Advances Of X Rays In Ic Packaging Engineering Essay

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X-rays have the capability of penetrating through the matter which had helped in the study of the internal structure of the IC devices without decapsulation of the device i.e., non destructive testing. The applications of x-rays are too broad in IC packaging ranging from process control to failure analysis covering almost the entire testing cycle process[3].The X-ray computed Tomography gives some most useful features of the X-rays that could be used in the IC testing process. High resolution 3D x-ray tomography provides critical new imaging capability for semiconductor packaging, including C4 ball grid arrays, stacked die, solder bumps and packaging interconnects. Defects such as solder non-wet, cracks, voids, delamination , opens and shorts are readily imaged [4].

X-ray radiation has following three fundamental advantages [1] that facilitate the IC failure analysis process

High energy x rays have very large penetration length to image internal structures of a IC device without deprocessing (non destructive testing).

The absorption and fluorescence emission depends strongly on the elemental composition of the sample, allowing high-sensitivity material analysis;

X-ray imaging causes little structural damage to integrated circuit samples and there is no charging effect.

The 3D structure of the IC using the X-ray tomography techniques [1] due to very deep penetration have resolution better than 50nm and subtlety to distinguish different elemental compositions. When combined with computed tomography (CT) technology, the full 3D structure of an IC can be obtained non-destructively at tens of nanometre accuracy, thus making x-ray nano-CT well suited for both metrology and failure analysis (FA) applications with 3D IC.

The x-ray CT [1] approach offers many important fundamental advantages for both FA and metrology applications:

Non-destructive 3D volumetric metrology on vertical walls, semi-closed, or even completely embedded structures.

Virtually no measurement distortion.

At 50 nm resolution, it provides up to parts-in-thousands geometric accuracy suitable for wide range of materials from polymers to metal with no charging effect or negligible radiation damage.

Process Overview:

To obtain the 3D structure of the IC, a number of projection images are acquired while rotating the sample to different tilt angles from the optical axis, and these projections are mathematically assembled to form a 3D image representing the sample's structure. This data acquisition process is non-contact and non-destructive.

Metrology of 3D IC with X-ray Microscopy Computed Tomography[1]:

Traditional processes are destructive and time-consuming in IC packaging. Using x-ray CT technique, however, the complete 3D structure of the void can be obtained non-destructively and with little sample preparation in less time. Since uncharged beam is used in the non contact measurements, it is compatible with both FA laboratory and production environments.

Figure 2 Main components of a typical high-

Figure 1 : TSV imaged with an SEM after cross-sectioning.

Additional measurements such as void volume, connectivity, and 3D surface profile can be measured directly from the 3D volume. The materials have different absorption that is used for obtaining high contrast images and voids can be very well captured.

Figure 3 X-ray CT images of a TSV. The imageacquiring a set of projections, a 3D data sets of via's structure can be reconstructed with 100 nm resolution.A "virtual cross-section" - a slice of the 3D data set was extracted and shown at the left. This images provides similar information to the traditional technique of physical cross-sectioning followed by SEM imaging, but from "virtual cross-section" image, one can also extract horizontal slice from the 3D data set, as shown at the right in fig 3

The CT explained in the [7] (published in the year 2000) has been found to be using the technique "Psuedo-Laminography" which had some inherent issues like it cannot find critical defects like the delamination and cracks. Also it has some issues in the thickness of the package being tested to be of fixed size. But it proved to be effective in the in-plane defects like the solder voids.

The example of the CT technology equipment in the [8] (published in 2002) used the microlaminography technique using which up to 10 µm were resolved with confidence .In this Microlaminography was used to identify bond-wire shorts in the plane of the solder resist of a ball grid array assembly.

A Novel X-ray Microtomography System[10] :

A very high resolution can be accomplished by a small x-ray source size and high magnification due to the reduced flux.In [10], there is a microtomography fully automated system, in which, "Xradia Inc.," attained 1µm resolution in few seconds of acquisitionThe system enabled the overlapping complex features of the chip to be captured in a non invasive manner which were not possible effectively using the 2D imagers.

Imaging geometry of projection-type x-ray microscopes.

The source to sample distance is ls, and the sample to detector distance is ld.The magnification is (ls + ld) /ls.

In this geometry, Each point casts a shadow

on the detector plane with the size (ld/ls)*s .This is essentially the point spread function. if the center of the object does not fall into the shadow of the other.

Therefore the resolution limit determined by the imaging geometry is: δ>=(M-1/M)s

Another main influence (particularly for

Illustration of the tomographic imaging process:

The sample is first imaged at different tilt angles from

the x-ray beam to obtain a series of tomographic projections.

These projections are then recombined mathematically

to form a 3D image representing the 3D structure of the sample.

high magnification) is the geometry of the X-ray source. The diameter of an ideal X-ray source is zero but it is not so in practice, hence there is an lack of sharpness present inherently in an X-ray source .Features such solder bumps, copper lines, plated through holes can be viewed clearly without any overlapping by the other layers.Because of its high throughput, quasi-real-time 3D imaging can be performed to study dynamic processes such as formation and propagation of cracks in a sample.

Use of NANOCT in Nanopackaging[11]:

Figure 2 3d overview of the BGA with wire bonds, solderThe digital flat panel detectors with a high number of pixels (1,500 x 1,900) and magnification of 1000 in the latest x-ray detectors. The oblique viewing is required for the better view of all the fine details of all the layers.A laser supported self mounting tool is used for the mounting that makes the process of X-ray CT more viable.

The [11] shows some of the inspection results of a BGA with 0.30mm ball diameternmicro vias in PCB and the Cracks in a 1206 Solder Joint

(left)Vertical view of a sample blocking the underlying

layers (right) obliquee view

manifesting the blocked regions

Figure 3Visualisation of tomographic voxel data, after automatic void detection. The surface of the metal is displayed transparently so that the voids are visible.Nanofocus tube technology and Nano CT :Nanofocus tube technology[12] and high resolution CT are the future inspection tools for IC packagesThey can be used for bond wire defects, copper bond wire inspection, Flip Chip solder interconnection and microvia inspection. The absorption of some materials like non-conductive die adhesives or copper bond wires reduce the feature extraction in the detectors.The metal suits a wide coverage for almost all the materials by operating at different energies,detactability and magnificationsFigure 43d view of a virtual cross section through a micro via

(diameter of the via's hole is 245 µm)

Based on the nanofocus tube technology nanoCT widely expands the spectrum of detectable microstructures in complex electronic devices and packages by 3D visualisation and slice-by-slice analysis. Thus nanofocus tube technology pushes computed tomography systems into application fields that very recently were exclusive to expensive synchrotron techniques.

NanoCT - Results:

In oblique view [12] at high magnification the vias and some internal solder bumps can be clearly seen. Hence the automatic statistical evaluation of size and volume of the voids is possible

X-Ray Inspection of Die Attach[9] :

 Inspection of die attach is performed using micro-focus high-magnification, high-resolution x-ray system.The X-ray detectors while scanning low silver epoxy compounds provide fine grayscale differences representing voids and an image processing software measures the percentage of void area relative to the percentage of die area The data will help the process engineer determine the manufacturing process parameters. 

X-Ray Inspection of Lid Seals.[9]: Epoxy seals are detectable during x-ray imaging due to the powdered silver that has been added to enable it to conduct electricity. Glass seals are typically larger and contain lead as an additive to improve reflow and adhesion properties. Metal seals are denser and produce excellent void details. Seal voids are typically small and require a high magnification high-resolution x-ray system.

If the chip size is 12 mm square and assuming 100% chip coverage, calculate the yield at each stage of the process and the overall yield (a) using the Poisson distribution (no clustering) and (b) using the negative binomial model with clustering. Show your calculations.

The fundamental X-ray physical properties, such as their small wavelength and their weak interaction with solid-state matter satisfy basic in-line metrology requirements. The X-ray based metrology methods are involved to monitor critical 65 and 45 nm processes like ion implant, nitride SiO2 gate dielectrics, NiSi, Cu/porous low-κ interconnects and MIM capacitors.It is important to monitor the various mOS processes at a time because of the sophistication involved in the processes nowadays. