Semiconductor Module

Description

Technical Field

The invention relates to the field of power electronics and more particularly to a method for producing a controlled punch-through semiconductor device according to the preamble of claim 1 and to a controlled-punch-through semiconductor device according to the preamble of claim 15.

Background Art

In US 6,482,681 B1 a punch-through (PT) insulated gate bipolar transistor

(IGBT) is described. Such a PT-IGBT is also schematically shown in fig. 1. The device is produced by using an n doped wafer, on top of which all processes for manufacturing layers on the emitter side 31 , also called cathode side are finished, i.e. all junctions and metallizations on the emitter side 31 are produced. Afterwards, the wafer is thinned and hydrogen ions are implanted on the collector side 21 of the wafer, also called anode side for forming an n+ doped buffer layer 15. Then p type particles are implanted for forming a collector layer 6. The wafer is then annealed at 300 to 400 ⁰ C in order to activate the hydrogen ions without damage to the structure on the emitter side 31. The buffer layer 15 can also be formed by multiple hydrogen implants of progressively shallower and progressively higher total dose in order to form one buffer layer 15 with increasing doping

concentration towards the collector and a peak dose concentration close to the collector. Due to the continuously rising doping concentration in the buffer layer 15 the reduction of the electric field during operation of the device increases within the layer. Thus, the buffer layer 15 serves, in the blocking case, for abruptly decelerating the electric field (shown in fig. 1 by the dotted line) before the collector and thus keeping it away from said collector, since the semiconductor element can be destroyed if the electric field reaches the collector.

DE 198 29 614 discloses a fabrication method for a soft punch-through power semiconductor element based on a PT type which makes it possible to fabricate relatively thin semiconductor elements without having to employ the epitaxy method. For this purpose, a buffer layer having a greater thickness than electrically necessary is introduced into a lightly doped wafer, process steps for embodying a cathode patterned surface of the semiconductor element are then carried out and only afterward the thickness of the buffer layer is reduced to the electrically required size by grinding or polishing. Thus, it is possible to carry out the cathode process steps on a relatively thick wafer, thereby reducing the risk of breaking. Nevertheless, by virtue of the subsequent thinning of the wafer, a semiconductor element having the desired small thickness can be produced. The minimum thickness of the finished semiconductor elements is no longer limited by a minimum thickness that can be achieved for its starting material. What is advantageous, moreover, is that the doping of the residual stop layer is relatively low, so that the emitter efficiency can be set by way of the doping of the collector.

JP 2004 193212 relates to a PT-IGBT with two buffer layers which are separated by a layer which has the same doping density as the base layer. The deeper buffer layer has a lower peak doping concentration than the shallow buffer layer. Such a device has a high leakage current and a low breakthrough voltage.

Disclosure of Invention

It is an object of the invention to provide a method for producing a soft controlled punch-through semiconductor device with improved electrical properties, which device is as thin as possible and easy to produce. This object is achieved by the method for producing a semiconductor module according to claim 1 and the device according to claim 15.

With the inventive method a controlled-punch-through semiconductor device with a four-layer structure comprising layers of different conductivity types and having a collector on a collector side and an emitter on an emitter side, which lies opposite the collector side, is produced. The steps for producing the semiconductor device are performed in the following order:

- on a wafer of a first conductivity type, which comprises a first side, which is the emitter side in the finalized semiconductor device, and a second side lying opposite the first side, steps for producing layers on the emitter side of the semiconductor device are performed,

- then the wafer is thinned on the second side,

- then particles of the first conductivity type are applied to the wafer on its second side by implantation or deposition of the particles of the first conductivity type, which particles form a first buffer layer in the finalized semiconductor device, the first buffer layer having a first peak doping concentration in a first depth, which first peak doping concentration is higher than the doping of the wafer,

- then particles of a second conductivity type are applied to the wafer on its second side by implantation or deposition, which particles form a collector layer in the finalized semiconductor device, and

- then an emitter metallization is formed on the second side.

At any stage particles of the first conductivity type are applied to the wafer on its second side by implantation of the particles, which particles form a second buffer layer in the finalized semiconductor device. The second buffer layer has in a second depth a second peak doping concentration, which is lower than the

first peak doping concentration of the first buffer layer and which is higher than the doping of the wafer.

Between the second depth of the second buffer layer and the first depth of the first buffer layer a third buffer layer is created with a minimum doping concentration, which is higher than the doping of the wafer and lower than the second and first peak doping concentration of the second and the first buffer layer. The minimum doping concentration of the third buffer layer is defined as the point of the lowest doping concentration of the third buffer layer, i.e. the point of the lowest doping concentration between the first and the second buffer layer.

