Refraction assisted illumination for imaging
Various embodiments are directed to systems and methods of imaging subsurface features of objects. An illumination source may be directed towards a surface of an object comprising subsurface features at a first angle relative to the normal of the surface. The object may have a portion between the subsurface features and the surface that has an index of refraction that is greater than the index of refraction of a surrounding medium. An imaging device may be placed with an objective lens oriented substantially normal to the surface. The first angle may be larger than an acceptance angle of the objective lens.
1. A method for generating an image of a subsurface feature of an object through a surface of the object, the method comprising:
directing illumination in an imaging wavelength range toward the surface at a first angle relative to a normal of the surface:
wherein the first angle is greater than an acceptance angle of an objective of an imaging device;
wherein the object is substantially transparent over the imaging wavelength range,
wherein the object has an index of refraction that is greater than an index of refraction of a surrounding medium that surrounds the object,
wherein a first portion of the illumination is reflected at the surface and is not incident on the objective,
wherein a second portion of the illumination is refracted through the surface towards the subsurface feature, is incident on the subsurface feature and is reflected by the subsurface feature back toward the surface over a range of angles,
wherein at least a portion of the range of angles is within an acceptance angle of the objective,
wherein the second portion of the illumination is refracted at the surface such that an attenuated component of the second portion is refracted outside of the acceptance angle of the objective and a filtered component of the second portion is incident on the objective;
wherein a third portion of the illumination is refracted through the surface towards a second subsurface feature, is incident on the second subsurface feature and is reflected by the second subsurface feature back towards the surface over a second range of angles,
wherein at least a portion of the second range of angles is within an acceptance angle of the objective,
wherein the third portion of the illumination is refracted at the surface such that an attenuated component of the second portion is refracted outside of the acceptance angle of the objective and a filtered component of the third portion is incident on the objective; and
capturing an image of the subsurface feature and the second subsurface feature with the imaging device based on the filtered component of the second portion received via the objective and the filtered component of the second portion received via the objective.
2. The method of claim 1 , wherein the imaging wavelength range comprises near infrared wavelengths.
3. The method of claim 1 , wherein the ratio of the index of refraction of the object over the index of refraction of the surrounding medium is about 3.5
4. The method of claim 1 , wherein the first angle is about 45°
5. The method of claim 1 , wherein the acceptance angle of the objective is about 28°, and wherein the first angle is between 28° and 90° relative to the normal of the surface.
6. The method of claim 1 , wherein the object is a semiconductor substrate and the surface is a backside surface of the substrate.
7. A system for generating an image of a subsurface feature of an object through a surface of the object, the system comprising:
an illumination source that directs illumination in an imaging wavelength range toward the surface at a first angle relative to a normal of the surface:
wherein the first angle is greater than an acceptance angle of an objective of an imaging device;
wherein the object is substantially transparent over the imaging wavelength range,
wherein the object has an index of refraction that is greater than an index of refraction of a surrounding medium that surrounds the object,
wherein a first portion of the illumination is reflected at the surface and is not incident on the objective,
wherein a second portion of the illumination is refracted through the surface towards the subsurface feature, is incident on the subsurface feature and is reflected by the subsurface feature back toward the surface over a range of angles,
wherein at least a portion of the range of angles is within an acceptance angle of the objective,
wherein the second portion of the illumination is refracted at the surface such that an attenuated component of the second portion is refracted outside of the acceptance angle of the objective and a filtered component of the second portion is incident on the objective;
wherein a third portion of the illumination is refracted through the surface towards a second subsurface feature, is incident on the second subsurface feature and is reflected by the second subsurface feature back towards the surface over a second range of angles,
wherein at least a portion of the second range of angles is within an acceptance angle of the objective,
wherein the third portion of the illumination is refracted at the surface such that an attenuated component of the third portion is refracted outside of the acceptance angle of the objective and a filtered component of the third portion is incident on the objective; and
an imaging device comprising the objective, wherein the imaging device captures an image of the subsurface feature and the second subsurface feature based on the filtered component of the second portion received via the objective and the filtered component of the third portion received via the objective.
8. The system of claim 7 , wherein the objective is oriented substantially normal to the surface of the object.
9. The system of claim 7 , wherein the objective comprises a confocal lens.
10. The system of claim 7 , wherein the imaging wavelength range comprises near infrared wavelengths.
11. The system of claim 7 , wherein the ratio of the index of refraction of the material over the index of refraction of the surrounding medium is about 3.5
12. The system of claim 7 , wherein the first angle is about 45° and the acceptance angle of the objective is about 28°.
13. The system of claim 7 , wherein the first angle has a value between that of the acceptance angle of the objective and 90° relative to the normal of the surface.