laminated vapor deposition film laminated in order from the ohmic contact 11 side. Here, it is assumed that the mounting electrode 12 is made of a Ti / Ni / Au laminated vapor deposition film.

  The substrate 2 is also not particularly limited, and a substrate having a structure in which the metal plate 21 is attached to the ceramic substrate 20, a ceramic substrate, a flexible substrate using polyimide or the like, a metal single metal substrate, etc. Applicable Further, it is general to provide a metal layer 22 of Ni, Pd or the like on the surface of the base of the substrate 2 (the surface on the side of the semiconductor element 1 in the metal plate 21 in the example of FIG. 1). By providing the metal layer 22 of Ni or Pd, the generation of unnecessary reaction products can be suppressed at the interface with the solder alloy 3, and the heat resistance is improved. Here, an example using the substrate 2 in which the metal layer 22 of Ni is plated on the surface on the semiconductor element 1 side of the Cu substrate 21 will be described. The thickness of the metal layer 22 of Ni is preferably 0.1 μm to 10 μm. More preferably, it is 3 μm to 5 μm. Furthermore, on the surface of the front side of the Ni metal layer 22, a thin noble metal coating layer may be provided to prevent oxidation and ensure wettability. The noble metal coating layer is made of, for example, an Au layer. Au can improve solder wettability even with a small amount. For this reason, in joining to the to-be-joined surface using the conventional Zn-Al type solder alloy, it is essential to use the board | substrate 2 with which Au layer was provided in the surface of the metal layer 22 of Ni. However, according to the bonding method of the embodiment of the present invention, a Zn-Al based solder alloy is directly stacked on the Ni surface of the substrate 2 and bonded to the semiconductor element 1 using the substrate 2 not provided with the noble metal coating layer. Is also derived on the premise that it is possible to prevent oxidation and ensure wettability. In this way, it is not necessary to coat the substrate with an expensive noble metal, and the number of processes can be reduced accordingly, and the cost can be reduced.

For the solder alloy 3, a Pb-free Zn-Al based solder alloy is used. The Pb-free Zn-Al based solder alloy is a solder alloy used for high temperature bonding, and examples thereof include a Zn-Al alloy and a Zn-Al-Ge alloy. The composition of Zn and Al in the Pb-free Zn-Al based solder alloy used in the bonding method of the embodiment of the present invention is a eutectic composition (Al concentration = 5 mass%), or a substantially eutectic composition (Al concentration = 5) It is preferable that it is ± 2 mass% or less.
In addition, even if the Pb-free Zn-Al based solder alloy contains a small amount (1 mass% or less) of Ge and other elements in order to improve the hardness of the solder material, the wettability during bonding, and the viscosity of the melt. Good.
Further, as the Pb-free Zn-Al based solder alloy, an alloy containing 80% by mass or more of Zn, which has a high melting point and is easily oxidized naturally on the surface of the alloy, can also be used. For example, a solder alloy in which Ge or Mg is added to a Zn-Al alloy (refer to JP-A-11-288955), a solder alloy in which Mg and Sn are added to a Zn-Al alloy (refer to JP-A-11-208487), Zn -A solder alloy in which Mg and In are added to Al alloy (refer to JP-A-11-172354), a solder alloy in which Ge, Sn and In are added to Zn-Al alloy (refer to JP-A-11-172353), Zn -A solder alloy in which Ge and Mg are added to Al alloy (refer to JP-A-11-172352, JP-A-2000-208533, JP-A-2000-61686), Zn-Al alloy added with Ge and P Solder alloy (refer to JP-A-2004-358540), Solder alloy obtained by adding Ge, Mg and P to Zn-Al alloy (JP-A-2004-358) 39 No. see Japanese) and the like are applicable as the Pb-free Zn-Al solder alloy used for the bonding method of the embodiment of the present invention.

  The thickness of the Pb-free Zn-Al based solder alloy is not particularly limited, but in practical use, the thickness is preferably approximately 50 μm to 300 μm or less. Since the bonding property is affected by the size of the semiconductor element and the volume of the Pb-free Zn—Al based solder alloy, the thickness is appropriately set to a thickness that can ensure the bonding property.

Next, a specific procedure of a method of bonding a semiconductor element and a substrate using a Pb-free Zn-Al based solder alloy will be described.
First, a bonding method using a conventional general reduced pressure reflow apparatus will be described. The semiconductor element 1, the substrate 2 and the Pb-free Zn—Al based solder alloy 3 described above are prepared. The surfaces to be bonded of the semiconductor element 1 and the substrate 2 are organically cleaned using a solvent such as acetone or isopropyl alcohol as necessary to remove contaminants attached to these surfaces.

As a reduced pressure reflow apparatus, one having an exhaustion capacity capable of reducing the pressure to about 200 Pa or less and configured to be capable of introducing an inert gas (nitrogen or argon gas) having a purity of 99.99% or more.
The Cu substrate with Ni plating is placed on the reflow table installed in the sample chamber of the reduced pressure reflow apparatus, with the Ni surface facing the bonding side, and then Ni plating is applied. The Pb-free Zn-Al based solder alloy and the semiconductor element with the bonding surface facing downward are sequentially placed on the site to be joined in the Cu substrate in a state in which the semiconductor element is directed downward.
Next, an inert gas is introduced into the sample chamber of the reduced pressure reflow apparatus to create an inert gas atmosphere, and the oxygen concentration in the sample chamber is reduced to 200 ppm or less. For example, nitrogen gas is used as the inert gas. In addition, argon gas etc. can also be used as an inert gas.
Then, the reflow stand or the entire sample chamber is heated to a predetermined temperature 50 ° C. to 70 ° C. higher than the liquidus temperature of the Pb-free Zn—Al based solder alloy. Then, when the predetermined temperature is reached, the temperature is held at the predetermined temperature for about 1 minute to 5 minutes. In the meantime, the Pb-free Zn-Al based solder alloy becomes liquid form over the melting point and joins the semiconductor element and the substrate. Thereafter, the temperature of the entire reflow table or sample chamber is lowered to room temperature, and the bonded product is taken out to complete the bonding process. Meanwhile, a small amount of inert gas may be continuously supplied to the sample chamber so as to maintain the oxygen concentration at 200 ppm or less.

  In the above example, the bonding of the semiconductor element and the substrate using the Pb-free Zn-Al based solder alloy is performed in an inert gas atmosphere, but may be performed in a vacuum if necessary. For example, an inert gas is introduced into the sample chamber before the temperature rise and the sample chamber is replaced with the inert gas to create an inert gas atmosphere, and then the temperature rise is started after reducing the pressure in the sample chamber to a vacuum state When the temperature reaches a predetermined temperature, the temperature may be maintained at that predetermined temperature for about 1 minute to 5 minutes, or an inert gas may be introduced into the sample chamber before the temperature rise to replace the sample chamber with the inert gas. Create an inert gas atmosphere, start raising the temperature, reduce the pressure in the sample chamber to a vacuum while raising the temperature, and maintain it at the predetermined temperature for about 1 to 5 minutes You may do so. When bonding is performed in a vacuum state, small voids existing in the Pb-free Zn-Al based solder alloy can be suppressed, and the wettability and spreadability can be improved.

  As described above, when a Pb-free Zn-Al based solder alloy is used as the solder alloy, the surface of the solder alloy is likely to be naturally oxidized, and the oxide film thereof is difficult to be reduced. Therefore, for example, in the bonding method of Patent Document 2, the Zn-Al solder alloy and the semiconductor element are superimposed on the substrate, and further, the load member is placed on the semiconductor element and heated. Then, by applying a load to the semiconductor element and melting the Zn-Al solder alloy, the foreskin of the natural oxide film of Zn or Al surrounding the Zn-Al-based solder melt is instantly destroyed, and the melt and the substrate are melted. And, the bonding reaction between the melt and the semiconductor element can be made to occur in a short time and with good reproducibility. Also, instead of applying a static load, the same effect can be obtained by rubbing the oxide film, such as scrubbing. However, applying a physical external force such as scrubbing or load to the solder alloy in the process of joining the surface to be joined and the solder alloy complicates the operation and also applies a load to the solder alloy. In some cases, it also causes variations in bonding strength.

  However, the bonding method according to the embodiment of the present invention is a bonding method that does not require the application of physical external force as described above, and is characterized by switching the atmosphere when the melting point of the solder alloy is exceeded during heating. There is. The details will be described below.

