High quality oxide on an ion implanted polysilicon surface

A method for growing a high quality oxide layer on the surface of a polysilicon film for use as an interpoly dielectric. The method comprises depositing a silicon film on a wafer and implanting the silicon film with phosphorous ions. The wafers are then sent into a diffusion tube to activate the dopant. This operation is carried out in an ambient of dry oxygen and the result is the silicon film is now polysilicon and an oxide layer has been grown on the polysilicon film. The wafers are then implanted using argon ions, the implantation is carried out through the oxide layer. The result is the surface layers of the polysilicon layer are rendered amorphous. The oxide layer grown on the polysilicon film is then removed. A new oxide layer is then grown on the polysilicon film. The result is an oxide layer with excellent physical and electrical characteristics.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the field of growing high quality oxides on the 
surface of ion implanted polysilicon. 
2. Prior Art 
In the field of growing oxide layers on the surface of polysilicon for use 
as an interpoly dielectric, it is desired to grow an oxide layer with 
improved electrical and physical characteristics. There are three 
generally followed methods of growing oxides on an ion implanted 
polysilicon layer having a variety of disadvantages. 
In a first method, the polysilicon is deposited by a chemical vapor 
deposition (CVD) process at a temperature of above approximately 
600.degree. and diffusion doped using a gaseous (generally POC1.sub.3) 
source. The oxide layer is then thermally grown on the film surface. This 
method suffers from several serious problems. First, the upper surface of 
the polysilicon film is rough, leading to poor electrical characteristics 
in the oxide layer grown on the polysilicon film due to localized field 
enhancement. Second, the heavy doping level leads to dopant segregtion at 
the grain boundaries. This leads to non-uniform oxide thickness and 
results in poor electrical characteristics of the oxide. Third, the 
quality of the oxide grown on this polysilicon film is poor due to the 
incorporation of phosphorous from the grain boundaries and results in a 
low breakdown voltage of the oxide. Fourth, the growth of the polysilicon 
grains through the subsequent thermal cycles causes residual stress in the 
oxide which reduces the breakdown voltage further. 
A second method of developing the oxide layer is to deposit silicon by a 
CVD process at a temperature below approximately 580.degree. C. The 
silicon is then diffusion doped using a gaseous (generally POC1.sub.3) 
source. The oxide is then thermally grown on the film surface. Although 
low temperature deposited silicon film is initially smooth, it 
recrystallizes during the doping cycle resulting in very large grains. The 
thickness of the oxide layer grown on such a film varies from grain to 
grain because different crystal faces are exposed to the oxidation ambient 
by different grains. It is well known that different crystallographic 
faces oxidize at different rates. The nonuniform oxide thickness results 
in non-uniform electrical characteristics. In addition, the "steps" caused 
as a result at grain boundaries can act as charge injection sites. 
A third method involves deposition of a silicon film by a CVD process above 
approximately 600.degree. C. A dopant is then introduced into the film by 
means of an ion implantation process. The thermal oxide layer is then 
grown on the surface of this polysilicon film. This method also results in 
a rough upper surface of the polysilicon film. The oxide grown on it, 
therefore, has a low breakdown voltage due to localized field enhancement. 
SUMMARY OF THE INVENTION 
A method for growing a high quality oxide layer on the surface of an ion 
implanted polysilicon for use as an interpoly dielectric is described. The 
method involves deposition of a silicon film on an oxide layer on a 
silicon wafer using a chemical vapor deposition (CVD) process. The silicon 
film is then implanted using phosphorous ions. The wafers are sent to a 
diffusion tube to activate the dopant. This operation is carried out in an 
ambient of dry oxygen. The dopant is activated and the film is now 
polysilicon as a result of the recrystallization. The wafers are then 
implanted using argon ions. The implantation is carried out through the 
oxide layer. The surface layers of the polysilicon film are rendered 
amorphous as a result of this implant. This oxide layer is then removed 
and the wafer is again sent through a diffusion tube in an ambient of pure 
dry oxygen resulting in growing another oxide layer. 
The polysilicon layer resulting from this method has a dual structure. The 
upper surface of the film is smooth with small equiaxed grains. The 
physical and electrical properties of the oxide grown on this film are, 
therefore, very uniform. The overall doping level is relatively low 
resulting in little or no dopant segregation. The lower layer of the film 
has very large grains and the dopant activation is high and resistivity is 
low. 
Overall, the polysilicon film uniformity is excellent which results in 
reproducible electrical and physical properties.

DETAILED DESCRIPTION OF THE PRESENT INVENTION 
A process for formation of a high quality thermally grown oxide on the 
surface of an ion implanted polysilicon film for use as an interpoly 
dielectric is described. In the following description, numerous specific 
details are set forth such as processing parameters, types of materials, 
etc., in order to provide a thorough understanding of the present 
invention. It will be obvious, however, to one skilled in the art that the 
present invention may be practiced without these specific details. In 
other instances, well known structures and techniques have not been shown 
in detail in order not to unnecessarily obscure the present invention. 
