The present invention relates to a process and an apparatus for producing a semiconductor, and more particularly to a process and an apparatus for producing a semiconductor, which can form a CVD film with a good step coverage at a high film-forming rate.
Production of semiconductor integrated circuits is now making much use of a process for forming a film, i.e. forming a thin film on a wafer by subatmospheric thermal CVD, where it is required to deposit a thin film of uniform thickness with uniform characteristics on a wafer. To fill step gaps around holes, vias, trench, etc. of a high aspect ratio (depth/opening diameter), a high step coverage is required. Voids, when formed in the deposition film on the step gaps due to a low step coverage, will give rise to lowering of reliability or failure of semiconductor integrated circuits.
Heretofore, a batch type CVD apparatus capable of treating a few tens to a hundred and a few tens wafers in one batch has been much used in the subatmospheric thermal CVD process. The batch type CVD apparatus itself has a high throughput and thus improvement of film thickness uniformity and higher step coverage is attained by forming films even at a somewhat low film-forming rate (a few nm/min. or so) under relatively low pressure, e.g. a few tens to a few hundred Pa.
On the other hand, shifting to a single wafer processing type CVD apparatus, which treats wafers one by one, is now gradually taking place. With the recent trend toward larger wafer diameter and more strict requirements for film thickness uniformity, the batch type CVD apparatus is confronting difficulties in keeping the film thickness uniformity not only within the in-place region of wafer, but also throughout wafers. The batch type requires a long treating time for one run and thus is not suitable for more flexible, short TAT (Turn Around Time) production. This is accelerating the shifting tendency.
A well known single wafer processing type CVD apparatus is a lamp heating type apparatus using a halogen lamp as a heat source and its structure is disclosed is JP-A-6-326078, JP-A-10-144619, JP-A-6-293595, etc. The lamp heating type apparatus requires cooling of the lamp by air injection to prevent the lamp durability from lowering due to excessive temperature elevation and thus the lamp cannot be provided in the subatmospheric treating chamber. Instead, the lamp is provided outside the treating chamber to irradiate a wafer or a susceptor (on which a wafer is mounted) with a lamp light through a light-transmissible window provided on one side of the treating chamber.
Another well known single wafer processing type CVD apparatus is an apparatus for heating a wafer mounted on a heater-incorporated stage, and its structure is disclosed in JP-A-9-45624. The stage is in such a structure that a plate type heater divided into a plurality of zones is fixed to the backside of a susceptor having a thickness of a few centimeters, composed of a material of high heat conductivity, and a pair of insulated electrodes are provided on the wafer-mounting side and an insulating film is further provided thereon to constitute an electrostatic chuck. Gas injection nozzles are provided in a circular region around the center of a shower plate, the circular region having a substantially the same diameter as that of the susceptor and a film is formed by injecting a gas through the nozzles, while securing the wafer onto the susceptor by electrostatic force. It is disclosed that the film thickness uniformity within the in-plane region of the wafer can be improved by balancing an injection gas rate against a gas consumption rate.
To make the throughput of the single wafer processing type CVD apparatus equal to that of the batch type, the film-forming rate must be increased to a few tens-a hundred and a few tens nm/min, and thus the film must be formed at a higher film-forming temperature under higher pressure than those of the batch type. The single wafer processing CVD apparatus has had such an inevitable problem as poor step coverage. JP-A-10-74703 discloses a method of improving the step coverage by using a high total pressure (20xe2x88x92300 Torr=2666xe2x88x9239990 Pa), a high silicon-containing gas partial pressure (4xe2x88x9240 Torr=533.2xe2x88x925332 Pa) and a low temperature (550xc2x0-620xc2x0 C.).
An object of the present invention is to solve the following problems of the prior art.
In the lamp heating type apparatus as disclosed in JP-A-6-326078, JP-A-10-144619, JP-A-6-293595, etc., the lamp cannot be provided too nearer to the wafer or susceptor as a heating target, so that it is difficult to appropriately control distribution of heating rate to the wafer or susceptor. For example, there is such a problem that even if the heating rate is increased only in the peripheral region of high heat radiation, its influence will be propagated even to the central region, resulting in difficulty in making the wafer temperature uniform.
In the heater-heating type apparatus as disclosed in JP-A-9-45624, it is necessary to increase the heat transfer rate to the wafer by elevating the temperature of susceptor (which is referred to as xe2x80x9cmounting basexe2x80x9d in the reference) in the peripheral region of high heat radiation to make the wafer temperature uniform, as mentioned in the reference. However, it is difficult to elevate the temperature only of susceptor having a high heat conductivity and a thickness as large as a few centimeters, and the temperature of the central region is also elevated to a higher level by heat transfer through the susceptor than as required. Furthermore, the heat capacity of the susceptor is so large that the susceptor surface temperature cannot be elevated rapidly even by increasing the heat generation rate of heater, and it is difficult to change a heat transfer rate to the wafer with a good responsibility. Therefore, it is difficult to make the wafer mounted on the susceptor reach a desired temperature for a short time and there is such a problem that the susceptor temperature is gradually changed during the continuous treatment of wafers. Still furthermore, no consideration has been paid to the fact that a film is deposition also on the susceptor surface acting to secure the wafer by electrostatic force. Particularly in case that a conductive film such as a phosphorus-doped silicon film is deposited on the upper side of a susceptor so as to cover electrodes having different polarities of a dipole type electrostatic chuck, charges migrate through the deposited film, resulting in such a problem failure to secure the wafer by electrostatic force.
As a result of numerical simulation studies based on information disclosed in JP-A-10-74703, the present inventors have found that only control of film-forming conditions within said range has a limit to improvement of step coverage, and no desired step coverage can be obtained at such film-forming rates as required by the present inventors.