  In the bonding method according to the first embodiment of the present invention, a Ni-plated Cu substrate, Pb-free Zn on the reflow stand of the sample chamber of the reduced pressure reflow device is used similarly to the conventional bonding method using the general reduced pressure reflow device. -After placing the Al-based solder alloy and the semiconductor element in an overlapping state, an inert gas is introduced into the sample chamber of the reduced pressure reflow apparatus to create a first inert gas atmosphere, and the oxygen concentration in the sample chamber is 200 ppm. Do the following. For example, nitrogen gas is used as the inert gas. Note that argon gas or the like may be used as the inert gas. In order to maintain the oxygen concentration at 200 ppm or less, a small amount of nitrogen gas may be continuously supplied into the sample chamber. The flow rate of nitrogen gas at this time is, for example, less than 5 ml / min, such as 1 to 2 ml / min.

  Then, the reflow stand or the entire sample chamber is heated to a predetermined temperature (second temperature) higher by 50 ° C. to 70 ° C. than the liquidus temperature of the Pb-free Zn—Al based solder alloy. During the temperature rise, when reaching a predetermined temperature (first temperature) higher than the liquidus temperature of the Pb-free Zn-Al based solder alloy, supply of inert gas (for example, nitrogen gas) at a predetermined flow rate anew To create a second inert gas atmosphere, and after creating the second inert gas atmosphere, supply of the new inert gas at a predetermined flow rate is continued. Thereafter, when a predetermined temperature (second temperature) higher than the liquidus temperature by 50 ° C. to 70 ° C. is reached, the temperature is maintained at the predetermined temperature (second temperature) for a predetermined time. The holding time is preferably 3 minutes to 5 minutes. In the meantime, the Pb-free Zn-Al based solder alloy becomes liquid form over the melting point and joins the semiconductor element and the substrate. Thereafter, as in the above-described conventional method, the temperature of the entire reflow stand or sample chamber is lowered to room temperature and the joined product is taken out.

  As described above, according to the bonding method of the first embodiment of the present invention, the inert gas is newly added when reaching the first temperature higher than the liquidus temperature of the Pb-free Zn-Al based solder alloy in the middle of the temperature rise. The supply of a predetermined flow rate (for example, nitrogen gas) is started, and an inert gas atmosphere (a second inert gas atmosphere) different from the inert gas atmosphere (the first inert gas atmosphere) before reaching the first temperature It is characterized by changing to an inert gas atmosphere).

In order to break the natural oxide film of the Pb-free Zn-Al solder alloy to secure the bonding strength, generally, external pressure such as bonding while applying a load in the solder bonding step is required. However, as a result of trial and error by the present inventors, when the Pb-free Zn-Al based solder alloy is melted, if the atmosphere is changed by newly supplying an inert gas (nitrogen gas) at a flow rate of a predetermined amount or more, It turned out that the same effect as external pressurization, such as joining, applying a load in a joining process, is acquired.
The timing at which the oxide film on the surface of the Pb-free Zn-Al-based solder alloy is easily broken is most effective when the Pb-free Zn-Al-based solder alloy melts. Therefore, the temperature (first temperature) at which an inert gas (nitrogen gas) is newly supplied is a predetermined temperature higher than the liquidus temperature of the Pb-free Zn-Al based solder alloy, and preferably, the liquidus temperature ~ 400 ° C. At this time, if an inert gas (nitrogen gas) is newly supplied to create a second inert gas atmosphere, the oxide film can be destroyed. The flow rate of inert gas (nitrogen gas) to be newly supplied when the first temperature above the liquidus temperature of the Pb-free Zn-Al solder alloy is reached needs to be at least 5 ml / min or more, preferably , 20 ml / min or better.

If the flow rate of the inert gas (nitrogen gas) to be newly supplied is less than 5 ml / min, the oxide film can not be broken and the bonding property can not be improved. If the flow rate of the inert gas (nitrogen gas) to be newly supplied is 20 ml / min or more, better bondability can be obtained.
In addition, after introducing an inert gas (nitrogen gas) into the sample chamber of the reduced pressure reflow apparatus first and continuously supplying a small amount (less than 5 ml / min) of the inert gas (nitrogen gas), In the first inert gas atmosphere in which the oxygen concentration in the sample chamber is maintained at 200 ppm or less, as in the bonding method of the first embodiment of the present invention, a Pb-free Zn-Al based solder alloy during heating When the first temperature above the liquidus temperature is reached, inert gas (nitrogen gas) is newly supplied at a flow rate of 5 ml / min or more to create a second inert gas atmosphere, and the second If the supply of the new inert gas at the above flow rate is continued even after the formation of the active gas atmosphere, the oxygen concentration in the sample chamber can be reduced to about 20 ppm or less, and the Pb-free Zn-Al based solder alloy Formed on the surface Since it is possible to suppress the progress of the oxide film, it is possible to positively influence the bonding properties.

