WO2008062341A1

WO2008062341A1 - A method of optically inspecting an integrated circuit through a lens

Info

Publication number: WO2008062341A1
Application number: PCT/IB2007/054609
Authority: WO
Inventors: Martijn Goossens
Original assignee: Nxp B.V.
Priority date: 2006-11-20
Filing date: 2007-11-13
Publication date: 2008-05-29

Abstract

A method is disclosed for optically inspecting an integrated circuit (IC) comprising a plurality of semiconductor devices (20) on a first surface of a substrate (10) through a lens (310, 410) in or on an area (40) of a further surface of the substrate (10) opposite the first surface. The method comprises a first step of reducing the thickness of the area (40) such that the area, at least when equipped with the lens (310, 410), has an increased transmittance for light having a predefined wavelength, preferably light having a wavelength in the range of 450-700 nm. In a next step, the lens (310, 410) is placed in or on said area (40). The lens may be formed in said area, or may be a separate lens placed on said area. Finally, a subset of the plurality of semiconductor devices (20) is inspected through the lens (310, 410). The method of the present invention has the advantage that, because of the relationship between optical resolution and light wavelength, a better resolution can be obtained when inspecting the IC.

Description

DESCRIPTION

A METHOD OF OPTICALLY INSPECTING AN INTEGRATED CIRCUIT

THROUGH A LENS

The present invention relates to a method of optically accessing an integrated circuit (IC) comprising a plurality of semiconductor devices on a first surface of a substrate through a lens in an area of a further surface of the substrate opposite the first surface.

The present invention further relates to an apparatus for preparing an IC for such an inspection method.

It is important for an integrated circuit (IC) manufacturer to optimize the manufacturing yield of the manufactured ICs, that is, to minimize the ratio between the number of good ICs and the number of faulty ICs manufactured. Firstly, due to the high cost involved with the production of integrated circuits (ICs), a high yield manufacturing process is essential to maintain competitiveness in a small-margin market. Moreover, the sale of a faulty IC can have a detrimental effect on the image of that product. For those reasons, an IC typically is tested rigorously before being sold to remove the faulty ICs from the manufactured batch.

In case of the detection of a faulty IC, it is important to determine the root cause of the fault, because this information can help to improve the yield of the IC manufacturing process. Unfortunately, IC tests are usually incapable of providing detailed enough information for the location of the fault on board the IC. Moreover, the IC test procedure often cannot identify every faulty IC in a batch, or an IC may break down during operation, which means that a faulty IC can be returned by a customer. In such cases, the fault on board the IC has to be located in a different way.

To this end, optical inspection or stimulation methods have been developed in which a lens is formed on the backside of the substrate to facilitate optical inspection of the semiconductor devices formed on the other side of the substrate, i.e. the internal side of the substrate. The lens has to be located at the backside of the IC substrate because the other side typically is covered by a number of metal layers that obscure the visibility of the internals of the IC. Alternatively, the lens may be used to optically excite a subset of semiconductor devices on the substrate, after which electrical measurements are performed involving the excited subset.

An example of such an inspection method is disclosed in US patent application US 2004/0203257. According to this method, a hemispherical cavity having a curved salient centre portion is etched in the backside of the substrate of the IC using a focussed ion beam etching technique. The cavity and its salient centre portion act as a solid immersion lens, which facilitates optical inspection of the internals of the IC. A disadvantage of this method is that a substantial part of the IC substrate has to be removed for the manufacturing of the solid immersion lens, which not only makes the method relatively costly but it usually also means that such a lens can only be formed once per substrate area. Consequently, if the solid immersion lens is formed in the wrong part of the substrate, and the fault is expected to be located in an area partially overlapping the area in which the solid immersion lens has been formed, the fault inside the IC cannot be detected, and the investment in the fault analysis has been wasted.

An improvement over this method is proposed in PCT patent application WO 2006/117765 A2, in which the solid immersion lens of the previous method is replaced by a diffraction lens, e.g. a Fresnel lens. Because the diffraction lens is much more shallow than a solid immersion lens, the manufacturing of the lens is significantly cheaper. Moreover, in case of the lens being formed in an incorrect location on the substrate, it can be easily polished off the substrate and reformed in a different location due to its shallowness. A problem associated with both methods is that the resolution of the optical inspection is limited by the transmittance of the substrate. For instance, a silicon (Si) substrate is opaque for wavelengths below 1 micron, which limits the resolution of the optical inspection to around 300 nm, since the resolution is directly proportional to the wavelength of the light used for the optical inspection. Such a resolution is not sufficient for submicron IC technologies, which have semiconductor feature sizes well below these resolutions.

