Focus variation is a method used to sharpen images and to measure surface irregularities by means of optics with limited

depth of field The depth of field (DOF) is the distance between the nearest and the furthest objects that are in acceptably sharp focus in an image captured with a camera. Factors affecting depth of field For cameras that can only focus on one object dist ...

Algorithm

The algorithm work as follows: # at first images with difference focus are captured. This is done by moving the sample or the optics in relation to each other. # then for each position the focus over each plane is calculated # the plane with the best focus is used to get a sharp image. the corresponding depth gives the depth at this position-

Optics

Focus variation requires an

optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviole ...

with very little

. This can be realized if a

microscopy Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of micr ...

like optics and a microscope objective is used. These objectives have a high

numerical aperture In optics, the numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. By incorporating index of refraction in its definition, NA has the proper ...

which gives a small depth of field.

Usage

The use of this method is for optical

surface metrology Surface metrology is the measurement of small-scale features on surfaces, and is a branch of metrology. Surface primary form, surface Fractal dimension, fractality, and surface finish (including surface roughness) are the parameters most commonly a ...

and

coordinate-measuring machine A coordinate measuring machine (CMM) is a device that measures the geometry of physical objects by sensing discrete points on the surface of the object with a probe. Various types of probes are used in CMMs, the most common being mechanical and l ...

. This means measuring form,

waviness Waviness is the measurement of the more widely spaced component of surface texture. It is a broader view of roughness because it is more strictly defined as "the irregularities whose spacing is greater than the roughness sampling length". It can ...

and roughness on samples.{{cite web , last1=Burmudez , first1=Carlos , title=Active illumination focus variation , url=https://www.spiedigitallibrary.org/conference-proceedings-of-spie/11056/2525981/Active-illumination-focus-variation/10.1117/12.2525981.short?SSO=1 , accessdate=22 October 2020 With optimized hardware and software components a

lateral resolution Lateral is a geometric term of location which may refer to: Healthcare * Lateral (anatomy), an anatomical direction *Lateral cricoarytenoid muscle *Lateral release (surgery), a surgical procedure on the side of a kneecap Phonetics *Lateral co ...

of 500 nm (limitation of wavelength of light) and a vertical resolution of several nm can be reached.

Advantages and disadvantages

Advantages: * can be used on samples with steep flanks. This is because a ring light can be used to extend the illumination aperture * can deliver color information * can measure on rough surfaces Disadvantages: * can not be used if the surface of the sample does not give structure in the image. This means it can not be used for wafers and glass

Standardisation

ISO ISO is the most common abbreviation for the International Organization for Standardization. ISO or Iso may also refer to: Business and finance * Iso (supermarket), a chain of Danish supermarkets incorporated into the SuperBest chain in 2007 * Iso ...

committee is working on a new series of ISO standards that is called the

ISO 25178 ISO 25178: Geometrical Product Specifications (GPS) – Surface texture: areal is an International Organization for Standardization collection of international standards relating to the analysis of 3D areal surface texture. Structure of the stan ...

series. The part 6 document describes the available methods for roughness measurement. Focus variation is one of the described methods.

References