Patent Application
20070145474
1. Field of the Invention
The present invention generally relates to semiconductor devices.
More specifically, the invention relates to vertical-gate MOS transistors.
2. Description of the Related Art In the last years, the demand of increasing the semiconductor device integration density has brought a reduction in the size of the elements used in integrated circuits.
A basic integrated circuit element is the transistor; particularly, in high-density integrated circuits, field-effect transistors are used. The use of integrated transistors in a number of relatively high power applications, such as liquid crystal display drivers and the like, has made it necessary to manufacture small size transistors that are nevertheless able to withstand relatively high voltages (for example, 10V-70V). Limitations in the manufacturing of small size field-effect transistors (for example, of the MOS type) often arise from the length of the transistor channel, i.e., the region between the source and drain regions.
A well-defined channel length is important for the correct operation of the MOS transistor; in fact, many electrical characteristic parameters, such as the transconductance, depend on the transistor channel length.
Moreover, as the channel length becomes smaller, the correct operation of the transistor as a whole may be impaired, due for example to short-channel effects, such as punch-through phenomena or a permanently short-circuited channel.
In particular, as far as MOS transistors for relatively high power applications are concerned, further electrical characteristic parameters that make the manufacturing of a small size MOS transistor troublesome are the voltages that it can withstand at its PN junctions and gate oxide layer; in particular, in order for the MOS transistor to withstand the desired high voltages, these must be lower than the breakdown voltages of both the PN junctions and the gate oxide layer.
As known, the breakdown voltage of the PN junction depends on a certain number of design and manufacturing process parameters, such as the dopant concentration of the regions forming the PN junction and the width of such regions. Particularly, the lower the dopant concentration of the regions forming the PN junction is the higher the breakdown voltage is. Moreover, in the case one or both of the regions forming the PN junction are lightly doped, the width of such regions must be enough to permit the extent of the depletion area in reverse bias condition, and this limits the possibility of reducing the integrated circuit area.
Likewise, the breakdown voltage of the gate oxide layer depends on a certain number of manufacturing process parameters, such as the thickness of such oxide layer. As known, the higher the thickness of the gate oxide layer is the higher the voltage withstood by the MOS transistor is. However, a higher thickness of the gate oxide layer reduces the saturation current of the MOS transistor. Thus, the thickness of such oxide layer should be kept relatively low, thereby reducing the voltages that can be withstood by the MOS transistor.
Vertical-gate MOS transistors (also known in the art as V-MOS, U-MOS, folded gate or trench gate MOS transistors) are less affected by short channel effects. In these devices, a trench is formed in a substrate region of a chip of semiconductor material wherein the MOS transistor is integrated. The walls of the trench are covered with a gate oxide layer, and the trench is then filled with a conductive material (typically, a polycrystalline silicon layer) adapted to form the gate electrode. The source and drain regions of the MOS transistor are formed in the chip at opposite sides of the trench.
This MOS transistor has a channel region developing along the vertical and bottom walls of the trench, between the source and drain regions. In such a way, even if the overall size of the vertical-gate MOS transistor is reduced, the channel region can be kept sufficiently long to prevent the short channel effects.
A vertical-gate MOS transistor is disclosed in the U.S. Pat. No. 4,455,740, which also describes a related manufacturing method.
The Applicants have observed that a vertical-gate MOS transistor realized according to the teachings of that patent is not able to withstand high voltages across the substrate-drain and substrate-source junctions, due to the fact that the drain and source regions are heavily doped (N+) diffusion layers. The high dopant concentration of the drain and source regions reduces the substrate-source and substrate-drain junctions breakdown voltages, and thus the voltages that can be withstood by such PN junctions. Moreover, the gate oxide layer is not able to sustain high voltages, due to its thin thickness. Both these features make the prior art vertical-gate MOS transistor not particularly adapted for power applications.
The U.S. Pat. No. 6,586,800 proposes a different vertical-gate MOS transistor wherein the drain region consists of a layer buried in the chip under the channel. The drain current is collected through a metallization formed at the bottom surface of the chip.
As an alternative to the bottom surface drain contact, a top-surface sinker adapted to collect the drain current may be provided, for example, as described in the U.S. Pat. No. 5,124,764.
