Method of Forming Polysilicon Having a Desired Surface Roughness

1. The Field of the Invention

The present invention relates to the manufacture of semiconductor devices. More particularly, the present invention is directed to a method of forming polysilicon having a crystal structure with a desired surface roughness useful in the manufacture of semiconductor memory devices.

2. The Relevant Technology

Polysilicon is used in a variety of ways in the design and fabrication of integrated circuits. Polysilicon is used in forming gates, contacts, capacitors, and many other circuit structures.

The capacitance of polysilicon is influenced significantly by its structure. The surface roughness of polysilicon must be consistently controlled to achieve desired capacitance. Capacitance of capacitors constructed with polysilicon generally increases with surface roughness because of increased surface area.

Consistency in achieving desired polysilicon surface roughness in the production fabrication environment has proved somewhat difficult.

An object of the present invention is to provide a method of producing polysilicon of a desired surface roughness, said method providing improved consistency and control.

Another object of the present invention is to provide a method of detecting the conversion of amorphous silicon to rough polysilicon during said conversion.

Still another object of the present invention is to provide a method of detecting the conversion of rough polysilicon to smooth polysilicon during said conversion.

Still another object of the present invention is to provide a method of forming polysilicon having a desired surface roughness by detecting the desired surface roughness during the formation thereof and stopping the formation process upon formation of the desired surface roughness.

In accordance with the method of the present invention, polysilicon of a desired surface roughness is reliably and repeatably formed by providing a layer of doped or undoped amorphous silicon on a substrate and heating the substrate while monitoring the emission of the substrate relative to an expected emission attributable to the heating regime employed. An increase in the monitored emission not attributable to the heating regime signals the transition from amorphous silicon to rough polysilicon. A decrease of the monitored emission not attributable to the heating regime signals a transition to smooth polysilicon. The increase and decrease in the monitored emission can be used to end the heating regime at the time at which the desired surface roughness of polysilicon is formed, or merely to passively monitor the process.

The in situ monitoring of the present invention provides the advantages of greater control, reliability, and repeatability in forming polysilicon having a desired surface roughness and electrical and other characteristics related thereto.

These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.

In order that the manner in which the above-recited and other advantages and objects of the invention are obtained may be more fully explained, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

FIG. 1 is a schematic representation of some materials and equipment useful in the method of the present invention.

FIG. 2 is a graph of an emission of seeded amorphous silicon measured by a single color pyrometer over time.

FIG. 3 is a graph of a reflectivity curve of the amorphous silicon resulting after the processing represented in FIG. 2.

FIG. 4 is a graph of a measured emission of seeded amorphous silicon over time, beginning with amorphous silicon and converting to rough polysilicon.

FIG. 5 is a graph of a reflectivity curve of the amorphous silicon resulting after the processing represented in FIG. 4, with processing terminating in a region A.

FIG. 6 is a graph of a reflectivity curve of the rough polysilicon resulting after the processing represented in FIG. 4, with processing terminating in a region C.

FIG. 7 is a graph of a measured emission of amorphous silicon over time, beginning with amorphous silicon and converting to smooth polysilicon.

FIG. 8 is a graph of a reflectivity curve of the silicon produced after the processing represented in FIG. 7, with processing terminating after the end of a region C.

The present invention provides an improved method for forming polysilicon having a particular desired crystal structure and surface roughness. The present invention is particularly useful in reliably producing hemispherical grain (HSG) polysilicon for improved capacitance.

According to a preferred embodiment of the method of the present invention, an amorphous silicon layer is provided on a substrate. The amorphous silicon layer may be doped or undoped, and implanted if desired. The amorphous silicon layer may be produced by any suitable means including CVD using a precursor such as Si.sub.2 H.sub.6, and PVD, The substrate is then heated to a preselected temperature and seeded for a length of time, then heated to and maintained at a second temperature of at least 100.degree. C., where the second temperature is preferably within the range of about 500.degree. to 1000.degree. C. The emission of the substrate with the amorphous silicon layer thereon is measured at a wavelength somewhere within the range of about 10.sup.-2 to 100 .mu.m, and is preferably within the range of about 1 to 10 .mu.m. The second temperature may optionally be the same as the seeding temperature.

The means employed to heat the amorphous silicon can be any conventional means including rapid thermal anneal (RTA) devices and conventional devices for providing deposition environments, such as CVD, PECVD, and LPCVD reactors and the like. The amorphous silicon may be heated in a vacuum or at ambient pressure. Typically known means other than seeding may also be used to induce crystal formation. Whatever heating means is employed, the temperature is preferably controlled by a temperature control means independent of any temperature feedback from the substrate or amorphous silicon layer itself, although feedback temperature control may optionally be used, as described hereinafter.

While the amorphous silicon layer is maintained at the second temperature, the emission of the amorphous silicon layer is monitored by suitable means, such as a single color pyrometer. As the amorphous silicon transforms to rough polysilicon (HSG) or to smooth polysilicon, the curve traced over time by the monitored emission undergoes specific changes. These changes allow the detection of the transition from amorphous silicon to rough polysilicon or from amorphous silicon to smooth polysilicon. This detection capability may be used merely to monitor a process for forming polysilicon, or the output of the emission measuring device may be coupled directly into a controller for closed loop feedback control of the surface roughness formation process. With feedback control, inherent process variability can be minimized or avoided by stopping the process and cooling the polysilicon at the moment a desired surface roughness is reached.

