This is a division of application Ser. No. 791,184 filed Apr. 27, 1977, abandoned.

The present invention relates to a MIS type semiconductor device (MIS type transistor) having a high breakdown voltage and a method of manufacturing the same.

In the first place, a brief description will be made on the accompanying drawings which will be helpful for a better understanding of the MIS type semiconductor device according to the invention as compared with those proposed heretofore. In the drawings,

FIG. 1 shows a structure of a MIS type transistor in a sectional view to illustrate the teaching of the invention;

FIG. 2 is a sectional view of an embodiment of the MIS type transistor constructed in accordance with the teaching of the invention;

FIGS. 3a to 3e show sections of a MIS transistor at manufacturing steps to illustrate an example of the manufacturing method according to the invention;

FIG. 4 is a circuit diagram of a MIS transistor;

FIG. 5 is a plan view showing an embodiment of MIS transistor according to the invention; and

FIG. 6 is a sectional view of an embodiment of a complementary type MIS semiconductor integrated circuit device to which the teaching of the invention is applied.

The inventors of the present application have already proposed a structure of a MIS type semiconductor device of a high integration density for a high performance which has a high breakdown voltage in Japanese patent application No. 11460/1976, filed on Feb. 6, 1976. An example of such device is shown in FIG. 1 in a sectional view. Referring to the figure, reference numeral 1 designates an N^- type semiconductor substrate having a surface in which oxide films 2 and 3 are formed through a selective oxidizing process so as to enclose a region where the transistor is to be formed and at the same time to isolate a source region and a channel region from each other. Formed under the oxide film 2 (known as field oxide film) is an N type semiconductor layer 4 which has an impurity concentration greater than that of the N^- type semiconductor substrate 1 and serves to prevent the occurrence of parasitic MOS effect. In this case, this parasitic MOS effect may occur as follows. With negative voltage applied to the lead on the field oxide film, this negative voltage converts the surface of the N^- type semiconductor substrate surface below the field oxide film 2 to P conduction type. This converted surface serves as a drain or source layer while the field oxide film between such source and drain regions serves as a gate oxide with the resultant MOS effect.

The drain and source regions are doped with a P⁺ type impurity thereby to form a drain layer 5 and a source layer 6, respectively. The drain and the source layers 5 and 6 are formed with oxide films 7 and 8 on the respective surfaces thereof and connected to a drain electrode 9 and a source electrode 10, respectively, through associated contacting apertures formed in the oxide films 7 and 8. A gate electrode 12 of an electrically conductive polycrystalline silicon layer is formed on the channel region through an interposed gate oxide film 11 with one edge portion of the gate electrode extending partially over the oxide film 3. Formed under the oxide film 3 is a P type semiconductor layer 13 which is connected to the drain layer 5 and has an impurity concentration smaller than that of the latter, whereby the avalanche breakdown possibly caused between the drain layer and the N⁺ type semiconductor substrate with a high voltage applied to the drain layer is prevented to assure high breakdown voltage.

With such structure of the high voltage-rated MIS semiconductor device, it has however been found that dielectric breakdown of the gate insulating film will occur concentrically at the peripheral portion b of the gate electrode located adjacent to the source layer 6, when a surge voltage is applied to the gate electrode of polycrystalline silicon or drain electrode. Such destruction of the gate isolating film at the peripheral portion will occur even when a gate protecting element such as a diode and/or a resistor connected to the gate electrode (not shown) is provided. It is believed that such undesirable phenomenon is ascribable to the fact that the electric field is likely to be concentrated at the peripheral portion of the gate electrode and the gate insulating film therearound is extremely thin.

In this connection, it is also noted that the formation of the source layer 6 is effected through an ion implantation or diffusion process by using the polycrystalline silicon layer of the gate electrode 12 as a mask. For this reason, the channel length a is determined by the width of the gate electrode 12. In practice, the width of the gate electrode 12 can be realized with a high accuracy. However, dimensional deviation on the order of ±2 μm will often occur in the mask alignment prior to the formation of the gate electrode 12. Such deviation of course provides a cause for variations in the channel length a because at least the source region is formed through diffusion or ion implantation technic by using the gate electrode as the mask therefor.

Accordingly, an object of the invention is to provide a MIS type semiconductor device having a high breakdown voltage which is immune to the destruction of the gate insulating film.

Another object of the invention is to provide a method of manufacturing the same.