FIG. 23 shows results of simulation of relations between film-forming rates and step coverage (film thickness at hole inside/film thickness in flat partxc3x97100%) when a silicon film is formed by introduction of monosilane (SiH4), phosphine (pH3) and hydrogen (H2), where hole aspect ratio is set to 2. Continuous line shows a case that total pressure is 13320 Pa and monosilane partial pressure is 600 Pa, dotted line shows a case that total pressure is 21310 Pa and monosilane partial pressure 1000 Pa, and alternate long and short dash line shows a case that total pressure is 42620 Pa and monosilane partial pressure is 2000 Pa. Mark xe2x80x9c◯xe2x80x9d shows film formation at the wafer temperature of 580xc2x0 C., xe2x80x9cxcex94xe2x80x9d shows film formation at the wafer temperature of 600xc2x0 C. and xe2x80x9cxe2x96xa1xe2x80x9d shows film formation at the wafer temperature of 620xc2x0 C. Comparison at the same film-forming rate reveals that the step coverage can be improved by increasing a monosilane partial pressure from 600 Pa to 1,000 Pa and further to 2,000 Pa and by decreasing a wafer temperature from 620xc2x0 C. down to 600xc2x0 C. and further down to 580xc2x0 C., but comparison atthe same temperature reveals that the step coverage is lowered with increasing monosilane partial pressure and the upper limit of step coverage is lowered with increasing film-forming rate, resulting in a failure to attain 90% or higher step coverage at such a film-forming rate of 30 nm-100 nm/min as required by the present inventors.
Such lowering of step coverage is due to the active gas formed by gas phase reaction of feed gases, that is, the intermediate having a higher deposition probability to the wafer than that of feed gases. In case of monosilane, silylene (SiH2) is formed as an intermediate by gas phase reaction, and the deposition probability of silylene is very high, i.e. about 1.0, irrespective of temperatures, whereas that of monosilane is 10xe2x88x926, 10xe2x88x927 at 600xc2x0 C. The intermediate of a high deposition probability deposits thick around the hole openings and deposition does not so much take place, at the inside of the holes, as disclosed in xe2x80x9cFlows of Atoms and Molecules, compiled by the Japan Society of Mechanical Engineers, pages 192-198 (1996)xe2x80x9d, and thus the step coverage is lowered. FIG. 24 schematically shows film formation at the inside of a hole, where modes of silicon film formation at the inside of the hole composed of silicon oxide film are shown. Actually, film formation by monosilane and that by silylene proceed simultaneously, but for simplicity""s sake the thickness of film formed by deposition of monosilane on the surface and that formed by deposition of monosilane on the surface and that formed by deposition of intermediate silylene are discriminately shown in FIG. 24. FIG. 24(a) relates to a case of less silylene in the gas phase, where the film formation by feed gas monosilane takes place predominantly, whereas the film formation by intermediate silylene takes place much less, resulting in a good step coverage. In case of FIG. 24(b) of more silylene, the film formation by feed gas monosilane takes place much less, whereas the film formation by intermediate silylene takes place predominantly, so that the film thickness around the hole openings becomes larger, whereas the film thickness at the inside of the hole becomes smaller, resulting in lowering of the step coverage. That is, to improve the step coverage, the formation rate of an intermediate must be reduced.
The higher the gas temperature, or the broader the gas high temperature zone, or the higher the pressure, the more easily the intermediate can be formed. The formation rate of the intermediate is more influenced by temperature than to pressure. Thus, as disclosed in JP-A-10-74703, it is also possible to increase the step coverage by elevating the pressure, so long as the wafer temperature is lower.
However, the step coverage attainable by such a method has a limit, resulting in a failure to satisfy such requirements as made by the present inventors, as already mentioned above.
Particularly in these years, the aspect ratio of hole or trench to be filled has been higher and higher and doping with an impurity such as phosphorus, etc. at a higher concentration ( greater than 1xc3x971020 atoms/cm3) has been in demand, while a much higher film-forming rate has been required. Thus, said limit to the step coverage has been a more and more important problem. A process and an apparatus for producing a CVD film with a distinguished step coverage at a high film-forming rate even against a high aspect ratio and a high impurity concentration uniformly with a good reproducibility have been desired in these situations.
The present invention has been established to solve the aforementioned problems. A first object of the present invention is to provide a process for producing a semiconductor with improved step coverage, film-forming rate, uniformity of film-forming rate in the in-plane region of a wafer (uniformity of temperature in the in-plane region of the wafer during the film formation and film-forming reproducibility at every wafers (temperature reproducibility).
A second object of the present invention is to provide an apparatus for producing a semiconductor suitable for conducting the process for producing the semiconductor.
These objects can be attained by forming a film under suitable pressure capable of attaining a desired film-forming rate while narrowing a gas phase zone were the gas is heated to high temperatures to control the formation rate of the intermediate.
For example, these objects can be attained in the film-forming step by placing a wafer on a susceptor provided in a treating chamber under pressure kept to 1,000-50,000 Pa, maintaining a susceptor temperature at 500xc2x0 C. or higher and supplying a feed gas under silicon-containing gas partial pressure of 200-5,000 Pa into the treating chamber from a position by 1-20 mm high above the wafer at a flow rate of 500-50,000 sccm.
The feed gas flow rate may be expressed by 0.7-80 sccm/cm2, a quotient of feed gas flow rate divided by wafer area on which a film is to deposit.
Since the feed gas is supplied at a high flow rate from a shower plate so as to be injected onto the wafer in the present process, the high temperature zone in the gas phase can be narrowed to control formation of the intermediate, thereby improving the step coverage. Furthermore, the necessary film-forming rate can be obtained by controlling the pressure during the film formation.