The present invention seeks to improve the resolution of the optical inspection method of the opening paragraph.

The present invention also seeks to provide an apparatus facilitating the implementation of such a method.

According to a first aspect of the present invention, there is provided a method of the opening paragraph, the method comprising reducing the thickness of the area such that the area, at least when equipped with the lens, has an increased transmittance for light having a predefined wavelength; forming the lens in said area; a focussing a light beam onto a subset of the plurality of semiconductor devices with the lens.

The present invention is based on the realization that the transmittance of the substrate of an IC, e.g. a Si-substrate for light having a wavelength for which the substrate is opaque at its normal thickness, can be improved by thinning the substrate prior to the formation of the lens thereon, e.g. to a thickness of only a few microns. This has the additional advantage that a smaller lens can be used because the lens is closer proximity to the semiconductor devices than in the aforementioned prior art methods. It is pointed out that it is known from: http://www.virginiaserni.corn/pdf/Optical%20Properties%20of%20Silicon

Z_,l_,5_,Q2_ι._ιpdf, as retrieved from the internet on 17 November 2006, that a variation in thickness of a Si wafer can influence the interaction, i.e. the absorption, reflectance and transmittance, of light of various wavelengths with the wafer. However, there is no hint in this document that below a certain thickness of an Si-substrate the transmittance would become substantially transparent for light well below 1000 nm, and that this could be favourably used to improve the resolution of optical inspection techniques applied to the IC having such a substrate. Therefore, the skilled person does not get an incentive from this document that would lead the skilled person to the present invention.

According to a further aspect of the present invention, an apparatus is provided for modifying a substrate of an integrated circuit comprising a plurality of semiconductor devices on a first surface of a substrate, the apparatus comprising a preprogrammed function for reducing the thickness of an area of a further surface of the substrate opposite the first surface such that the area, at least when equipped with a lens, has an increased transmittance for light having a predefined wavelength.

Such an apparatus facilitates the at least partial preparation of ICs for inspection using the method of the present invention.

Preferably, the apparatus comprises a further preprogrammed function for forming the lens in the area of reduced thickness, such that the apparatus facilitates a fully automated preparation of the IC for the inspection method of the present invention.

The invention is described in more detail and by way of non-limiting examples with reference to the accompanying drawings, wherein:

Fig. 1 shows the wavelength dependent transmission characteristics of Si-substrates of different thicknesses;

Fig. 2 shows the wavelength dependent refractive index of Si; Fig. 3 shows a first application of the method of the present invention to an IC; and Fig. 4 shows another application of the method of the present invention to an IC.

It should be understood that the Figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the Figures and their detailed description to indicate the same or similar parts. Fig. 1 , shows the transmittance curves for Si-substrates having a thickness of 2, 3, and 4 microns respectively for light having a wavelength λ from 400-700 nm. From these curves, it can be seen that for Si-substrates having a thickness of no more than only a few microns, the substrate becomes substantially transparent for light having a wavelength in the visible spectrum, in particular light having a wavelength no smaller than 450 nm. It can also be seen that thinner substrates have an increased transparency for light at the blue part of visible spectrum, e.g. light at 450 nm. Therefore, the thinner the substrate can be made, the better the resolution of the inspection method of the present invention will be, because of the correlation between wavelength of light used for the inspection and the resolution of the images obtained by the inspection method.

Fig. 2 shows the refractive index of Si as a function of light wavelength. It can be seen that the refractive index is higher for light at the blue part of the visible spectrum. This can be advantageously utilized when using a solid immersion lens, because the resolution, i.e. the minimum feature size detectable, of such lenses is inversely proportional to the refractive index of the material used for the lens.

Fig. 3 shows a first example of an IC prepared by the method of the present invention. In a first step, an area 40 of the IC has been reduced to a thickness D of no more than a few microns. This can be realized using known Si-removal techniques such as dry etching, wet etching, milling and so on. Next, a diffractive lens 310 is formed in the area 40. The lens 310 may be formed by any of the methods described in the PCT patent application WO 2006/117765 A2. Finally, a subset 30 of the semiconductor devices 20 on the substrate 10 of the IC may be optically inspected through the lens 310 on the area 40, for instance by using a confocal laser scanning microscope operating at a wavelength between 500-700 nm, e.g. 550 nm. Other ranges such as 700-1000 nm are also feasible but yield a lower resolution in the optical inspection method. Alternative optical inspection principles may also be feasible, as well as IC probing methods where the laser is used to optically excite a part of the substrate, after which the IC including the excited area is subjected to electrical measurements.