With the above objects in view, there is provided according to a fundamental aspect of the invention a MIS type semiconductor device in which a source and a drain layer are selectively formed in a surface of a semiconductor substrate while a gate electrode is formed over a channel region between the source layer and the drain layer in the substrate surface through an interposed gate insulating film, wherein the semiconductor substrate is formed with selectively buried insulation films between the source and drain layers and the channel portion in thickness greater than that of the gate insulating film and further under the selected insulating films formed with a first region of the same conduction type as that of the source layer for connecting the channel portion and the source layer to each other and with a second region of the same conduction type as that of the drain layer for connecting the drain layer and the channel portion to each other, the second region having an impurity concentration greater than that of the drain region, and the gate electrode is formed as extending over the selected insulating films.

Now, detailed description will be made on preferred embodiments of MIS type semiconductor device according to the invention.

FIG. 2 shows an embodiment of MIS type semiconductor device according to the invention in a sectional view. The structure shown in FIG. 2 differs from the one shown in FIG. 1 in that a buried thick oxide film 14 is formed at the side of the source region in a similar manner as the drain region side and additionally a P type semiconductor layer 15 is formed in the semiconductor substrate under the oxide film 14 for connecting the source layer 6 to the channel region lying immediately below the gate insulating oxide film 11.

In the following, an example of the method of manufacturing the MIS semiconductor device of the above structure will be described by referring to FIGS. 3a to 3e.

Referring to FIG. 3a, at first, a thermally oxidized film 16 (SiO₂) having a thickness of 800 A is formed in the plane (100) on a surface of a N^- type substrate (Si) having resistivity of 2 Ω-cm. Further, a nitride (Si₃ N₄) film 17 is formed on the SiO₂ film 16 in thickness of 1400 A. This nitride film is obtained from reaction: 4NH₃ +3SiH₄ →Si₃ N₄ +12H₂. A photoresist film of 8500 A thick is formed on the nitride film 17 and subjected to a photoresist treatment comprising selective exposure and etching steps. Subsequently, the nitride film 17 as well as the thermally oxidized film 16 are selectively etched by using the thus formed photoresist film as the mask for the etching treatment, thereby to form apertures for forming P type semiconductor regions 13 and 15 and expose the corresponding surface regions of the semiconductor substrate 1. The etching treatment of the nitride film may be effected through a plasma etching process. Subsequently, the exposed surface of the substrate 1 is implanted with boron ions at the rate of about 2×10^-- atoms/cm² with the photoresist film being left as it is, whereby the P type regions 13 and 15 are formed. Then, the photoresist film is removed and a new photoresist film 18 is formed and subjected to a photoresist treatment in a similar manner as the removed photoresist layer. The nitride film (Si₃ N₄) 17 and the thermally oxidized film (Si O₂) are selectively etched away by using the photoresist film 18 as the mask therefor, thereby to form an opening for the formation of an N type semiconductor layer 4 to surround the P type semiconductor regions 13 and 15 and expose the associated surface portion of the substrate 1. The thus exposed surface of the semiconductor substrate 1 is implanted with phosphor ions at the rate of about 2×10¹² atoms/cm² thereby to form the N type semiconductor layer 4. FIG. 3a shows the structure of the MIS semiconductor device at the step where the N type semiconductor layer 4 has just been formed.

Referring to FIG. 3b, the photoresist film 18 is removed and the surfaces of P type semiconductor regions 13 and 15 as well as N type layer 4 are selectively oxidized with SiO₂ film 16 and Si₃ N₄ film 17 being used as the mask for the selective oxidation, which may be conducted at 1000° C. for 6 to 7 hours in the atmosphere of mixed oxygen and nitrogen. As a result of the oxidation step, buried thick oxide (SiO₂) films 2, 3 and 14 of about 1.2μ thick are formed. It will be noted that the N type semiconductor layer 4 and the P type semiconductor regions 13 and 15 having surface impurity concentration of about 10¹⁶ -10¹⁷ atoms/cm² are located immediately below the SiO₂ films 2, 3 and 14.

At the step shown in FIG. 3c, the Si₃ N₄ film 17 is removed through chemical etching using H₃ PO₄ aqueous solution, at 175° C. with the etching rate of 50 A/min. or plasma etching, SiO₂ film 16 remaining as it is. It is to be noted that a portion of SiO₂ film 16 constitutes the gate insulating oxide film of the MIS transistor. Alternatively, the SiO₂ film 16 is also removed and a fresh thermally oxidized film (SiO₂ film) may be formed to be used as the oxide film for the gate.

