1. Field of the Invention The present invention relates to a semiconductor device having at least one semiconductor element disposed on a substrate, and a method of manufacturing the semiconductor device. 2. Description of the Related Art In recent years, in a power conversion apparatus for electric vehicles, industrial equipment and the like, semiconductor elements are used as driving elements such as IGBTs (Insulated Gate Bipolar Transistors) and MOSFETs (Metal Oxide Semiconductor Field-Effect Transistors). By using the semiconductor elements as such driving elements, driving of a motor and the like can be performed by a comparatively small current. Therefore, an excellent heat radiation performance is required of the semiconductor device for driving the motor. A high-density and high-thermal conductivity substrate is generally used for realizing such high heat radiation performance. In addition, in order to achieve an even better heat radiation performance, heat has to be transferred in an appropriate manner between the semiconductor elements that are disposed on the substrate and the substrate, and in that case, a heat spreader is sometimes disposed between the semiconductor elements and the substrate. Examples of a manufacturing method of a conventional semiconductor device for driving a motor include one disclosed in Japanese Laid-Open Patent Publication No. 2011-229082 (Patent Document 1). In Patent Document 1, a metal substrate having a thickness of about 2 mm is used as a heat radiation material, and then, a semiconductor chip is flip-chip connected to the metal substrate via solder bumps, and further, a heat spreader made of copper is disposed over the metal substrate. In addition, although there is no detailed description about it, this semiconductor device also uses the metal substrate as a substrate on which semiconductor elements are disposed. In the conventional semiconductor device for driving the motor, in addition to flip-chip bonding between the semiconductor chip and the substrate via the solder bumps, wire bonding between the semiconductor elements and the substrate is also possible. Further, when a plurality of semiconductor elements are disposed on the substrate, wire bonding can be performed also between the semiconductor elements and the substrate via wires. Moreover, when a heat spreader is disposed between the semiconductor elements and the substrate, the semiconductor elements and the substrate may be bonded via thermosetting resin or the like. When such wire bonding or the like is performed, the semiconductor elements may not be disposed at desired positions. Furthermore, for example, when a through hole or a via hole is formed on the substrate and a lead wire is passed through the through hole or via hole, semiconductor elements and bonding wires may interfere with each other in some cases. In such a case, also the semiconductor elements cannot be disposed at desired positions. In addition, when soldering or brazing is performed between an IC (Integrated Circuit) and a metal substrate, or when thermal diffusion is performed by high-temperature heating, a high-temperature thermal stress is applied to the semiconductor elements, and as a result, an adverse influence is exerted on the operation of the semiconductor elements, and/or characteristics of the semiconductor elements are deteriorated. In order to solve the aforementioned problems, in Japanese Laid-Open Patent Publication No. 2013-141244 (Patent Document 2), for example, the following has been proposed: that is, a plurality of semiconductor chips are flip-chip connected to a carrier substrate via bumps, and further, by disposing the carrier substrate on which the semiconductor chips are flip-chip connected in a face-down state, a heat spreader is provided between the carrier substrate and the semiconductor chips, and thereby, heat is efficiently transferred to the carrier substrate and a heat radiation performance is improved. Further, an inner connection unit is formed by flip-chip bonding a heat transfer wiring on the carrier substrate to the heat spreader via the bumps, and thereby, heat radiation performance of the semiconductor chips is improved, and heat is transferred from the heat transfer wiring, which is disposed in a face-down state on the carrier substrate, to the heat spreader, and thereby, heat radiation performance of the semiconductor chips is improved. Japanese Laid-Open Patent Publication No. 2014-4903 (Patent Document 3) discloses a semiconductor device in which a plurality of semiconductor elements are flip-chip connected via solder bumps to a printed substrate, and further, an underfill resin is interposed between the semiconductor elements and the printed substrate. In addition, in Patent Document 3, the printed substrate, which is used in the semiconductor device, is obtained by disposing a metal plate on one face of a build-up substrate. However, even though the semiconductor elements are flip-chip connected to the substrate via the solder bumps, when wire bonding or the like is performed, the semiconductor elements cannot be disposed at desired positions as has been described above. Moreover, when the thermal diffusion is performed by high-temperature heating, the characteristics of the semiconductor elements are deteriorated. In addition, in the case where a printed substrate is used as a substrate and the carrier substrate is disposed on the back side of the printed substrate, a distance from the semiconductor elements to the carrier substrate via the printed substrate is long and there is a concern that heat transfer performance may be insufficient. Therefore, in this case, it is difficult to maintain a sufficient heat transfer performance. In addition, in this case, in order to provide sufficient heat transfer between the printed substrate and the carrier substrate via the solder bumps, for example, solder bumps having an increased solder area are formed, and thus, heat transfer performance of the printed substrate or the like is improved. However, a large amount of solder is required in this case. Furthermore, in the case where the printed substrate is used as the substrate, heat transfer performance of the semiconductor elements becomes insufficient, for example, when the semiconductor elements generate heat. On the other hand, in Patent Document 3, a heat radiation member made of aluminum having high heat conductivity is provided in the heat radiation path between the printed substrate and the carrier substrate. However, the heat radiation member may fail to improve the heat transfer performance to a sufficient extent in the case where the printed substrate is used as the substrate and where heat is generated from the semiconductor elements. In this case, it is difficult to maintain a sufficient heat transfer performance. Therefore, there is a possibility that the characteristics of the semiconductor elements are deteriorated and a breakdown thereof is caused. In addition, as has been described above, in the case where the printed substrate is used as the substrate and the carrier substrate is disposed on the back side of the printed substrate, a distance from the semiconductor elements to the carrier substrate via the printed substrate is long and there is a possibility that heat transfer performance may be insufficient.