In both the solutions, the dopant concentration of the drain region (but not of the source region) is chosen according to the desired breakdown voltage of the drain-substrate junction.
As a result, the breakdown voltages are relatively high for the substrate-drain junction, but low for the substrate-source junction. Thus, the proposed MOS transistor is inherently asymmetric; this may be a disadvantage, because in many applications (e.g., pass transistors) the source and drain regions should be interchangeable.
In any case, the thin gate oxide layer does not allow withstanding high voltages supplied between the gate and drain/source terminals of the MOS transistor.
One embodiment of the present invention provides a trench gate structure with an insulation layer having a differentiated thickness.
One embodiment of the present invention proposes a vertical-gate MOS transistor integrated in a semiconductor chip of a first conductivity type having a main surface. An insulated trench gate extends into the semiconductor chip from the main surface to a gate depth. The trench gate includes a control gate and an insulation layer (for insulating the control gate from the semiconductor chip). A source region and a drain region of a second conductivity type are formed in the semiconductor chip; one or both of the source region and drain region are adjacent to the insulation layer and extends into the semiconductor chip from the main surface to a region depth (lower than the gate depth). The insulation layer includes an outer portion (extending into the semiconductor chip from the main surface to a protection depth less than the gate depth) and an inner portion. The outer portion has first thickness and the inner portion has a second thickness lower than the first thickness.
Another embodiment of the present invention proposes a corresponding method for manufacturing a vertical-gate MOS transistor.
The present invention, as well as further features and the advantages thereof, will be best understood by reference to the following detailed description, given purely by way of a non-restrictive indication, to be read in conjunction with the accompanying drawings, wherein:
In the following description, it should be noted that the Figures are not drawn to scale. Relative dimensions and proportions of parts of drawings have been increased or reduced in size for the sake of clarity.
Referring to
The trench gate 110 is formed into a trench 170 excavated in the semiconductor region 120 from its main surface, and it includes an insulation layer 180 and a polycrystalline silicon layer 190. The insulation layer 180 has an outer portion 180a, covering the walls of the trench 170 from the main surface of the semiconductor region 120 to a (protection) depth d4, and an inner portion 180b, covering the deep walls of the trench 170. The outer portion 180a includes an edge section 180c extending from the main surface of the semiconductor region 120 and which is inclined outwardly with respect to a remaining section of the outer portion 180a. For example, the edge section 180c is inclined at an angle preferably ranging from 30° to 45°, and more preferably from 35° to 40° (for example, 37°). Typically, the depth of the edge section 180c ranges from 10% to 20% and more preferably from 11% and 15% (for example, 13%) of the depth of the whole outer portion 180a. The outer portion 180a and the inner portion 180b have respective thicknesses d5 and d6. The polycrystalline silicon layer 190 fills the trench 170 so covered by the insulation layer 180.
A field oxide layer 191 covers the main surface of the semiconductor region 120 except for two active area windows 191S and 191D over the contact regions 150 and 160, respectively, (where source and drain contacts are realized) and a window 191G over the free surface of polycrystalline silicon layer 190 (where the gate contact is realized). Metallizations 192S, 192G, and 192D fill the windows 191S, 191G, 191D and form the source, gate, and drain terminals S, G and D, respectively, of the MOS transistor 100.
In the MOS transistor 100, a channel region is formed by a portion of the semiconductor region 120 between the internal source region 130 and the internal drain region 140, which channel develops along the vertical and bottom walls of the insulated trench gate 110. The MOS transistor 100 thus has a vertical-gate structure, which allows achieving a relatively high channel length at the same time saving integrated circuit area. In particular, it is possible to shrink the lateral dimension of the MOS transistor 100, without for this reason incurring in short-channel effects (because the channel length can be increased by increasing the depth d1 of the trench gate 110).
The insulation layer 180 has the inner portion 180b with the thickness d6 that is lower than the thickness d5 of the outer portion 180a. Preferably, the thickness d6 ranges from 15% to 40% and more preferably from 20% to 30% (such as about 25%) of the thickness d5. For example, the thickness d6 is about 90 nm and the thickness d5 is about 350 nm.
The presence of the thin inner portion 180b (for example, a gate oxide) along the walls of the trench 170 adjacent to the channel ensures a high saturation current of the MOS transistor 100. In fact, as mentioned above, the lower the thickness d6 of the internal portion 180b, the higher the saturation current of the MOS transistor 100.