At the step illustrated in FIG. 3d, the whole surface of the semiconductor substrate 1 having the structure shown in FIG. 3c is formed with a polycrystalline silicon layer 12' of 3500 A thick at temperature of 600° C. in the atmosphere of mixed nitrogen and silane (SiH₄). Subsequently, the polycrystalline silicon layer 12' is removed through a selective etching treatment by using a photoresist layer formed thereon as the etching mask. The remaining polycrystalline silicon layer 12' is used for a gate electrode 12 as well as for a wiring or metallization layer (not shown). The polycrystalline silicon layer 12' destined to constitute the gate electrode is so selectively etched that end portions thereof extend over the buried thick oxide (SiO₂) layers 3 and 14, as is shown in FIG. 3d.

At the step shown in FIG. 3e, the oxide film (SiO₂) 16 is removed away without using a photoresist film and openings are formed to provide contact regions for leading-out electrodes for the source and the drain layer. The reason why the photoresist layer is not used at this step can be explained by the fact that the oxide (SiO₂) films 2, 3 and 14 are remarkably thicker than the oxide film 16 and therefore the oxide films 2, 3 and 14 may remain with a sufficient thickness even after the oxide film 16 have been etched away to expose the surface of the semiconductor substrate 1. Next, P type impurity, say boron, is diffused into the N^- type substrate 1 at 1050° C. for about 30 minutes to form a P⁺ type drain layer and a P⁺ type source layer having a surface impurity concentration of 10²⁰ atoms/cm² in the N^- type semiconductor substrate 1 using SiO₂ films 2, 3 and 14 as masks. As a result of such diffusion step, the drain and the source layer 5 and 6 are formed with thermally oxidized (SiO₂) films 7 and 8 at the respective surfaces thereof. Further, the polycrystalline silicon layer 12' contains P type impurity to exhibit a low resistivity. Accordingly, the layer 12' can be used as the gate electrode 12.

Next, the SiO₂ films 7 and 8 are selectively removed through etching treatment and formed with contact openings at appropriate portions thereof. Subsequently, the whole surface is formed with an aluminum layer through evaporation, which is thereafter etched away selectively through photoresist treatment, whereby the drain electrode as well as the source electrode 10 connected to the drain layer 5 and the source layer 6, respectively, are formed as is shown in FIG. 2. The removal of the SiO₂ films 7 and 8 through the etching treatment can be carried out without using the photoresist film by taking advantage of the relation in thickness of the SiO₂ films 2, 3, 14 and 7, 8, as described hereinbefore.

With the structure of the MIS semiconductor device described above, the aimed object of the invention can be accomplished for the following reasons:

(1) By virtue of such structure that a thick selectively buried oxide (SiO₂) film 3 is provided between the P⁺ type drain layer 5 and the channel portion (i.e. the surface portion of the semiconductor substrate 1 underlying immediately below the gate oxide film 12) and that the P type semiconductor region 13 is provided which has an impurity concentration lower than that of the drain layer to serve to connect the drain layer 5 and the channel portion, the P type semiconductor region 13 can function as a resistance in the drain layer 5. In other words, MIS transistor of the structure described above has an electrical structure such as shown in the equivalent circuit diagram of FIG. 4. The P type semiconductor region 13 will serve as a saturatable resistance element, as a result of which the voltage applied to the drain electrode D will be held at a level lower than the pinch-off voltage of such saturatable resistance at the point F. In this manner, the voltage rated for the drain layer 5 can be set at a higher level than the surface breakdown voltage across the cylindrical PN junction between the drain layer 5 and the semiconductor substrate 1 and increased approximately upto the breakdown voltage of the planar PN junction between the drain layer 5 and the substrate 1. Thus, the MIS semiconductor device according to the invention can handle a significantly high voltage.

(2) Since the peripheral edge portions of the gate electrode 12 made of polycrystalline silicon layer extend partially over the buried oxide (SiO₂) films 3 and 14, possibility of the gate insulating film being destroyed at the edge portions of the gate electrode 12 is positively suppressed.

(3) The channel length is determined by the distance between the P type semiconductor regions 13 and 15 before the formation of the polycrystalline silicon gate electrode 12, while the distance in turn is determined by the mask pattern as used as well as the conditions such as temperature and time duration required for forming the buried oxide films. Thus, variations in the above distance due to deviation in the mask alignment upon forming the gate electrode can be utterly excluded. In other words, although the polycrystalline silicon layer destined to constitute the gate electrode is used as the diffusion mask (formed by self-alignment) for the formation of the source and drain layers, the channel length between the source and the drain is determined before the gate electrode is formed. Accordingly, even when deviation on the order of ±2 μm should occur in the position of the gate electrode 12 at the formation thereof due to a misalignment of the mask, no influence is exerted onto the channel length. Thus, the channel length can be positively set.

