## Introduction

In the differential geometry, the countdown of the curvature is the radius of curvature, ie r = 1 / k. The curvature of the planar curve is to define the rotation rate of the arc length for a certain point in the curve, defined by differential, indicating that the curve deviates from the straight line. For a curve, it is equal to the radius of the arc closer to the curve at this point. For surface, the radius of curvature is the radius of a circle that is suitable for normal cross sections or thereof.

The radius of curvature is mainly used to describe the degree of curve change in curve on the curve. Special as: the degree of bending degree on the circle is the same, the radius of curvature is the radius of the circle; straight line is not Bending, and the radius of the circle in this point can be arbitrarily, so the curvature is 0, so the line does not have a radius of curvature, or the radius of the curvature is

The larger the radius of the circle, the smaller the degree of bending, the more like a straight line. Therefore, the larger the radius of curvature, the smaller the curvature, and vice versa.

If a point equal to the curvature is found for a certain point, the radius of the curvature on the curve is the radius of the circle (note, the radius of curvature of this point) Other points have other radius of curvature). It can also be understood that it is as possible to differentiates the curve until the last approximation is a circular arc, which is the radius of the arc is the radius of curvature on the curve.

## Formula Detecting

In the case of a space curve, the radius of curvature is the length of the curvature vector. In the case of a planar curve, then R is to take an absolute value.

where S is the arc length of the fixed point on the curve, α is tangential angle, K is the curvature.

If the curve is expressed as

If the curve is given by the function

If

As special case, if f (t) is a function from R to R, the radius γ (T) of the figure thereof is = (t, f (t))

### semicircle

For the half round of the upper half-plane radius A:

For the half circle of the upper half-plane radius A:

radius A circle of curvature is equal to A.

### Ellipse

In the ellipse having long axis 2a and the short axis 2b, the vertices on the long axis have any point of the minimum curvature radius,

and the vertices on the short axis have any point of the maximum radius of curvature

## Application

(1) For applications, see the CESàro equation;

(2) for the radius of curvature of the Earth (approximated by elliptical ellipse Please refer to the radius of curvature of the Earth;

(3) The radius of curvature is also used in the bending three parts equation of the beam;

(4) radius of curvature (optical).

(5) Stress in the semiconductor structure:

relates to the stress in the semiconductor structure of the evaporation film is typically from thermal expansion (thermal stress) during the manufacturing process. The occurrence of thermal stress is because the membrane deposition is usually over room temperature. When cooling from the deposition temperature to room temperature, the difference in thermal expansion coefficient of the substrate and the film causes thermal stress.

When the atom is deposited on the substrate, the microstructure formed by the film causes inherent stress. Since the atoms pass through the void attractive interaction, the micropores in the film produces tensile stress.

The stress in the film semiconductor structure causes warpage of the wafer. The radius of curvature of the stress structure is related to the amount of stress tensor in the structure, and can be described with amended STONEY formula. The morphology of the stress structure including the radius of curvature can be measured using an optical scanner. The modern scanner tool has the ability to measure the full picture of the substrate and measure the radius of two main curvature, and provides 0.1% accuracy of the radius of the radius of 90 meters and above.