On the other hand, the thick outer portion 180a is able to sustain higher voltages before breaking. In such a way, the voltage which can be withstood by the MOS transistor 100 (i.e., between the gate terminal G and the drain/source terminals S/D) without breaking the insulation layer 180 is increased.
The depth d4 of the outer portion 180a is lower than the depth d2 of the source and drain regions 130, 140 (and then lower than the depth d1 of the trench gate 110). Preferably, the depth d4 ranges from 20% to 60% and more preferably from 30% to 50% (such as 35%) of the depth d1. For example, the depth d1 is 3.6 μm, the depth d2 is about 2.4 μm and the depth d4 ranges from 1.5 μm to 1.8 μm (such as 1.5 μm). In such a way, as better described with reference to
Moreover, the fact that the contact regions 150 and 160 are spaced apart (at the distance d3) from the trench gate 110 further reduces the stress voltage at the insulation layer 180. In fact, the voltage that is applied to the insulation layer 180 is reduced by an amount equal to the voltage drop at the portion of the semiconductor region 120 extending horizontally along the distance d3. Preferably, the distance d3 ranges from 0.3 μm to 2 μm according to the requested breakdown voltage, and more preferably from 0.5 μm to 1.5 μm. For example, the distance d3 is about 1 μm.
The internal source region 130 and the internal drain region 140 are relatively lightly doped; for example, they have a dopant concentration preferably ranging from 1*1015 ions/cm3 to 1*1017 ions/cm3, and more preferably from 5*1015 ions/cm3 to 5*1016 ions/cm3 (such as about 1*1016 ions/cm3). Thus, a breakdown voltage of the junctions between the source/drain regions 130, 140 and the semiconductor region 120 is kept relatively high; in this way the vertical-gate MOS transistor 100 is capable of withstanding high voltages.
The contact regions 150 and 160 are instead heavily doped; for example, they have a dopant concentration preferably ranging from 1*1019 ions/cm3 to 1*1021 ions/cm3, and more preferably from 5*1019 ions/cm3 to 5*1020 ions/cm3 (such as about 1*1020 ions/cm3). In this way, it is ensured that the contacts with the metallizations 192S, 192D are low-resistance, non-rectifying (i.e., ohmic) contacts.
Referring to
The diagram 200 has the breakdown voltage BV on the axis of the ordinates and the depth d4 on the axis of abscissas. As it can be noted, the breakdown voltage BV has a constant value BVc (about 43 V) when the depth d4 ranges from 0 to 0.6 μm. Then, the breakdown voltage BV linearly increases up to reach a saturation breakdown voltage BVs (about 62 V) when the external portion 180a extends to a depth d4 of about 1.8 μm. For depths d4 higher than 1.8 μm, the breakdown voltage BV remains approximately constant and equal to the saturation breakdown voltage BVs. Therefore, in order to reach the maximum allowable breakdown voltage BV, the depth d4 should be at least equal to 1.8 μm. However, it is preferable not to exceed this value; indeed, any increase of the depth d4 has no effect on the breakdown voltage BV but it reduces the saturation current of the MOS transistor 100.
Referring now to
Considering in particular
Thereafter, the silicon nitride film 303 is selectively etched and removed, using a conventional photoetching process, as shown in
Moving to
As shown in
Moving to
Thereafter, the first trench portion 307 is extended down to the depth d4 (for example, 1.5 μm) by selectively etching the silicon layer 301, with the layers 304, 305 and 306 used as a hard mask, as shown in
The etching is performed by two processes, each one having a corresponding isotropic degree with respect to two directions X (lateral) and Y (vertical).
More in detail, the isotropic degree (in the following referred to also as AD) is defined through the following expression:
AD=x/y
wherein y is the etching depth along the direction Y and x is the etching depth along the direction X.
In particular, the first etching has an isotropic degree preferably equal to 90%-100%, and more preferably equal to 95%-100%, such as about 100% (meaning that the epitaxial layer 301 is equally etched laterally and vertically so as to form a sloped portion 308 adapted to accommodate the edge section of the insulation layer). In other words, the sloped portion 308 generates an undercut below the oxide layer 304.