FIG. 1 shows some equipment and materials useful in the method of the present invention. A substrate 12 in the form of a silicon wafer has had a layer of amorphous silicon deposited on both the front and back sides thereof. Substrate 12 and the amorphous silicon layer are heated by suitable means, and crystal formation is induced by suitable means. An emission detector 20 in the form of a single color pyrometer is placed so as to detect emissions from the back side of substrate 12. Front side emission detection may also be used. Emission detector 20 may be coupled to a controller 22 for closed loop feedback process control.

FIG. 2 shows a measured emission curve E measured by a single color pyrometer arranged as illustrated in FIG. 1. The substrate with a layer of amorphous silicon thereon was first seeded for 20 seconds at a first temperature of 630.degree. C., then annealed for 4 minutes at a second temperature of 685.degree. C. Measured emission curve E is shown on a y-axis scale of degrees Celsius according to the calibration of the single color pyrometer, with the x-axis representing seconds.

Measured emission curve E of FIG. 2 is flat or nearly flat during the time the substrate is held at the second temperature. Measured emission curve E of FIG. 2 is thus characteristic of the heating regime employed. No changes appear in measured emission curve E which are not accounted for by the heating regime.

FIG. 3 shows a measured percent reflectance curve R, as a function of wave length, of an amorphous silicon layer after processing according to the heating regime represented in FIG. 2. An upper characteristic curve U and a lower characteristic curve L for the reflectance of smooth polysilicon are also displayed. The x-axis is scaled in nanometers (nm). Reflectance curve R between 200 and 400 nm shows the characteristics of amorphous silicon, demonstrating that the processing according to the heating regime of FIG. 2 did not convert amorphous silicon into rough or smooth polysilicon.

FIG. 4 is another graph of a measured emission curve E measured by a single color pyrometer arranged as in FIG. 1. A substrate having an amorphous polysilicon layer thereon was seeded for 30 seconds at a first temperature of 660.degree. C. and then annealed for 80 seconds at a second temperature of 685.degree. C. At the beginning of the anneal at the second temperature, measured emission curve E, in region A thereof, is the same as the characteristic curve expected due to the temperature regime alone. But in region B of measured emission curve E, the measured emission increases. In region C, the measured emission remains at the higher level reached in region B. The increase in the measured emission at region B has no corresponding increase in the characteristic emission curve, which is the emission expected from the heating regime employed. The emission change measured in region B is related to the formation of rough polysilicon, as shown below.

FIG. 5 is a graph of the measured reflectance of an amorphous polysilicon film processed according to the heating regime illustrated in FIG. 4, but with the processing not completed as shown in FIG. 4, but terminated instead in region A of measured emission curve E. Reflectance curve R of FIG. 5 is characteristic of amorphous silicon, showing that the amorphous silicon layer remains amorphous during region A of FIG. 4.

FIG. 6 is a graph of the measured reflectance of an amorphous polysilicon film processed according to the heating regime illustrated in FIG. 4, with the processing terminated in region C of FIG. 4. The extremely low reflectance exhibited between 200 and 400 nm is characteristic of rough polysilicon or HSG, showing that by the time of region C of the measured emission curve of FIG. 4, the amorphous silicon layer has been converted into rough polysilicon.

FIG. 7 is yet another graph of a measured emission curve E measured by a single color pyrometer arranged as in FIG. 1. A substrate having an amorphous polysilicon layer thereon was seeded for 5 seconds at a first temperature of 710.degree. C. and then annealed for 45 seconds at a second temperature identical to the first, 710.degree. C. At the beginning of the anneal, measured emission curve E, in region A thereof, is the same as the characteristic curve expected due to the temperature regime alone. But in region B of measured emission curve E, the measured emission increases. In region C, the measured emission decreases.

The increase in the measured emission at region B has no corresponding increase in the characteristic emission, which is the emission expected from the heating regime employed. Likewise, the decrease region C has no corresponding decrease in the characteristic emission expected from the heating regime employed. The emission change measured in region B is related to the formation of large grained polysilicon, as noted above, while the emission changes measured in region C are related to the formation of small grained or smooth polysilicon, as shown below. A decrease such as that in region C may also be observed without any prior increase.

FIG. 8 is a graph of the measured reflectance of an amorphous polysilicon film processed according to the heating regime illustrated in FIG. 7, with the processing completed as shown in FIG. 7. Reflectance curve R of FIG. 8 is characteristic of smooth polysilicon, demonstrating that the amorphous silicon layer undergoes a transition during region C of FIG. 7 from the rough polysilicon formed in region B to smooth polysilicon.

The increases and decreases in the measured emission illustrated above may be used to detect the formation of the desired smoothness or roughness of polysilicon, upon which the heating of the substrate and polysilicon layer may be terminated. Alternatively, the measured emission may be monitored only as a control to evaluate the process, without direct feedback. Other heating regimes than those above may be employed.

The exact mechanisms for causing the characteristic increase in emission during the transition to rough polysilicon illustrated above have not been determined. The increase in emission seen in region B may be due mostly to an increase in emissivity, or to increased emissivity and an actual increase in temperature. Both the emissivity and the absorption characteristics of a material can change with changes in the surface thereof. It is thus possible that an increase in both temperature, due to increased absorption, and an increase in emissivity may occur. While a single color pyrometer is currently preferred for monitoring the emission changes, other instruments, including dual or multi-color pyrometers may be employed.

While a heating mechanism employing environmental temperature control is presently preferred, rather than one employing feedback temperature control using feed back from the substrate, a feedback temperature control system may nonetheless be employed in the present invention. It is preferred that the feedback temperature control be sensitive to actual temperature only, and not to emissivity changes, while the emission detector employed is sensitive to all emission changes. Even with a constant temperature maintained at the substrate by a feedback controller, the emissivity changes would then be detected by the emission detector as an increase in emission from the substrate, signalling the formation of rough polysilicon.

The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.