System for Controlling Ion Implantation Dosage in Electronic Materials

FIG. 1 is a functional block diagram of the wafer process control system according to the invention;

FIG. 2 is a fragmentary plan view of one geometry for the ion beam masking and the current sensor apparatus for the system in FIG. 1;

FIG. 3 is a schematic diagram of the ion beam current sensing and integrating circuitry of FIG. 1.

Referring now to FIG. 1, there is shown a motor driven wafer supporting member or tray 10 which carries a plurality of individual wafers, such as wafers 12 and 14, which have been previously loaded onto member 10 at desired locations by an operator. This support member 10 is operatively positioned to move beneath a mask 16 having an opening 18 therein of predetermined geometry as will be further described below. Both the mask 16 and a pair of current sensors 20 and 22 are exposed as shown to an impurity ion beam 24 of a predetermined pattern or raster such that, for a given beam current, the average number of ions over each portion of the raster per unit area per unit time is uniform. Only the ions passing through the mask opening 18 are implanted in the wafers on the tray 10.

The two current sensors 20 and 22 may advantageously be Faraday cups which have well-known current sensing characteristics and which collect sampled portions of the ion beam 24 currents. These currents are conducted through a common electrical circuit node 26 and into the input of an integrator 28. While the separate peripheral sensors 20 and 22 are preferred, the current collected by support member 10 could also be used to drive the integrator 28. The above construction which employs the separate sensors 20 and 22 removes the wafers from the electrical circuit of the integrator 28, permitting the member 10 to be grounded, and eliminating any effects of secondary electrons emanating from the wafers. Thus both mechanical and electrical simplicity are enhanced by the use of this preferred embodiment of the invention. The schematic details of this integrator are disclosed below with reference to FIG. 3.

The integrator 28 provides multiple successive integrations on the signal current received from the common input node 26 and generates a plurality of corresponding output pulses 30 on line 31 as shown. A pulse 30 is generated each time the voltage level at the input of the integrator 28 exceeds a predetermined threshold level. The control pulses 30 are utilized to drive a conventional transistor logic circuit stage 32, which stage 32 may advantageously include a Schmitt trigger. The Schmitt trigger is a well-known circuit which switches from one to another of two discrete output voltage levels once its input switching threshold is exceeded. Simultaneously, the logic circuitry 32 provides a feedback discharge pulse via line 33 to the integrator 28 and there serves to bias appropriate semiconductior circuitry to conduction and discharge the integrating capacitors of stage 28. This action enables the integrator 28 to provide many successive integrations of the input signal current received from circuit node 26. In order to program a desired ion implanted dose, the exact value of dose represented by each of these integrations may be varied in accordance with the integrator calibration techniques described below with reference to FIG. 3.

After suitable amplification, the output pulses 33 of the digital logic circuitry 32 are coupled to the input drive terminal of a conventional stepping motor 34 which incrementally drives the movable wafer support member 10 by suitable mechanical means 36. This means may be a motor shaft (not shown) which is directly or indirectly coupled to the support member 10 to rotate same at a prescribed rate in response to the mechanical output of the stepping motor 34. The closed loop control system of FIG. 1 is calibrated by a simple adjustment of the integrator 28, so that each integration represents a definite incremental ion dose delivered to the implanted target area through which the wafers pass. The length of time that wafers are exposed to the mask opening 18 for each incremental step is inversely proportional to the average current flowing from the input node 26 to the integrator 28. This current is in turn proportional to the ions per square centimeter beam dosage received by the Faraday cup current sensors 20 and 22, so that the ion beam exposure time for the semiconductor wafers 12 is incrementally controlled as a direct function of the ion beam dosage received by these wafers.

The support member 10 may, for example, be a straight continuous conveyor belt, in which case the mask opening 18 will be rectangular. In the alternative, the movable member 10 may be an annular belt or table which is rotatable about a central axis, and in the latter example, the opening 18 will have a pie-shaped geometry, such as that of the opening 18' in FIG. 2. The advantageous features of this annular construction are described below.

Referring now to FIG. 2, the annular support member or tray 10 rotates about a central axis 37, and the pie-shaped mask opening 18' accommodates the radial differences in linear speed of the wafers 12 on tray 10. The sides of the pie-shaped opening 18' should intersect at the rotational axis 37 as shown in FIG. 2. Thus, the wafers closer to the rotational axis 37 move slower through a given ion beam pattern and therefore need to be exposed to a smaller segment of a given pattern than do the more remote wafers. In this manner, all of the wafers 12 will receive the same total ion dosage while they pass beneath the mask opening 18'. The mask 16 may be secured to a stationary housing 40 by any suitable means. Wafers may be packed as densely as possible onto the exposed annular area of the tray 10. However, in order to have room to set up and start the ion implantation process, a pie-shaped area on the tray corresponding to the opening 18' must initially be left empty. Thereafter, no other spaces need be left empty as long as the ion implantation continues. In the case of a continuous straight conveyor belt, a corresponding rectangular space should be left empty initially.