The second etching has instead an isotropic degree theoretically equal to about 0, such as lower than 5% and more preferably lower than 2% (meaning that the epitaxial layer 301 is only vertically etched so as to form a straight portion 309 adapted to accommodate the remaining section of the insulation layer).
Then, a thermal oxidation should be performed for growing the desired insulation layer. However, the preceding etching process may affect the quality of the oxide that will be subsequently grown on the walls of the trench portions 308 and 309. In order to avoid this problem, the wafer is now subjected to a thermal annealing process into a room providing an environment saturated with hydrogen (H2), at an annealing temperature preferably ranging from 900° C. to 1100° C., and more preferably from 950° C. to 1050° C. (for example, 1000° C.). During this phase, the wafer is heated by a thermal process (for example, by Rapid Thermal Process) up to the annealing temperature and subsequently cooled in the same room (including H2 only). In such a way, the thermal annealing process allows the silicon molecules belonging to the walls of the trench portions 308 and 309 to arrange along their preferential directions, so resolving the above mentioned problem (without requiring any further etching of the epitaxial layer 301 or re-oxidation of the silicon).
A thermal oxidation is now performed for growing a first oxide layer 310 on the walls of the trench portions 308 and 309, as shown in
As described in the foregoing, also these etching process may affect the quality of the oxide which will be subsequently formed on the walls of the trench 170. In order to avoid this problem, the wafer is subjected to a further thermal annealing process with similar operative parameters as in the preceding case. In such a way, the quality of a subsequent grown oxide or of an interface with a subsequent deposited oxide will be improved for the desired purpose.
Moving to
It should be noted that, thanks to the preceding annealing steps, it is possible to obtain an insulation layer with a quite uniform thickness even along the curvature near the bottom wall of the trench 170; for example, this thickness is 15%-30% greater than the one that would have been obtained without the annealing steps.
Alternatively, in order to form the inner portion of the insulation layer of the trench gate, the second oxide layer can be thermally grown along the walls of the trench 170. In this case, however, the thickness of the second oxide layer where the trench 170 is already covered by the first oxide layer 310 is lower than the one where the trench 170 is completely exposed; more specifically the thickness of the second oxide layer depends non-linearly (for example, according to a square root law) on the thickness of the first oxide layer 310. Moreover, the first oxide layer 310 can be subjected to a certain numbers of surface treatments that reduce its thickness before the growing of the second oxide layer. For these reasons, the thickness of the first oxide layer 310 should be higher than the difference between the thicknesses of the outer portion and of the inner portion of the insulating layer for the trench gate (for example, 350 nm in the example at issue).
Thereafter, as shown in
A first dopant implantation process is performed, for forming the internal source and drain regions 130 and 140; for example, in order to form N-type source and drain regions, arsenic or phosphorus dopant ions may be used.
Particularly, the first implantation process is performed at a relatively high energy, for example up to 2 or 3 MeV, in order to cause the dopant ions to penetrate the field oxide layer 304 and the epitaxial layer 301, down to the depth d2 (whereas where the mask 313 is present the dopants do not reach the surface of the epitaxial layer 301).
Preferably, the dopants, after having been implanted, are simply activated by a high thermal budget Rapid Thermal Process (RTP), without being made to diffuse into the epitaxial layer.
Alternatively, the source and drain regions may be graded doped junctions. In this case it is possible to perform more than one dopant implantation process, at different, relatively high energies, for example, 200 keV, 400 KeV, 1000 keV and 2500 keV.
Successively, still using the mask 313, a second dopant implantation process is performed in order to form the heavily doped contact regions 150 and 160 of the N-type. In particular, the second dopant implantation process is performed at an energy sufficiently high to cause the dopants to penetrate the thinner portions of the field oxide layer 304, but too low to cause the dopants to penetrate the thicker portions of the field oxide layer 304. For example, arsenic ions are implanted at an energy of approximately 50 KeV, adapted to concentrate the dopant distribution close to the surface of the wafer.
Thereafter, as shown in
Afterwards, a metallization layer, (for example, Al or Ti/TiN plus a W-plug and an Al layer) is deposited on the oxide layer 314, and the source, drain and gate contacts are formed by patterning thereof, so as to obtain the structure described above with reference to
In
In particular, the profile 401 shows the dopant concentration of the region under the drain/source terminals, while the profile 402 shows the dopant concentration of the regions under the field oxide layer.