  Further, in the bonding method of the second embodiment of the present invention, the temperature reached the first temperature equal to or higher than the liquidus temperature of the Pb-free Zn-Al-based solder alloy during the temperature rise in the first inert gas atmosphere. When the pressure in the sample chamber is reduced to a vacuum, the pressure is reduced to 200 Pa or less, and then the pressure reduction is stopped. At the same time, inert gas (nitrogen gas) is newly supplied at a flow rate of 5 ml / min or more. After the second inert gas atmosphere is created and the second inert gas atmosphere is created, the supply of the new inert gas at the above flow rate is continued. The time for performing this pressure reduction is about one minute or less. In addition, when a predetermined temperature (second temperature) of not less than the first temperature and not more than the liquidus temperature + 50 ° C. to 70 ° C. of the Pb-free Zn—Al solder alloy is reached, the temperature is set to the second temperature. Hold time. The holding time is preferably 3 minutes to 5 minutes.

  According to the bonding method of the second embodiment of the present invention, the first temperature above the liquidus temperature of the Pb-free Zn-Al based solder alloy is reached during heating up under the first inert gas atmosphere Since pressure reduction is sometimes performed to a vacuum state, the Pb-free Zn-Al based solder alloy melted in a liquid state is expanded by being reduced to a vacuum state. Thereafter, by stopping the pressure reduction, an inert gas (nitrogen gas) is newly supplied at a predetermined flow rate to the second inert gas atmosphere where the expansion of the Pb-free Zn-Al based solder alloy has been solved. By continuously supplying the new inert gas at a predetermined flow rate even after the second inert gas atmosphere is created, the diffusion of the newly supplied inert gas (nitrogen gas) is rapidly accelerated. As a result, the load is indirectly applied, and the oxide film on the surface of the Pb-free Zn-Al based solder alloy can be further destroyed. The flow rate of the newly supplied inert gas (nitrogen gas) at this time needs to be at least 5 ml / min or more.

  In addition, if the sample chamber is temporarily evacuated under reduced pressure during heating, small voids present in the Pb-free Zn-Al-based solder alloy can be suppressed, and the Pb-free Zn-Al-based solder can be suppressed. The wettability and spreadability of the alloy is improved.

  Furthermore, in the bonding method of the first and second embodiments of the present invention, as shown in Patent Document 2, a weight may be placed on a semiconductor element and a direct load may be applied. The jointability is further improved by using external pressure together.

In the bonding method described in Patent Document 2, the solder alloy has a shape smaller than that of the semiconductor element, and a load is applied to the entire natural oxide film surrounding the solder melt, thereby breaking the oxide film in a short time with high reproducibility. It is indicated that you can do it. However, if the solder alloy is smaller than the semiconductor element as in the bonding method of Patent Document 2, the alignment becomes difficult.
On the other hand, in the bonding methods according to the first and second embodiments of the present invention, the sizes of the Pb-free Zn-Al based solder alloy and the semiconductor element are not limited as long as there is a volume capable of maintaining the bondability. . Therefore, according to the bonding methods of the first and second embodiments of the present invention, it is possible to make the Pb-free Zn-Al based solder alloy larger than that of the semiconductor element, and the Pb-free Zn-Al based solder alloy Both the semiconductor element and the semiconductor element can be visually confirmed, and the workability such as alignment of the bonding point is improved.

  In addition, according to the bonding method of the first and second embodiments of the present invention, sufficient bondability and solderability are secured after solder bonding even when using a Ni-plated Cu substrate. it can. Therefore, as described in Patent Document 2, Ni plating is performed on the surface of the substrate being used, and plating for forming a highly wettable noble metal coating film (Au or Pd) or the like on the surface is performed. There is no need to perform sputtering and deposition. Moreover, in the technique described in Patent Document 1, plating, sputtering, and deposition of a metal film (Cu or Ag) that is easy to reduce is performed on the surface of the Zn-Al solder alloy, but it is not necessary. For this reason, according to the bonding methods of the first and second embodiments of the present invention, it is not necessary to coat expensive noble metals, the number of processes can be reduced accordingly, and cost can be reduced.