Furthermore, at any stage after applying the particles for the first buffer layer, the second buffer layer and/or the collector layer to the wafer at least one wafer thermal treatment for forming the first buffer layer, the second buffer layer and/or the collector layer in the finalized semiconductor device is performed.

In the inventive semiconductor device, the electric field is stopped in the second buffer layer. In the third buffer layer, the bipolar transistor current gain of the device, being defined as the ratio of the collector and base current, is reduced. The first buffer layer further reduces the bipolar current gain and makes the device insensitive to variations of the depth of the second buffer layer. By having a device, which is less sensitive to the position of the second buffer layer, manufacturing of such a device is simplified and the switching properties and electrical properties of the device are improved. Furthermore, by having the inventive three buffer layer structure, the buffer layers can be made thinner, resulting in much lower losses than in the prior art devices.

Further preferred embodiments of the inventive subject matter are disclosed in the dependent claims.

Brief Description of Drawings

The subject matter of the invention will be explained in more detail in the following text with reference to the attached drawings, in which:

FIG 1 shows the doping profile of a semiconductor module for a typical prior art punch-through semiconductor device according to the invention;

FIG 2 shows the doping profile of a semiconductor module for a semiconductor device according to the invention;

FIG 3 shows the doping profile of Fig. 2 schematically in more details; and

FIG 4: shows a cross sectional view on semiconductor module according to the invention.

The reference symbols used in the figures and their meaning are summarized in the list of reference symbols. Generally, alike or alike- functioning parts are given the same reference symbols. The described embodiments are meant as examples and shall not confine the invention.

Modes for Carrying out the Invention

As shown in fig. 4 the semiconductor module according to the invention is a controlled-punch-through semiconductor device 1 with a four-layer structure, typically an IGBT, having a collector 2 on a collector side 21 and an emitter 3 on an emitter side 31 , which lies opposite the collector side 21.

A base layer 4 is located between the emitter 3 and collector 2. A second buffer layer 8 lies on the collector side 21 between the base layer 4 and the collector 2. This buffer layer 8 has in a second depth 81 a second peak doping concentration 82, which is higher than the base layer 4. In the second buffer layer 8, an electric field is stopped during operation of the device 1.

A first buffer layer 5 lies between the second buffer layer 8 and the collector 2. As schematically shown in Fig. 3 this first buffer layer 5 has in a first

depth 51 , which depth is smaller than the second depth 81 , a first peak doping concentration 52, which is higher than the second peak doping concentration 82 of the second buffer layer 8. The first and second buffer layer 5, 8 are formed in such a way that a third buffer layer 9 is formed between the second depth 81 and the first depth 51 with a doping concentration, which is lower than the first and second peak doping concentrations 51 , 81 of the first and second buffer layer.

The third buffer layer 9 has a minimum doping concentration 92, which is higher than the wafer doping, preferably at least 1 * 10 ¹³ cm ^"3 and in particular 2 * 10 ¹⁴ cm ^"3 , and in particular the minimum doping concentration 92 is in a range of 5 * 10 ¹⁴ cm ^"3 up to 7 * 10 ¹⁴ cm ^"3 . The first peak doping concentration 52 of the first buffer layer 5 lies in a first depth 51 between 0 to 2 μm, in particular 0.8 to 1.2 μm and in particular 0.9 to 1.1 μm. The second peak doping concentration 82 of the second buffer layer 8 lies in a second depth 81 between 3 to 10 μm, in particular 5 to 8 μm. The distance 91 between the second depth 81 and the first depth 51 , the depths of the second and first peak doping concentration 81 , 51 of the second and first buffer layer is in a range between 1 μm and 10 μm and in particular 4 μm and 7 μm.

For producing the inventive controlled-punch-through semiconductor device 1 steps for producing the semiconductor device being performed in the following order: - on an (n-) doped wafer, which comprises a first side, which is the emitter side 31 in the finalized semiconductor device 1 , and a second side lying opposite the first side, steps for producing layers on the emitter side 31 of the semiconductor device 1 are performed. Such layers are typically a p doped channel region 10, which surrounds an n doped source region 11. These regions 10, 11 are in electrical contact with an emitter electrode 12. A gate electrode 13 is manufactured on

top of the wafer, electrically insulated from the layers by an insulation layer 14. After performing such steps on the emitter side 31 ;

- the wafer is thinned on its second side, which is the collector side 21 in the finalized semiconductor device 1 , in particular it is thinned to a minimum wafer thickness during manufacturing;