Both the profiles are chosen in order to optimize a number of transistor parameters, such as the current capability, the on-resistance, the safe operating area (SOA) and the breakdown voltages at the PN junctions, as better described in the following.
In particular, referring to the profile 401, the dopant concentration of the regions closer to the surface of the structure is higher than that in the deeper regions. This allows realizing the ohmic contacts of the MOS transistor. Moreover, the lower dopant concentration of the deeper regions allows increasing the breakdown voltage of the MOS transistor.
Moving now to the profile 402, it should be noted that the dopant concentration of the regions under the field oxide layer is always lower than the dopant concentration of the drain/source contact regions. Also this choice allows increasing the breakdown voltage.
In this way, it is possible to obtain a MOS transistor capable of withstanding voltages ranging from 50 to 60V, having a pitch lower than 3.5 μm, whereas conventional horizontal-gate structures are capable of withstanding the same voltages but with a pitch of about 11.5 μm.
Referring to
As can be seen, a working characteristic 510 is obtained for a MOS transistor having the profiles of the dopant concentration of the drain and source regions shown in
As it can be noted, the working characteristic 510 has a saturation current density J ranging from 350 μA/μm to 450 μA/μm and a breakdown voltage about 55V. Both values are higher than the saturation current density and the breakdown voltage of the other working characteristics (for example, the working characteristic 520 has a saturation current density ranging from 250 μA/μm to 350 μA/μm and a breakdown voltage about 40V).
Thus, by choosing the dopant concentration of the drain and source regions (i.e., the energy for the implantation processes) it is possible to optimize the trade-off between the saturation current density J flowing in the MOS transistor and the voltage which is withstood by the transistor.
Finally, in
Naturally, in order to satisfy local and specific requirements, a person skilled in the art may apply to the solution described above many modifications and alterations. Particularly, although the present invention has been described with a certain degree of particularity with reference to preferred embodiments thereof, it should be understood that various omissions, substitutions and changes in the form and details as well as other embodiments are possible; moreover, it is expressly intended that specific elements and/or method steps described in connection with any disclosed embodiment of the invention may be incorporated in any other embodiment as a general matter of design choice.
For example, although in the preceding description reference has been made to an N-channel MOS transistor, the conductivity types of the various regions may be reversed, so as to form a P-channel MOS transistor.
In addition, the external portion of the insulation layer might also reach the channel of the MOS transistor (even if its performance would be reduced).
In any case, a different extension of the external portion of the insulation layer (with respect to the depth of the drain/source regions) is contemplated.
It should be noted that the proposed thicknesses of the different portions of the insulation layer must not be interpreted in a limitative manner.
Moreover, the solution of the invention is also suitable to be implemented in MOS transistors having one only between the source and drain regions adjacent to the insulated trench gate (while the other region is buried under the channel).
In addition, it is possible to use other profiles of the dopant concentrations.
In any case, the use of alternative processes for realizing the proposed MOS transistor are possible.
For example, it is not necessary to grow the epitaxial layer or to form the silicon nitride layer.
Alternatively, both the first oxide layer and the second oxide layer can be formed by means of deposition processes.
In addition, the drain and source regions may be formed in another way, (e.g., by means of an epitaxial growth), or the insulated trench gate may be formed by using a metal material instead of polycrystalline silicon.
In addition, it is also possible to manufacture the vertical-gate MOS transistor easily in a structure also including standard horizontally-channel MOS transistors (with the channel region developing horizontally between the drain and source regions).
Moreover, the (sloped) edge section of the trench may extend at a different angle and can have a different shape; alternatively, the same result may also be achieved with another isotropic degree of etching, or with equivalent techniques. In any case, the implementation of the MOS transistor with a trench that is completely vertical (i.e., without any sloped edge section) is not excluded in some applications.
Similar considerations apply if the steps of annealing are carried out at different temperatures and/or in another environment. In any case, those steps are not strictly necessary and may be omitted (at the cost of a degradation of the performance of the MOS transistor).
All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheetare incorporated herein by reference, in their entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 05110577.3 | Nov 2005 | EP | regional |
