JP2503628B2 - Method for manufacturing bipolar transistor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for manufacturing a bipolar transistor using molecular beam growth, and more particularly to a method for producing a polysilicon emitter.

(Prior Art) In recent years, polysilicon has been used as an emitter material for NPN bipolar transistors. Haren is an IEE transaction on electron device ED-32, page 1307 (P.Halen and DL
Pulfrey, IEEE Transaction on Electron Devices, ED-3
2 (1985) 1307.), it is known that the use of a polysilicon emitter improves the current amplification factor by up to 20 times compared to the case of using a single crystal emitter.
This is Patton's IEE Transaction on Electron Device ED-33, page 1754 (GLPatton, JCBravman and JDPlummer, IEEE).
Transaction on Electron Devices, ED−33 (1986) 175
As described in 4.), when depositing polysilicon by vapor phase epitaxy (CVD), a natural oxide film on the surface of the single crystal base is buried in the interface between the single crystal base and the polysilicon emitter, and It is believed that this is because the thin oxide film acts as a hole barrier. But,
The tunneling probability of the barrier of the oxide film against electrons and holes greatly depends on the oxide film thickness. For example, green is solid. State. Electronics Volume 17 Page 551 (MAGreen, FDKing and J.Shewchun, Solid State
As described in Electron., 17 (1974) 551.), if the oxide film thickness changes by 2Å, the tunneling current changes by an order of magnitude. Therefore, in order to use an oxide film as a hole barrier of a bipolar transistor, the film thickness is controlled to 2 Å or less in a thickness region of about a natural oxide film, which does not work as a barrier for electrons but for holes. It must be a barrier. At present, such precise control is difficult, and there is a problem that the current amplification factor greatly changes due to differences in the method of cleaning the substrate before depositing polysilicon.

Further, in recent years, for the purpose of application to high-speed bipolar devices, microwave devices, superlattice structure devices, etc., growth has been performed at a lower temperature than that of conventional silicon thin film growth techniques, and growth at the atomic layer level has been achieved. Silicon molecular beam growth (SiMBE) technology in a high vacuum, which has the characteristic of being controllable, is being actively researched and developed. In this MBE technique, it is possible to form a thin SiO ₂ film whose film thickness is extremely precisely controlled by simultaneously irradiating an oxygen molecular beam and a silicon molecular beam. A similar film can be formed by low-pressure thermal oxidation in which a heated substrate is irradiated with oxygen. However, by these methods, a SiO ₂ film with a film thickness equal to or smaller than that of the natural oxide film was precisely controlled was formed on the base layer, and then a polysilicon emitter was formed by the conventional method of CVD. In this case, when the wafer is taken out of the MBE chamber into the atmosphere, the oxidation on the base layer progresses, and since it is exposed to high temperature and high pressure in the CVD device, there is no difference compared to the case where a natural oxide film is used. There is a problem that it ends up.

(Problems to be Solved by the Invention) An object of the present invention is to eliminate the above-mentioned conventional defects and to form a hole barrier of an extremely thin silicon oxide film whose thickness is precisely controlled at the interface with the base layer. It is to provide a method of manufacturing a polysilicon emitter provided in.

(Means for Solving Problems) The present invention forms a silicon oxide film by cleaning an exposed substrate surface of a base layer and then simultaneously irradiating the surface of the base layer with a silicon molecular beam and an oxygen molecular beam. And the second step of forming polysilicon by irradiating a substrate including a silicon oxide film with a silicon molecular beam in the same vacuum chamber or in a vacuum chamber connected by a vacuum. A method for manufacturing a bipolar transistor characterized by performing a method, and forming a silicon oxide film on the surface of the base layer by low pressure thermal oxidation of irradiating oxygen while heating the substrate after cleaning the surface of the substrate having the base layer. In the same vacuum chamber, the first step and the second step of forming polysilicon by irradiating a substrate including a silicon oxide film with a silicon molecular beam Alternatively, the present invention provides a method for manufacturing a bipolar transistor, which is characterized in that it is continuously performed in a vacuum chamber connected by a vacuum.

