Non-contact wafer thickness measurement of capacitance sensor circuit based on CAV 424

Non-contact wafer thickness measurement with the CAV424 capacitance sensor special integrated circuit and arc pole plate capacitor sensor has good stability and linearity under low capacity of the bottom of sensor and low C condition. This method has a high technical advantages and practical value.Two capacitance sensors Cb, Ca measurement spacing 4mm install at the same axis which constitutes the size condition for measuring thickness. The static capacity of Ca and Cb is a constant value. The capacity of Cb and Ca will change when the silicon wafer is involved. This change is checked by the CAV424 capacitive sensor which has better linearity and higher thickness


INTRODUCTION
Semiconductor silicon non-contact thickness measurement has higher difficulty and practical value.
Since the silicon ingot cutting with a silicon wafer has the thickness difference even after polishing.At present it is hard to measure the thickness from several points of the silicon using the mechanical method which has a large error.Also, the force is not easy to control and it will damage the silicon easily.The application of capacitor non-contact measurement is one of the nondestructive technique [1].FUNDAMENTAL There are many kinds of measuring circuit for capacitance sensor [1]   According to the formula, the capacitance C of sensor is changed by the distance, dielectric constant and area.
When the dielectric constant and the area of the plate are fixed, the distance is the single value function of capacitance variable C. The characteristic capacitance sensor applications can measure effectively. [9] [10]

BRIEF INTRODUCTION OF CAV424
CAPACITANCE SENSOR CIRCUIT [4] In Figure 3, almost all of the capacitance sensors use differential capacitance method to obtain a linear degree and a larger signal since the capacity of the capacitance sensor varies in a range of pF or up to several hundred Pf.In some applications, the unit of C is also several pF.In general, a fixed capacitor is chosen as a reference, and the other one is used to measure and amplify the differential signal.

THE WORKING PRINCIPLE OF CAV424 [4]
A reference oscillator with adjustable capacitor COSC frequency drives two symmetric structure of integrator and synchronizes in the same time and phase.

APPLICATION OF CAV424 4.1. APPLICATION DESIGN OF CAV424
The circuit is shown in Figure 6.After the decimal point is retained 3, the thickness of the test is 0.243mm, which is 0.001mm larger than the sample.
According to formula (7) test results show that silicon in the D timeline does not affect the accuracy of the test data.Therefore, the method based on CAV424 capacitance sensor circuit of non-contact wafer thickness measurement has high technical feasibility and practical value.

Conclusion
After practical application, CAV424 capacitance sensor application specific integrated circuit of non-contact wafer thickness testing system design and test device structure has high precision, simple circuit, convenient adjustment, and extremely high performance price ratio.
The design has been applied in non-contact thickness measurement function of multifunctional solar silicon wafer tester.
such as Tightly coupled inductor arm bridge, transformer bridge,double T bridge circuit, differential pulse width modulation circuit, frequency modulation circuit.In this paper, the principle of non contact thickness measurement of capacitance sensor is shown in Figure 1.Two capacitive sensors a and b are spaced at a fixed distance D. At this point the capacitance of a and b is a based static value Ca and Cb.The thickness of the silicon wafer in the D interval is T. It raises the capacitance of Ca and Cb which is processed by the CAV424 capacitance sensor analog circuit and gets an voltage output signal Vb out and Va out.The difference of voltage indicates the difference of capacity Ca and Cb which is related to the thickness T. Since the CAV424 circuit is the differential input and Vc constant current integral form, its linearity is excellent and the resolution can reach 0.1um.D is a fixed interval.To obtain the thickness T of the silicon wafer: 1 The a and b in the formula are the distance from the capacitance sensor a or b to the silicon wafer.

Figure 3 the internal structure of capacitance sensor circuit 3 . 1 CAPACITANCE INTEGRATOR [ 4 ]
Figure 3 the internal structure of capacitance sensor circuit

Figure 4
Figure 4 the output voltage of Zeit= two integrator First, the voltage (VCM) and VCR of two capacitors Cm and Cr are subtracted through the signal processing circuit.Adder-subtractor with the function of rectification also eliminated clamp fixed voltage VCLAMP.A DC voltage signal VTPAS is generated at the output end after filtering.

Fig. 5
Fig. 5 current block diagram with signal processing

Figure 6
Figure 6 application circuit diagram Figure 7 CAV424 application PCB diagram

Figure 9 4 According to the formula 1 T
Figure 9 linear regions of test data