Hereinafter, examples of the present invention will be described.
In the bonding method of Example 1, a 5.0 mm square SiC element was prepared as a semiconductor element. As the substrate, a Cu substrate having a thickness of 3 μm was used. The Pb-free Zn-Al-based solder alloy used was a Zn-Al-Ge alloy having a melting point of 382 ° C, in which Al is 5 wt%, Ge is 0.1 wt%, and the balance is Zn. The size of the Zn-Al-Ge alloy was 6.0 mm square and the thickness was 0.1 mm.
Next, a bonding test was performed using a reduced pressure reflow apparatus. First, the solder alloy and the SiC element were placed on top of the substrate in this order in a superimposed state. These were mounted on the reflow stand of the sample chamber of a pressure reduction reflow apparatus. Thereafter, the atmosphere in the sample chamber was replaced with nitrogen gas to create a first inert gas atmosphere, and the oxygen concentration in the sample chamber was reduced to 200 ppm or less. Nitrogen gas at a small flow rate of about 1 to 2 ml / min was continuously supplied into the sample chamber, and the temperature was raised while maintaining the oxygen concentration of 200 ppm or less. The temperature was raised until 435 ° C. (second temperature) was reached, and after reaching 435 ° C., the temperature was kept at 435 ° C. for 5 minutes.
In addition, when reaching 400 ° C. (first temperature) in the middle of the temperature rise, the pressure reduction was started, and the pressure was reduced to a vacuum state of 200 Pa or less. After reaching a vacuum state of 200 Pa or less, the pressure reduction was stopped. It took about 1 minute during this period. At the same time as stopping the pressure reduction, nitrogen gas was newly supplied at a flow rate of 10 ml / min to create a second inert gas atmosphere. Nitrogen gas was continuously supplied at a flow rate of 10 ml / min even after the second inert gas atmosphere was created.
After holding at 435 ° C. for 5 minutes, temperature lowering of the sample chamber was started. The temperature was lowered to room temperature, the semiconductor element and the substrate joined through the Pb-free Zn-Al based solder alloy were taken out from the sample chamber, and the joining process was completed.

  In the bonding method of Example 2, when reaching 400 ° C. (first temperature) in the middle of the temperature rise, the pressure reduction was started, and the pressure was reduced to a vacuum state of 100 Pa or less. Further, the holding time at 435 ° C. after raising the temperature to 435 ° C. (second temperature) was set to 3 minutes. The other conditions were substantially the same as the bonding method of the first embodiment.

  In the bonding method of Example 3, when reaching 400 ° C. (first temperature) in the middle of the temperature rise, the pressure reduction is started, the pressure is reduced to a vacuum state of 200 Pa or less, and then the pressure reduction is stopped, At the same time, nitrogen gas was supplied at a flow rate of 5 ml / min. The other conditions were substantially the same as the bonding method of the first embodiment.

  In the bonding method of Example 4, the substrate, the solder alloy, and the SiC element were placed in order on the reflow stand of the sample chamber of the reduced pressure reflow apparatus, and a 20 g weight was placed thereon. . Further, the holding time at 435 ° C. after raising the temperature to 435 ° C. (second temperature) was set to 3 minutes. The other conditions were substantially the same as the bonding method of the first embodiment.

  In the bonding method of Example 5, the thickness of the Pb-free Zn-Al-based solder alloy is 0.2 mm. Further, the holding time at 435 ° C. after raising the temperature to 435 ° C. (second temperature) was set to 3 minutes. The other conditions were substantially the same as the bonding method of the first embodiment.

  In the bonding method of Example 6, the thickness of the Pb-free Zn-Al-based solder alloy is 0.2 mm. Other conditions were substantially the same as the bonding method of the fourth embodiment.

  In the bonding method of Example 7, a Zn-Al alloy having a melting point of 382 ° C., which comprises 5 wt% of Al and the balance of Zn, is used as the Pb-free Zn-Al based solder alloy. The other conditions were substantially the same as the bonding method of the first embodiment.

  In the bonding method of Example 8, the thickness of the Pb-free Zn-Al-based solder alloy is 0.2 mm. The other conditions were substantially the same as the bonding method of Example 7.

  In the bonding method of Example 9, the thickness of the Pb-free Zn-Al-based solder alloy is 0.2 mm, and when reaching 382 ° C. (first temperature) in the middle of the temperature raising, the pressure is not reduced and anew. Nitrogen gas was supplied at a flow rate of 5 ml / min to create a second inert gas atmosphere. The other conditions were substantially the same as the bonding method of the first embodiment.

  In the bonding method of Example 10, the flow rate of nitrogen gas newly supplied when reaching 382 ° C. (first temperature) was set to 10 ml / min during heating. Other conditions were substantially the same as the bonding method of Example 9.

  In the bonding method of Example 11, the flow rate of nitrogen gas newly supplied when reaching 382 ° C. (first temperature) was set to 20 ml / min during heating. Other conditions were substantially the same as the bonding method of Example 9.

  In the bonding method of Example 12, the flow rate of nitrogen gas newly supplied when reaching 382 ° C. (first temperature) was set to 30 ml / min during heating. Other conditions were substantially the same as the bonding method of Example 9.

  In the bonding method of Example 13, the thickness of the Pb-free Zn-Al-based solder alloy is 0.2 mm, and when the temperature reaches 382 ° C. (first temperature) in the middle of temperature rising, decompression starts and 200 Pa or less The vacuum was reduced to a vacuum state of 200 Pa. After reaching a vacuum state of 200 Pa or less, the vacuum was stopped. At the same time, nitrogen gas was newly supplied at a flow rate of 5 ml / min to create a second inert gas atmosphere. The other conditions were substantially the same as the bonding method of the first embodiment.

  In the bonding method of Example 14, the pressure was reduced to a vacuum state of 200 Pa or less, and after reaching a vacuum state of 200 Pa or less, the flow rate of nitrogen gas newly supplied was stopped at the same time as stopping the pressure reduction. The other conditions were substantially the same as the bonding method of Example 13.

  In the bonding method of Comparative Example 1, when the temperature reaches 400 ° C. (first temperature) during heating up, the pressure reduction to 200 Pa or less, the pressure reduction stop after reaching 200 Pa or less, the new pressure simultaneously with the pressure reduction stop. The temperature was held at 435 ° C. for 5 minutes when 435 ° C. (second temperature) was reached, without any supply of nitrogen gas at 10 ml / min. The other conditions were substantially the same as the bonding method of the first embodiment.

  In the bonding method of Comparative Example 2, the thickness of the Pb-free Zn-Al-based solder alloy was 0.2 mm. The other conditions were substantially the same as the bonding method of Comparative Example 1.

  In the bonding method of Comparative Example 3, the atmosphere in the sample chamber was replaced with nitrogen gas before the temperature rise to create a first inert gas atmosphere, and the oxygen concentration in the sample chamber was 200 ppm or less. Thereafter, the sample chamber was depressurized to 200 Pa or less. The other conditions were substantially the same as the bonding method of Comparative Example 1.

  In the bonding method of Comparative Example 4, the thickness of the Pb-free Zn-Al-based solder alloy was 0.2 mm. The other conditions were substantially the same as the bonding method of Comparative Example 3.

  In the bonding method of Comparative Example 5, when reaching 400 ° C. (first temperature) in the middle of temperature rising, the pressure reduction is started, the pressure is reduced to a vacuum state of 200 Pa or less, and after the vacuum state of 200 Pa or less is reached , The decompression was stopped. At this time, new nitrogen gas was not supplied, and a second inert gas atmosphere was not created. The other conditions were substantially the same as the bonding method of the first embodiment.

Evaluation Test of Bondability and Wettability Next, according to the bonding method of each of the above Examples and Comparative Examples, a Cu substrate subjected to Ni plating and a SiC element were bonded through a Pb-free Zn-Al based solder alloy. The samples were evaluated for bonding and wettability.

The bondability was evaluated by measurement using a bond tester. In this evaluation test, three samples in each of which a Cu substrate subjected to Ni plating and a SiC element were joined via a Pb-free Zn-Al based solder alloy used in the wettability evaluation test described later The shear strength was measured using each of them, and the average value of the measured values of the shear strength of three samples was calculated as the shear strength of the samples.
The specific shear strength measurement method is as follows. That is, in order to fix the joined body of each sample with the upper surface of the SiC element up on the work holder (the part to fix the sample) of the shear strength measuring instrument and apply a shear stress to the Pb-free Zn-Al based solder alloy A share tool was applied to the side of the SiC device. Then, load is applied to the shear tool from the side of the SiC element of the joined body by automatic measurement, load is applied until the Pb-free Zn-Al based solder alloy or the SiC element breaks, and the measured value of the load when broken Was taken as the shear strength of the sample.
Very good “○” when the shear strength is 19.6 MPa or more, “good” when it is 7.84 MPa or more and less than 19.6 MPa, “Δ”, defective when the SiC element is peeled or less “x” And

  The wettability of the solder is visually confirmed at the joint between the Pb-free Zn-Al solder alloy and the Ni-plated Cu substrate and at the joint between the Pb-free Zn-Al solder alloy and the SiC element in each sample. Rated. If there is no wetting at all, or if the solder is shrinking, a defect "x", if it spreads out from the four sides of the SiC element (the solder is thin and spreads out from the four sides of the SiC element) It evaluated as favorable "(circle)."

The bonding conditions and the evaluation results in the bonding method of each example and each comparative example are shown in Table 1.

As can be seen from Table 1, in the bonding methods of Examples 1 to 14, a bonded body having good bonding properties and wettability was obtained. In the bonding method of Examples 9 to 12, the oxygen concentration is replaced with nitrogen to create a first inert gas atmosphere, and the temperature is raised to 435 ° C. (second temperature) while being maintained at about 200 ppm, and then raised. When the melting point temperature of the Pb-free Zn-Al-based solder alloy reaches 382 ° C. (first temperature) during heating, nitrogen gas is newly supplied at a flow rate of 5 to 30 ml / min to perform second inactivation By forming a gas atmosphere and holding the temperature at 435 ° C. for 5 minutes after reaching a temperature of 435 ° C., it was possible to form a joined body having good bonding properties and wetting and spreading properties. In addition, according to the evaluation results of the bonding methods of Examples 9 to 12, when reaching 382 ° C. (first temperature) in the middle of the temperature raising, a new nitrogen gas is supplied without performing the pressure reduction and the second In the case of creating an inert gas atmosphere, it was found that by supplying nitrogen gas at 20 ml / min or more, the bondability becomes better.
As shown in the evaluation results of the bonding methods in Examples 1 to 3, 5, 7, 8, 13 and 14, the oxygen concentration in the sample chamber is replaced with nitrogen to create a first inert gas atmosphere, and approximately 200 ppm. The temperature is raised to 435 ° C. (second temperature) while maintaining the pressure, and when reaching 400 ° C. or 382 ° C. (first temperature) during the temperature rise, the pressure is reduced to 200 Pa or less and then reduced pressure The second inert gas atmosphere is created by supplying new nitrogen at a flow rate of 5 to 30 ml / min at the same time as stopping the reaction, and maintaining the temperature at 435 ° C for 5 minutes after reaching 435 ° C. A bonded body having good wettability and spreadability could be formed. In addition, in the joining method which newly supplies nitrogen and makes it into a 2nd inert gas atmosphere, after setting it as a vacuum state 200 Pa or less, evaluation of the joining method of Examples 1-3, 5, 7, 8, 13 As shown in the results, the bondability was good even when the flow rate of fresh nitrogen supplied to create the second inert gas atmosphere was 5 to 10 ml / min.
In addition, as shown in the evaluation results of the bonding methods of Examples 4 and 6, the wettability and the bondability are further improved by using a load on the SiC element in combination, and the temperature of 435 ° C. (the second temperature) The retention time in) could be shortened.