(Operation) First, the principle of the present invention will be described. Single crystal Si as the base layer introduced in the ultra-high vacuum chamber in FIG.
A natural oxide film 11 exists on 12 (a). When this is heated in an ultrahigh vacuum, the natural oxide film 11 evaporates and a clean surface of Si appears (b). When Si molecular beam and O ₂ molecular beam are simultaneously vapor-deposited on this clean surface or low-pressure thermal oxidation is carried out to send oxygen to a heated substrate, extremely thin SiO ₂ 13 is formed (c). If it is taken out from the vacuum chamber into the atmosphere in this state, oxidation on the surface of the base proceeds and the oxide film thickness becomes the same as the natural oxide film. Therefore, the inventor, without taking out from the vacuum chamber,
If the Si molecular beam is continuously irradiated after exhausting O ₂ , polysilicon 14 is formed on this SiO ₂ film in the form of embedding a thin SiO ₂ film as shown in (d), and this oxide film is formed. Found for the first time that they act as a barrier to the hall. Once filled, there is no change in SiO ₂ even when exposed to the atmosphere, and an emitter is formed if As diffusion is performed in the upper polysilicon layer. Since the effective mass for electrons of the thin SiO ₂ barrier between the emitter and the base formed by the above method is smaller than the effective mass for holes, it is possible to suppress holes through only electrons if the barrier thickness is appropriately selected. It can be done. The step of forming SiO ₂ and the step of forming polysilicon can be performed in different vacuum chambers, but in this case, both vacuum chambers are connected by a vacuum tunnel so that the substrate does not come into contact with the atmosphere and oxidize. I have to

(Example) Next, the Example of this invention is described concretely. First, formation of an extremely thin SiO ₂ film embedded in Si will be described. The MB is equipped with a 40cc electron gun Si vaporizer.
E equipment was used. A 4-inch n-type and p-type Si (100) 0.01 to 0.02 Ωcm substrate was used as a sample wafer. The sample wafer is transferred to the forming chamber after the normal RCA cleaning and then a
After -Si was deposited, a cleaning treatment was performed at 800 ° C for 1 minute to expose a clean surface, and an epitaxial layer as a buffer layer was grown at 3000 Å at a growth temperature of 500 ° C. After lowering the substrate temperature to room temperature, oxygen with a purity of 99.9999% leaked from the nozzle into the formation chamber, and the electron deposition type Si vaporizer converted the SiO ₂ formation rate to 0.55.
An oxide film was formed on the clean surface by irradiating Å / s Si molecular beam.
The oxygen partial pressure is 5 × 10 ^-5 Torr, and the formed film thickness is 2.5Å to 60
I changed it to Å. Furthermore, the oxygen leaked into the growth chamber was exhausted, the substrate temperature was raised to 500 ° C again, and the degree of vacuum was set to 1 × 10
^-9 Torr atmosphere, S on the oxide film at a growth rate of 1.0Å / s
After i was deposited, the change in crystallinity of the upper Si layer depending on the oxide film thickness of the lower layer was investigated by RHEED.

The change in RHEED pattern was observed when Si was grown again after growing 5Å SiO ₂ on the (100) clean surface.
After superlattice pattern indicating a clean surface is SiO ₂ growth,
It completely disappeared and became a 1 × 1 pattern. The upper Si thickness is 70
At Å, the RHEED pattern became a pattern peculiar to three-dimensional epitaxial growth, and it was found that the upper layer Si was epitaxially grown. The growth mode is island growth instead of layered growth in homoepitaxial growth. Furthermore, when the upper layer Si is 500 Å or more, it becomes 2 × 1 again
It was found that the two-dimensional pattern appeared and the epi islands coalesced into a flat surface again. On the other hand, on a clean surface
It was found that when Si was grown again after growing SiO ₂ at 7.5 Å, the RHEED pattern of the upper Si layer became a ring pattern and the upper Si layer became polycrystalline. SiO ₂ film thickness
Above 7.5 Å, the upper Si layer was all polycrystalline.

Moreover, the formation temperature of the upper layer Si must be 100 ° C. or higher. At 100 ° C or lower, the upper layer Si does not become polycrystalline but becomes amorphous. Further, when the formation temperature of the upper layer Si becomes 860 ° C. or higher, the irradiated Si reacts with SiO _{2 of the} substrate to become SiO and evaporates.

A similar phenomenon was found in low pressure thermal oxidation in which a heated Si substrate was irradiated with only O ₂ molecules. When the Si (100) clean surface is oxidized at a substrate temperature of 500 ° C and an oxygen partial pressure of 5 × 10 ^-5 Torr for 20 minutes, and then Si is grown again, the RHEED pattern becomes a pattern peculiar to three-dimensional epitaxial growth, and the upper layer Si
It was found that the film had grown epitaxially. However, when the oxidation time exceeds 30 minutes, RHEE of the upper layer Si
The D pattern was a ring pattern, and the upper layer was Si polycrystal. FIG. 3 shows the oxidation time dependence of the AES oxygen peak measured at the stage after the low pressure thermal oxidation and before the formation of the upper layer Si. As can be seen from this figure, the oxygen peak was saturated and there was no significant change in the oxygen peak at the point where the epitaxial growth changed to polysilicon formation. This indicates that the oxygen concentration on the surface is saturated, and it is considered that the upper layer Si becomes polycrystalline when the thickness of the oxide film formed by low pressure thermal oxidation exceeds a certain value.