It is emphasized that the light travelling though the outer regions of the lens will travel to more substrate material than the light travelling to the centre of the lens. As can be seen from Fig. 1 , this will have a significant impact on the amount of light that will travel through the lens, especially when the thickness of the substrate is in the regions indicated in Fig. 1. In order to optimize the resolution of the visualization of the semiconductor devices under the lens, it may be necessary to compensate for this effect by reducing the intensity of the part of the light beam travelling through the centre of the lens, such that the collected light has a substantially homogeneous intensity distribution over the whole width of the lens. For instance, in case of a confocal laser being used to provide the light beam, a grey filter may be placed in the beam to reduce the intensity of the centre of the beam. In the context of the present invention, the phrase 'lens' is intended to include any optical element capable of focussing incident light onto a set of semiconductor devices 30 on the substrate 10.

It should be appreciated that the initial step of reducing the thickness of the area 40 may take into consideration that a further reduction of thickness is required to form the lens 310 in the area 40, such that the initial thickness reduction reduces the thickness to D+d, with d being the further thickness reduction of the substrate caused by the formation of the lens therein.

Fig. 4 shows an alternative example of an IC prepared by the method of the present invention. In a first step, the whole substrate 10 is thinned to a thickness D of no more than a few microns. A Si-solid immersion lens 410, which typically is only a few microns in diameter, is encapsulated in an UV- hardened injection moulded resin such as an epoxy resin to protect the lens from breaking. The lens 410 is placed in its holder 420 on the thinned substrate 10 of the IC for facilitating optical inspection of some of the semiconductor devices 20 on the substrate 10 and covered by the metal layer 15 (the metal layer 15 has been omitted from Fig. 3 for reasons of clarity only). Preferably, the thickness D of the substrate should not exceed 2 micron, because the combined thickness of the substrate and the lens 410 would otherwise become too large, causing a too large attenuation of the light travelling through the combined Si material.

The lens 410 and its holder may be mounted on a movable device, e.g. the laser head of a DVD or CD player to facilitate repositioning of the lens on the substrate 10.

The initial position for placing or forming a lens on or in the substrate 10, i.e. the determination of the global area of interest, may be determined using conventional laser-based failure analysis techniques. Examples of such techniques can for instance be found in Soft Defect localization on ICs, Bruce et al. Proc. 28^th Intl. Symposium for Testing and Failure Analysis 2002, p. 21 - 27, or in US patent application US6549022.

Advantageously, the thinning of the substrate upon detection of the global area of interest can be done automatically. To this end, an apparatus such as a focussed ion beam emitter may be extended with a preprogrammed function in which for instance the determined area of interest, the required thickness of the substrate and/or the size of the area 40 are specified, after which the apparatus creates the thinned area automatically. This can be routinely implemented in software. In case the lens is also to be formed in the substrate 10, e.g. a diffractive lens 310, the apparatus may be further extended with a preprogrammed function to form the lens in the thinned area. A possible implementation of such a function is disclosed in PCT patent application WO 2006/117765 A2.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention can be implemented by means of hardware comprising several distinct elements. In the device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. A method for optically accessing an integrated circuit comprising a plurality of semiconductor devices (20) on a first surface of a substrate (10) through a lens (310, 410) in or on an area (40) of a further surface of the substrate (10) opposite the first surface, the method comprising: reducing the thickness of the area (40) such that the area, at least when equipped with the lens (310, 410), has an increased transmittance for light having a predefined wavelength; placing the lens (310, 410) in or on said area (40); and focussing a light bundle onto a subset of the plurality of semiconductor devices (20) with the lens (310, 410).

2. A method as claimed in claim 1 , wherein the step of placing the lens (310, 410) in or on said area (40) comprises creating the lens in said area.

3. A method as claimed in claim 1 , wherein the lens (410) is a solid immersion lens.

4. A method as claimed in claim 1 or 2, wherein the lens (310) is a diffraction lens.

5. A method as claimed in any of the preceding claims, wherein the predefined wavelength is smaller than 700 nm.

6 A method as claimed in claim 5, wherein the predefined wavelength lies in the range of 450-700 nm.

7. An apparatus for modifying a substrate (10) of an integrated circuit comprising a plurality of semiconductor devices (20) on a first surface of a substrate (10), the apparatus comprising a preprogrammed function for reducing the thickness of an area (40) of a further surface of the substrate opposite the first surface such that the area (40), at least when equipped with a lens (310, 410), has an increased transmittance for light having a predefined wavelength.

8. An apparatus as claimed in claim 6, wherein the apparatus comprises a further preprogrammed function for forming the lens (310, 410) in the area (40) of reduced thickness.