Apparatus for and a method of determining surface characteristics

Apparatus for determining surface structure element characteristics, wherein the apparatus comprises a coherence scanning interferometer and a data processor, the data processor comprising a structure determiner operable:

• to determine a frequency transform ratio relating to a ratio between:
• structure intensity data comprising a first series of intensity values resulting from a measurement operation by the coherence scanning interferometer on a surface structure sample area using a low numerical aperture objective such that the spread of light from a light source of the coherence scanning interferometer at the sample surface is greater than a pitch of surface structure elements of the surface structure sample area;
• and
• non-structure intensity data comprising a second series of intensity values resulting from a measurement operation on a second sample surface area not having the surface structure;
• to relate the frequency transform ratio to a relationship representing the amplitude and phase of the electric field at the image plane of an objective of the interferometer in terms of a first structure measure representing a ratio between the width and pitch of the surface structure elements and a second structure measure representing the size of the surface structure elements in a direction transverse to the sample surface;
• and
• to determine values for at least one of said first and second structure measures as a result of relating the frequency transform ratio to the relationship.

A method of determining information relating to a sample surface structure, the method comprising the steps of:

• carrying out respective measurement operations on a structure surface sample area having a surface structure comprising surface structure elements having a pitch and a non-structure surface sample area not having the surface structure by, at least for the measurement operation on the structure surface sample area, using light having a point spread function with a spread at the sample surface greater than the pitch of the surface structure elements and by, in each measurement operation, directing the light along a sample path towards a region of the sample surface and along a reference path towards a reference surface such that light reflected by the region of the sample surface and light reflected by the reference surface interfere, and causing relative movement along a measurement scan path between the sample and the reference surface while a sensing device senses light intensity representing interference fringes produced by the sample surface region at intervals during the relative movement to provide structure intensity data comprising a series of intensity values resulting from a measurement operation on the structure surface sample area and non-structure intensity data comprising a series of intensity values resulting from a measurement operation on the non-structure surface sample area;
• determining a frequency transform ratio corresponding to a ratio between the structure and non-structure intensity data;
• and
• relating the frequency transform ratio to a relationship involving a first structure measure representing a ratio between the width and pitch of the surface structure elements and a second structure measure representing the size of the surface structure elements in a direction transverse to the sample surface to determine values for at least one of said first and second structure measures.

Apparatus for determining information relating to a sample surface structure, the apparatus comprising:

• light directing means for directing light along a sample path towards a region of a surface of a sample and along a reference path towards a reference surface such that light reflected by the region of the sample surface and light reflected by the reference surface interfere, the light directing means being operable to provide light having a point spread function with a spread at a sample surface greater than a pitch of surface structure elements providing a surface structure of the sample;
• moving means for effecting relative movement along a measurement scan path between a sample and the reference surface;
• sensing means for sensing light representing the interference fringes produced by a sample surface region during relative movement along the measurement scan path;
• control means for carrying out a measurement operation by causing the moving means to effect relative movement while the sensing means senses light intensity at intervals to provide a series of intensity values representing interference fringes produced by a sample surface region during the relative movement;
• and
• data processing means for receiving and processing i) structure intensity data comprising a set of intensity values resulting from a measurement operation on a structure surface sample area using light having a point spread function with a spread at the sample surface greater than a pitch of surface structure elements providing the surface structure of the structure surface sample area, and ii) non-structure intensity data comprising a set of intensity values resulting from a measurement operation on a non-structure surface sample area not having the structure, the data processing means comprising:
• frequency transform ratio determining means for determining a frequency transform ratio corresponding to a ratio between the structure and non-structure intensity data;
• and
• structure providing means for relating the frequency transform ratio to a relationship involving a first structure measure representing a ratio between the width and pitch of the surface structure elements and a second structure measure representing the size of the surface structure elements in a direction transverse to the sample surface to determine values for said first and second structure measures.