The present invention discloses a method of forming a polysilicon film 
which has a dual structure, the upper surface of which is smooth with 
small grains. This surface allows an oxide grown on the film to have 
excellent physical and electrical properties. The lower layer of the film 
has very large grains as a consequent of low temperature deposition. The 
dopant activation is high and resistivity is low, thus this part of the 
film carries the major part of any current flowing through the film. In 
addition, the resistivity of the polysilicon film is very uniform across 
the wafer and the magnitude of the resistivity is a considerably lower of 
that obtained by a similar implant dose into silicon deposited in the 
polycrystalline form. This is due to the high level of dopant activation 
achieved using this approach. The low resistivity results in a significant 
advantage in creating high density circuits. 
Referring now to FIG. 1, a wafer as may be utilized by the present 
invention is disclosed. In the preferred embodiment, an oxide layer 11 is 
deposited on a single crystal silicon wafer 10 using any of a number of 
well known processing methods. The prior processing history of the wafer 
is immaterial to the present invention. 
An amorphous silicon film 12 is then deposited on the oxide layer as shown 
in FIG. 2. The deposition of the silicon film is carried out in any 
commercially available chemical vapor deposition (CVD) reactor by 
employing a deposition temperature of approximately 560.degree.. The 
thickness of the silicon film 12 depends on the application, in the 
preferred embodiment the silicon film is approximately 3000 angstroms 
thick. 
The amorphous silicon film 12 is then implanted using phosphorous ions as 
illustrated in FIG. 3. The implantation may be done with any commercially 
available ion implantation machine. In the preferred embodiment the 
phosphorous ions are accelerated to approximately 75 keV and the flux of 
the ions used in the preferred embodiment is approximately 6E14/cm.sup.2. 
The wafers are then sent into a diffusion tube to activate the dopant. This 
operation is carried out in an ambient of dry oxygen at a temperature of 
approximately 920.degree. C. As illustrated in FIG. 4, an oxide film 13 is 
grown as a result and in the preferred embodiment is approximately 280 
angstroms thick. The dopant is activated and the film 14 is now 
polysilicon as a result of recrystallization. 
As shown in FIG. 5, the wafers are then implanted using singly charged 
argon ions at an energy of approximately 40 keV in a flux of approximately 
6E14/cm.sup.2. The implantation of the argon ions is carried out through 
the oxide layer 13 and the surface layers of the polysilicon film 14 are 
rendered amorphous as a result of this implant. 
The oxide layer 13 is then removed as illustrated in FIG. 6. In the 
preferred embodiment the oxide layer 13 is removed by dipping the wafers 
in a solution of 5:1 water:HF. It will be obvious to one of ordinary skill 
in the art that the exact ratio of water to HF may be varied and that 
other methods of removing the oxide layer 13 may be utilized without 
departing from the spirit of the invention. 
The wafers are then sent through a diffusion tube and a new oxide layer is 
grown on the polysilicon film. In the preferred embodiment, the 
temperature in the diffusion tube is maintained at approximately 
1000.degree. C. and the ambient during the oxidation is pure dry oxygen. 
In the preferred embodiment of the present invention, argon is chosen as 
the implant species because it is an inert element and, therefore, does 
not affect the resistivity of the film. Since argon's atomic radius is 
significantly different than that of silicon, incorporating argon atoms 
into the silicon lattice results in a large amount of strain which retards 
the growth of silicon grains during subsequent high temperature 
processing. The argon ions are implanted through the oxide layer because 
implanting through the oxide results in the incorporation of small amounts 
of oxygen atoms in the silicon lattice. The amorphous oxide film prevents 
any channeling effect of Argon atoms during implant and allows the implant 
process to produce an amorphous layer with uniform thickness. The oxygen 
also stabilizes the small grains up to temperatures above 1000.degree. C. 
This helps to maintain the small grain size through subsequent processing. 
The second oxide layer grown on the polysilicon film has extremely uniform 
and excellent electrical properties for several reasons. First, the 
amorphous polysilicon film presents random orientations to the oxidizing 
ambient and the oxide grown on this surface is, therefore, extremely 
uniform. The wave length and amplitude of any undulations in the oxide 
thickness is very small. Second, the method precludes the segregation of 
dopant (phosphorous) at grain boundaries and therefore the oxide grown on 
the polysilicon is of high quality. The oxide sustains higher breakdown 
voltage and the number and density of oxide defects is minimized. 
An alternative to the preferred embodiment of the present invention 
consists of implantation of the argon ions without first growing the oxide 
layer on the polysilicon film. The polysilicon film is then implanted with 
oxygen atoms and an oxide layer is grown on the polysilicon film. This 
method still results in the incorporation of small amounts of oxygen atoms 
in the silicon lattice which helps to maintain the small grain size of the 
polysilicon film through subsequent processing. 
Thus, a method for growing a high quality oxide layer on a polysilicon film 
for use as an interpoly dielectric is disclosed.