On the other hand, in the bonding method of Comparative Examples 1 and 2, the oxygen concentration in the sample chamber is replaced with nitrogen to form a first inert gas atmosphere, and the temperature is raised to 435 ° C. while being maintained at about 200 ppm. Although the second inert gas atmosphere was not created during heating, the temperature was maintained at 435 ° C. for 5 minutes after reaching 435 ° C. However, both the bonding property and the wettability were problematic in use.
In the bonding methods of Comparative Examples 3 and 4, the temperature in the sample chamber is reduced to a vacuum of 200 Pa or less before the temperature rise, and the temperature is raised to 435 ° C. while maintaining the oxygen concentration at about 200 ppm. A second inert gas atmosphere was not created by the supply of fresh nitrogen gas, and was maintained at 435 ° C. for 5 minutes after reaching 435 ° C. By reducing the pressure before raising the temperature, it was in a state close to a vacuum state, and the wettability and spreadability of the solder improved, but the bondability was poor.
In the bonding method of Comparative Example 5, the pressure is reduced to a vacuum state of 200 Pa or less when reaching 400 ° C. during heating up, and then the pressure reduction is stopped, but at the same time, the second by the supply of new nitrogen gas The inert gas atmosphere was not created, and was maintained for 5 minutes after reaching 435 ° C. However, the bonding property and the wettability were both bad as in the bonding methods of Comparative Examples 1 and 2, and there was a problem in use.

Reference Signs List 1 semiconductor element 2 substrate 3 solder alloy 11 ohmic contact 12 mounting electrode 20 ceramic substrate 21 metal plate 22 metal layer

Claims (6)

In a bonding method for bonding a semiconductor element and a substrate in which at least the main element on the surface is Ni, using a Pb-free Zn-Al based solder alloy containing Zn as a main component and containing no Pb.
The Pb-free Zn-Al based solder alloy and the semiconductor element are sequentially placed on the substrate in a superimposed state,
Raise the temperature under the first inert gas atmosphere;
During the temperature rise, when the first temperature above the liquidus temperature of the Pb-free Zn-Al based solder alloy is reached, an inert gas is newly supplied at a predetermined flow rate to produce a second inert gas atmosphere. Continue the supply of the new inert gas at the predetermined flow rate even after the second inert gas atmosphere is created,
A semiconductor device using a Pb-free Zn-Al based solder alloy, characterized in that when the second temperature is reached, the second temperature is maintained for a predetermined time, and the semiconductor device and the substrate are joined. How to bond to the substrate. In a bonding method for bonding a semiconductor element and a substrate in which at least the main element on the surface is Ni, using a Pb-free Zn-Al based solder alloy containing Zn as a main component and containing no Pb.
The Pb-free Zn-Al based solder alloy and the semiconductor element are sequentially placed on the substrate in a superimposed state,
Raise the temperature under the first inert gas atmosphere;
When the temperature reaches a first temperature above the liquidus temperature of the Pb-free Zn-Al based solder alloy in the middle of the temperature rise, the pressure reduction is started and the pressure reduction is stopped after reducing the pressure to a vacuum state of 200 Pa or less. The inert gas is newly supplied at a predetermined flow rate simultaneously with the stop of the pressure reduction to create a second inert gas atmosphere, and the second inert gas atmosphere is created, and the predetermined amount of the new inert gas is maintained. Continue supply at flow rate,
A semiconductor device using a Pb-free Zn-Al based solder alloy, characterized in that when the second temperature is reached, the second temperature is maintained for a predetermined time, and the semiconductor device and the substrate are joined. How to bond to the substrate. Raising the temperature under the first inert gas atmosphere having an oxygen concentration of 200 ppm or less,
When the temperature reaches a first temperature above the liquidus temperature of the Pb-free Zn-Al based solder alloy in the middle of the temperature rise, the pressure reduction is started and the pressure reduction is stopped after reducing the pressure to a vacuum state of 100 Pa or less. 3. The Pb-free Zn—Al system according to claim 2, wherein the supply of the new inert gas at the predetermined flow rate for creating the second inert gas atmosphere is performed simultaneously with the termination of the pressure reduction. A method of bonding a semiconductor element and a substrate using a solder alloy.   The method of bonding a semiconductor element and a substrate using a Pb-free Zn-Al based solder alloy according to claim 2 or 3, wherein the time for performing the pressure reduction is within about 1 minute.   The Pb-free Zn-Al-based solder alloy according to claim 1 or 2, wherein the first temperature is a predetermined temperature not less than the liquidus temperature of the Pb-free Zn-Al-based solder alloy and 400 ° C or less. Method of bonding a semiconductor element to a substrate using   The Pb-free Zn-Al based solder alloy according to claim 1 or 2, characterized in that the new inert gas for creating the second inert gas atmosphere is supplied at a flow rate of 5 ml / min or more. The bonding method of the used semiconductor element and a substrate.

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