As described above, SiO ₂
When is thin, the epitaxial information is transmitted on SiO ₂ . There are three possible explanations for this. First is thin
If there is a pinhole in the SiO ₂ film and the epitaxial information is transmitted through this pinhole, the second is that the Si and SiO ₂ react at the initial stage of growth of the upper layer Si and the thin SiO ₂ film nucleates to form the lower layer Si.
Is exposed and epitaxial information is transmitted, the third
Is the case where thin SiO ₂ has some epitaxial information. However, since the epitaxial information is not transmitted when the SiO ₂ film thickness is 7.5 Å or more, it is considered that the lower layer Si and the upper layer Si are separated by SiO _{2 at} least above this film thickness. It is thought to work effectively. Actually embedded in these Si
Was a SiO ₂ was observed lattice image of the cross-section TEM, intended film thickness of the SiO ₂ is 5 Å SiO ₂ is a precipitate diameter of about 12 Å, also the portion without a SiO ₂ between these precipitates Observed, the epitaxial information is read from this SiO ₂ opening.
It was transmitted to the Si layer. In addition, in this case, the SiO ₂ / Si interface was not flat, and the interface layer was 20 Å. While in the sample the upper Si becomes poly-silicon SiO ₂ put 7.5 Å, specific image is observed between the lower single-crystal Si and the upper polysilicon amorphous SiO _2, upper Si and a lower Si by this SiO ₂ Were completely separated. Also, SiO ₂ / Si
The interface was extremely flat. Furthermore, the width of this SiO ₂ image is
It was about 8Å calculated from the lattice spacing of Si, which almost corresponded to the design value. The SiO ₂ image corresponds to the thickness of the SiO ₂ images growing molecular beam could be controlled from 8Å to 60 Å.

Next, it was investigated whether the thin SiO ₂ film embedded in Si electrically functions as a hole barrier. Experiment P ⁺ (100)
P-type and N-type epitaxial buffer layers are grown on the N ⁺ (100) substrate for 3000 Å and cooled to room temperature.
SiO ₂ is formed by co-evaporation with O _2, and the substrate temperature is set again.
Raise the temperature to 500 ° C and remove the P-type and N-type doped Si layers.
000Å has grown. B was used for P-type doping and Sb was used for N-type doping. The doping of B is carried out by heating HBO ₂ in the crucible of PBN, and the doping of Sb is carried out by heating Sb.
The heating was performed in the crucible of the PBN, and the voltage of −400V was applied to the sample substrate to increase the sticking coefficient of Sb. The film thickness of SiO ₂ was 20Å to 60Å. At this time, it was confirmed that the upper Si layer was all polycrystalline Si. P on top of Si
A contact was made with Al for the mold and AuSb for the N mold, and using this electrode as a mask, the surrounding Si was etched and removed until the substrate was exposed. The electrode diameter was 0.6 mm. Back contact was made with In. 4 a) and 4 b) compare the SiO ₂ film thickness dependence of the tunnel current when holes and electrons are injected from the lower single crystal Si. In both N-type and P-type, the amount of tunnel current decreases as the film thickness of SiO ₂ increases. N-type has a small Sb doping amount and upper poly-Si
Because of the resistance was high, but by way of SiO ₂ film thickness 20Å has a value lower than the P-type, is more of N-type tunnel current to the above 30Å towards SiO ₂ barrier comes heard than the series resistance P It is one digit higher than the type. From the above experiments, it was found that this oxide film functions as a hole barrier. However, if the oxide film thickness is made thicker, electron injection is also reduced, so that the oxide film thickness of 10Å to 20Å is appropriate. It was confirmed that the SiO ₂ film obtained by low-pressure thermal oxidation in which a heated Si substrate is irradiated with only O ₂ molecules also functions as a hole barrier by performing the same experiment as above.