The implanted area on the member 10 exposed at any instant in time by the mask opening 18' is limited in one dimension by the corresponding dimenson of the ion beam raster 24. This dimension is in a direction at right angles to the mechanical sweep of the moving support member 10. However, in the direction of the mechanical sweep, the implanted area is unlimited, which means that the entire annular surface region of the annular member 10 in FIG. 2 can, if desired, be implanted in one continuous operation. Furthermore, in a linear system using an endless conveyor belt, this dimension of implant exposure is equally unrestricted, and so far as I am aware, these implant area dimensional features of my invention as illustrated in FIG. 2 are totally novel.

Since the ion beam 24 defining the raster is swept in both the X and Y mutually perpendicular directions, then the uniformity of the ion implantation density and area distribution is enhanced, and this latter feature can be particularly advantageous in the fabrication of certain high grade ion implanted semiconducted devices. Additionally, the net relative motion between the raster and the moving tray 10 provides further enhancement of the doping uniformity. However, in instances where it is preferred not to employ the two-axis high voltage ion beam scanners required for the X-Y raster, one may choose to rely entirely on scanning the ion beam in only a single direction which is at right angles to the mechanical sweep of the moving support member 10.

Referring now to FIg. 3, the Faraday cup 20' represents one or more of the ion beam current sensors, and the current from the latter charges one or more input integrating capacitors 42 and 44 connected as shown at one differential input terminal 46 of a conventional differential operational amplifier 48. When the voltage level on the input terminal 46 exceeds the DC threshold level on the other input terminal 50, an output pulse is generated on the output line 31 and is coupled therefrom to the input of the digital logic circuitry 32. As previously mentioned, the logic circuitry 32 preferably includes a known Schmitt trigger-type circuit which provides a square wave digital output pulse train in response to the nominally triangular shaped pulses at the output of the operational amplifier 48.

Simultaneously, the logic circuitry 32 is utilized to generate a capacitor discharge feedback pulse on line 54 which couples into the gate electrode of a junction field. effect transistor 56. This field effect transistor 56 is connected as shown in parallel with the input capacitors 42 and 44 of the operational amplifier 48, and this FET 56 conducts and thus rapidly discharges these active capacitors each time the logic circuitry 32 is switched by the output signal of the operational amplifier 48. In this manner, rapid incremental integration of the input signal current from the Faraday cup 20' is achieved; and the number of integrations per unit time provided by the integrator 28 and thus the total dose delivered to the wafers can be closely controlled by the appropriate choice of the size or number (or both) of the integrating capacitors 42 and 44. These capacitors are only representative of any desired plurality of input capacitors corresponding to the necessary number of calibration range settings of the integrator 28.

The input terminal 50 of the differential operational amplifier 48 is connected to a variable tap 57 on a level adjustment potentiometer resistor 58. By varying the position of this tap 57, the DC threshold voltage on the input terminal 50 and thus the switching level of the operational amplifier 48 can be varied. Thus, the time at which the output pulse 30 appears at the output of the operational amplifier 48 may be controlled both by varying its input capacitance and by changing its DC set point or reference voltage applied to terminal 50.

In accordance with the present invention, it is possible to make an absolute calibration of the integrator 28, and one needs to know only the value of shunt capacitance at the input of the operational amplifier 48 and the voltage to which this capacitance is being charged. The latter quantities, of course, can be directly correlated to the ion doping density, so that an unskilled operator may select a particular ion dosage range by choosing a corresponding capacitor setting which connects one or more of the capacitors 42 and 44 to the operational amplifier 48. Thereafter, the potentiometer tap 57 may be varied to change the ion doping density within this previously chosen range, and this potentiometer tap 57 may be easily controlled by a dial or the like. This extreme simplicity of both the integrator 28 design and its above calibration is particularly advantageous, both from the standpoint of system design costs and operational costs as well.

In a control system of the type described above which was actually built and successfully operated, each incremental dose was 1/1400 of the required programmed dose for the ion implanted wafers 12. Thus, the ion dose necessary to step the motor 34 once was a small but definite fraction of the total required dose, and this above fraction may be increased or decreased by design for a particular fabrication process. As the limit of the incremental ion dose becomes infinitely small, the above ion dosage control system becomes continuous, and in a truly continuous system, the motor stepping rate becomes a smooth actual velocity and the incremental dose measurement becomes a measure of instantaneous current. Thus, the present system may, if desired, be modified by those skilled in the art to operate as a smooth continuous system, and such modification would clearly be within the spirit and scope of this invention.

The present invention is not limited to the ion implantation in semiconductive materials, and may be used to control any process where it is desired to control the introduction (dosage) of accelerated charged particles into electronic or other materials to change their electrical characteristics.

It should also be understood that the present invention is not limited to the particular relative movement set up in FIG. 1 wherein a target is moved relative to a fixed ion beam. Obviously, the present invention could be employed for use with a fixed wafer support member 10 and a movable on beam by altering the mechanical drive coupling 36 so that the output of the stepping motor 34 would drive the ion source rather than the wafer support member 10. 9n