(4) Since the P type semiconductor regions 13 and 15 are subjected to the further diffusion treatment for a sufficiently lengthened time when these regions 13 and 15 are selectively oxidized, no lengthened diffusion will take place upon formation of the drain and source layers 5 and 6 through diffusion. Namely, the semiconductor substrate undergoes the treatment for selectively forming the selective oxide films for 6 to 7 hours at a temperature of 1000° C. In contrast thereto, the time duration required for forming the drain and the source layer 5 and 6 through diffusion at 1050° C. is only on the order of only 30 minutes. Thus, the channel length is fixed at the termination of the selective oxidation treatment and held at the fixed length without being changed in the succeeding processes. It is also noted that evaporation of aluminium for the electrodes or wiring is carried out at the substrate temperature on the order of 300° C. In this connection, when the wiring layer (polycrystalline silicon layer) is deposited on the upper surfaces of the oxide films 3 and 14 in perpendicular to the plane of the drawing, flexibility is allowed in the design of the wiring or metallization, involving an enhanced integration density. For example, the polycrystalline layer 19 may be so deposited as to extend over the selectively oxidized film 3 on the P type semiconductor region 13, as is shown in FIG. 5.

FIG. 6 shows a complementary type MIS semiconductor integrated circuit which can be constructed by implementing therein MIS semiconductor devices according to the invention. In the figure, reference numeral 21 denotes an N^- semiconductor substrate, 22 a P^- type well region, 23 a P type semiconductor layer imparted with an impurity concentration higher than that of the P^- type well region with a view to preventing a parasitic MOS structure from being formed, 24 an N type semiconductor layer having a higher impurity concentration than the substrate 21 for the just above mentioned reason, 25 and 26 designate, respectively, a drain and a source layer of an N channel MIS (MOS), 27 and 28 denote a drain and source layer of a P channel MIS (MOS), respectively, 29 and 30 denote N type semiconductor regions each having a lower impurity concentration than the drain and source layers 25 and 26, 31 and 32 P type semiconductor regions each having a lower impurity concentration than the drain and the source layer 27 and 28, 33 denotes a selectively buried oxide (SiO₂) film, 34 and 35 denote gate oxide films and numeral 36 denotes a gate electrode formed of a polycrystalline silicon.

When the complementary type MIS semiconductor integrated circuit described above and illustrated in FIG. 6 is fabricated, the P type semiconductor layer as well as the same conduction type regions 31 and 32 are formed simultaneously through an ion implantation or diffusion process. On the other hand, N type semiconductor layer 24 as well as same conduction type semiconductor regions 29 and 30 are also formed simultaneously through an ion implantation or diffusion technique. After the above semiconductor layers and regions have been formed, the sub-structure of MIS semiconductor integrated circuit is subjected to the treatments described hereinbefore in conjunction with FIGS. 3b to 3e.

As will be appreciated from the above description, the complementary type MIS semiconductor integrated circuit implementing the MIS semiconductor devices according to the invention can be easily and inexpensively accomplished without increasing the number of the manufacturing steps.

When a complementary MIS semiconductor integrated circuit device comprising a plurality of the complementary type MIS devices of the structure described above is to be fabricated, a plurality of P channel MOS's having different work function difference φMS (metal-semiconductor work function difference) can be obtained by providing masked portions and unmasked portions for the gate electrodes of the plural P channel MOS's upon forming the N⁺ type semiconductor layers (drain and source layers) 25 and 26. It is known that such work function difference φMS is a parameter for determining the threshold voltage (Vth) of the MOS circuit. Accordingly, it is possible to form the P channel MOS's having different threshold voltage Vth in a single semiconductor substrate of the complementary type MIS semiconductor integrated circuit device. On the other hand, when masked portions and unmasked portions are present for the gate electrodes of the plural N channel MOS's, then N channel MOS's having different threshold values Vth can be realized in a single semiconductor substrate in a similar manner as described above.

In the disclosed embodiments, it has been assumed that the teachings of the invention are applied to the silicon gate MIS type transistors. However, the invention is never restricted to such transistors but can be equally applied also to aluminium gate MIS type transistors.