Finally, the result of actually making a prototype of a bipolar transistor by using this method for producing a polysilicon emitter will be described. Fig. 1 shows a conceptual diagram of a transistor prototype process. The thickness of the collector 43 is 4000Å, carrier concentration is 5 × 10 ¹⁶
cm ⁻³ phosphorus was used and was formed on the high-concentration buried layer 42 to be an electrode by using the epitaxial growth method by CVD. After element isolation by the Locos method, a base portion was opened (FIG. 1 (a)). Next, a SiMBE method is used to form a base layer 46 having a carrier concentration of 5 × 10 ¹⁸ cm ^-3 boron at a growth temperature of 700 ° C. and a carrier concentration of 5 × 10 ¹⁸ cm ^-3 . After removing the polysilicon on the field oxide film 44, the CVD method is used. An oxide film 45 for passivation was formed, and a portion to be an emitter was opened (FIG. 1 (b)). Furthermore, after cleaning the surface in SiMBE, at room temperature 0.2 Å / s Si molecular beam and 5 × 1
Simultaneous irradiation with molecular oxygen beam of 0 ^-5 Torr results in SiO ₂ 4
After forming 7 and exhausting O ₂ , the substrate temperature was raised to 500 ° C., and a Si molecular beam was irradiated to form 3000 Å polysilicon 48, and the polysilicon was removed on the oxide film 45 (see FIG. 1 ( c)). Arsenic was implanted into the polysilicon layer 48 by 5 × 10 ¹⁵ cm ^-2 by an ion implantation method with an acceleration of 100 KeV, and indentation diffusion was performed by lamp annealing. The base electrode portion was opened, and boron was implanted at 5 × 10 ¹⁵ cm ^-2 by an ion implantation method of ¹⁵ KeV to form an ohmic contact with the base layer 46. When the static characteristics of the prototype transistor were evaluated as described above, the current amplification factor when SiO ₂ was not sandwiched was 15, but when the SiO ₂ layer thickness was 10Å, it was 150, which was about 10 times that of the current amplification factor. An increase in current gain was observed.
When the thickness of the SiO ₂ layer was 5Å, the current amplification factor did not increase and it did not work as a hole barrier. The thickness of the SiO ₂ layer is
At 20Å, the emitter resistance increased and the collector current decreased. On a heated Si substrate. The same experiment was performed on the SiO ₂ film obtained by low-pressure thermal oxidation in which only O ₂ molecules were irradiated, and it was confirmed that the current amplification factor increased. From the above experiments, it became clear that the polysilicon emitter produced by this method effectively acts as a hole barrier and increases the current amplification factor of the bipolar transistor.

The method for cleaning the base surface is not limited to the one described above, and the case of evaporating the natural oxide film on the surface only by heating, the case of ion sputtering of Ar or the like, and the case of etching by Si molecular beam were performed. But the results were all the same.

In the present embodiment, a silicon wafer was targeted, but the method of the present invention uses SOS (Silicon
on Sapphire) substrate and more generally SOI (Silicon on Insu)
Of course, it also applies to substrates.

(Effects of the Invention) As described in detail above, according to the present invention, a SiO ₂ film, which is thinner than a natural oxide film whose film thickness is extremely precisely controlled, is provided between the base of the NPN bipolar transistor and the polysilicon emitter. Since it can be embedded, and such a thin SiO ₂ film acts as a hole barrier, reverse injection of holes from the base can be suppressed, and the current amplification factor can be increased.

[Brief description of drawings]

1 (a) to 1 (c) are conceptual diagrams of a transistor prototype process, FIGS. 2 (a) to 2 (d) are diagrams for explaining the operation of the present invention, and FIG. 3 is a diagram for low pressure thermal oxidation. FIG. 4 shows the relationship between the oxidation time and the AES oxygen peak, and FIGS. 4 (a) and 4 (b) show the dependence of the tunnel current on the SiO ₂ film pressure when holes and electrons are injected from the lower single-crystal Si, respectively. Is. In the figure, 11 ... Natural oxide film, 12 ... Silicon single crystal base layer, 13 ... Hole barrier oxide film, 14 ... Polysilicon emitter, 41 ... P-type silicon substrate, 42 ... High concentration buried layer, 43 ... Collector layer , 44 ... field oxide film, 45 ... passivation oxide film, 46 ... base layer, 47 ... hole barrier oxide film, 48 ... polysilicon emitter.

Claims (2)

(57) [Claims] 1. A first step of forming a silicon oxide film by cleaning a surface of a substrate having a base layer and then simultaneously irradiating the surface of the base layer with a silicon molecular beam and an oxygen molecular beam; and a silicon oxide film. For forming polysilicon by irradiating a silicon molecular beam on a substrate containing silicon
The method of manufacturing a bipolar transistor, wherein the first and second steps are continuously performed in the same vacuum chamber or in a vacuum chamber connected by a vacuum. 2. A first method for forming a silicon oxide film on the surface of a base layer after cleaning the surface of the substrate having the base layer, by low pressure thermal oxidation of irradiating oxygen while heating the substrate.
And a second step of forming polysilicon by irradiating a substrate including a silicon oxide film with a silicon molecular beam. The first and second steps are performed in the same vacuum chamber. Alternatively, the method for manufacturing a bipolar transistor is characterized in that the steps are continuously performed in a vacuum chamber connected by a vacuum.