- afterwards n type particles are either implanted or deposited on the wafer on its second side. These particles will form a first buffer layer 5 in the finalized semiconductor device 1. The first buffer layer 5 has a first peak doping concentration 52 in a first depth 51 , which first peak doping concentration 52 is higher than the doping of the wafer;

- then p-type particles are either implanted or deposited on the wafer on the collector side 21 , which particles form a collector layer 6 in the finalized semiconductor device 1 ; and

- then a collector metallization 7 is formed on the second side. - At any stage n type particles are either implanted or deposited on the wafer on the collector side 21 , which particles form a second buffer layer 8 in the finalized semiconductor device 1. The second buffer layer 8 has in a second depth 81 , which depth is greater than the first depth 51 , a second peak doping concentration 82, which is lower than the first peak doping concentration 52 of the first buffer layer 5 and which is higher than the doping of the wafer. The second buffer layer 8 is arranged in such a depth 81 and comprises such a doping that an electric field can be stopped during operation of the finalized semiconductor device. By introducing the second buffer layer 8 there is a third buffer layer 9 arranged between the second depth 81 of the second buffer layer 8 and the first depth 51 of the first buffer layer 5. This third buffer layer 9 has a minimum doping concentration 92, which is lower than the first and second peak doping concentration 51 , 81 of the first and second buffer layer 5, 8. - At any stage after applying the particles for the first buffer layer 5, the second buffer layer 8 and/or the collector layer 6 to the wafer at least

one wafer thermal treatment for finalizing the first buffer layer 5, the second buffer layer 8 and/or the collector layer 6 is performed. The second peak doping concentration 82 of the second buffer layer 8 in the finalized semiconductor device 1 lies in a second depth 81 between 3 to 10 μm and in particular 5 to 8 μm. The distance 91 between the first depth 51 of the first peak doping concentration 52 of the first buffer layer 5 and the second depth 81 is in a range between 1 μm and 10 μm and in particular between 4 μm and 7 μm.

In a preferred embodiment the thermal treatment for any of the first buffer layer 5, second buffer layer 8 or collector layer 6 or for two or even all layers together is performed at a temperature in a range between 350 up to 550 ⁰ C and preferably between 400 ⁰ C up to 500 ⁰ C. That means that the thermal treatment can be done for one layer after the other or the thermal treatment is done simultaneously for two or all layers.

The particles for forming the first buffer layer 5 are implanted at an energy in a range between 50 keV up to 600 keV. These particles are applied in a dose range between 1 ^* 10 ¹² cm ^"2 up to 1 ^* 10 ¹⁶ cm ^"2 . Preferably, phosphorous, arsenic or antimony are used as particles for forming the first buffer layer 5. In another preferred embodiment, the particles for forming the first buffer layer 5 are driven into the wafer by the thermal treatment up to a depth of 0.5 up to 5 μm and/or the particles have a maximum doping level of 5 ^* 10 ¹⁵ cm ^"3 up to 1 ^* 10 ¹⁷ cm ^"3 , in particular 1 ^* 10 ¹⁶ cm ^"3 up to 5 ^* 10 ¹⁶ cm ^"3 . The first peak doping concentration 52 of the first buffer layer 5 in the finalized semiconductor device lies in a first depth 51 between 0 to 2 μm, in particular 0.8 to 1.2 μm and in particular 0.9 to 1.1 μm.

In another preferred embodiment, the particles for forming the first buffer layer 5 are applied to the second side of the wafer by depositing an amorphous silicon layer. The silicon layer preferably has a depth in a range

between 0.5 μm up to 10 μm and/or a doping concentration above 1 * 1015 cm ^"3 .

In another preferred embodiment, the particles for forming the collector layer 6 are implanted with a maximum doping level of 1 ^* 10 ¹³ cm ^"3 up to 1 10 ¹⁶ cm ^"3 . Preferably, boron particles are used. These particles can be activated by a thermal treatment like low temperature activation or laser annealing.

Alternatively, the particles for forming the collector layer 6 are applied to the second side of the wafer by depositing an amorphous silicon layer from a silicon source, which is pre-doped with boron. The silicon layer has a depth in a range between 5 nm up to 1 μm and/or a doping concentration above 1 ^* 10 ¹⁵ cm ^"3 . Using the method of depositing the layer 6 has the advantage that the particles do not penetrate into the region, in which the particles for the first buffer layer 5 are, so that the full thickness of the first buffer layer 5 is maintained.

In a further preferred embodiment, the particles for forming the second buffer layer 8 are protons, which are implanted, in particular in a dose range between 1 * 10 ¹² cm ^"2 up to 1 * 10 ¹⁵ cm ^"2 and/or an energy in a range between 200 keV up to 1000 keV. The particles for forming the second buffer layer 8 are driven into the wafer by the thermal treatment up to a depth of 2 up to 15 μm, in particular 2 μm up to 8 μm and in particular 2 μm up to 5 μm. The particles have a maximum doping level of 1 ^* 10 ¹⁵ cm ^"3 up to 5 ^* 10 ¹⁶ cm ^"3 and in particular 5 ^* 10 ¹⁵ cm ^"3 up to 5 ^* 10 ¹⁶ cm ^"3 . Preferably helium, deuterium, phosphorous, arsenic or antimony are used as particles for forming the second buffer layer 8. The particles are implanted at an energy of more than 1 MeV. It is advantageous to use phosphorous, because phosphorous has a comparably small particle size so that the particles can deeply penetrate into the wafer. Furthermore, it is possible to have phosphorous not only single charged, but double, triple or even more

heavily charged, which makes it possible to achieve deeper lying layers. There are also no special precautions necessary when applying phosphorous to the wafer. As both first and second buffer layer 5, 8 can be made with phosphorous only one kind of particle material is needed, thus simplifying the manufacturing of the device.

Fig. 8 shows the doping concentration of the base layer 4 and the buffer layers 8, 9, 5 through a cut along the line A - A in fig. 4. The minimum value for the doping concentration 92 of the third buffer layer 9 in Fig. 8 is obtained by the implantation of n-type species for the first and second buffer layers 8 and 5 followed by a low temperature thermal anneal (300 - 550)°C . The lowest doping concentration point 92 in the third buffer layer 9 of 5 ^* 10 ¹⁴ cm ^"3 is achieved due to the overlapping of the n-type dopings of the first and second buffer layers 8 and 5 as shown in Fig. 8.

Fig. 5 shows in comparison semiconductor devices with a first buffer and a second buffer. In the figure there is shown the influence of the minimum doping concentration of the third buffer layer. The dotted line 93 shows the case of a semiconductor device which has no third buffer layer, i.e. in case that there is a layer between the first and second buffer layer which has the same dose concentration as the base layer 4 or wafer, respectively as is known e.g. from JP 2004 193212. The continuous line 95 shows the case of a semiconductor device, in which there is no drop in doping concentration between the first buffer layer 5 and the peak of the second buffer layer 82. The dashed line 94 shows the case on an inventive semiconductor device, in which the third buffer layer 9 has a minimum doping concentration 92, which is higher than the doping of the base layer 4 and lower than the second peak doping concentration 82 of the second buffer layer.

Figure 6 shows a graphics of voltage versus current of the device. The current Is is the leakage current and V _br is the breakthrough voltage. In Fig. 7, short

circuit behaviour of the devices is shown for a line voltage of 900 V and a gate emitter voltage V _ge = 20 V at room temperature (300 K).

The device, in which the layer between the first and second buffer layer has the same doping concentration as the base layer 4 (dotted line 93), has a good short circuit behaviour (see fig. 7), but it has a high leakage current of 8.8 A and a low breakdown voltage of 1152 V.

For the device without a drop in doping concentration between the first buffer layer 5 and the peak of the second buffer layer 82 (continuous line 95; the peak dose concentration 82 of the second buffer layer and therefore, in this case also the minimum doping concentration of the third buffer layer 9 being 1 * 10 ¹⁶ cm ^"3 ) the leakage current is low (3.2 A) and the breakthrough voltage is higher (1200 V). However, as shown in Fig. 7, for such a device the electrical field is close to flip from the anode to the cathode, especially if the gate emitter voltage is slightly increased resulting in a failure of the device.

The inventive semiconductor device with a third, intermediate buffer layer 9, has good short circuit behaviour, i.e. the electric field as shown in Fig. 7 can be kept low, while the leakage current (6.7 V) is also low and the breakdown voltage is enhanced (1162 V) compared to the device with no intermediate buffer layer (Fig. 6).

In the preceding description n doped layers have been used as an example of a layers of a first conductivity type and p doped layers as an example for layers of a second conductivity type, but the layers can also be of the opposite conductivity type.

Reference list

1 semiconductor device

2 collector

21 collector side

3 emitter

31 emitter side

4 base layer

5 first buffer layer

51 first depth

52 first peak doping concentration

6 collector layer

7 collector metallization

8 second buffer layer

81 second depth

82 second peak doping concentration

9 third buffer layer

91 distance

92 minimum doping concentration

10 channel region

11 source region

12 emitter electrode

13 gate electrode

14 insulation layer

15 buffer layer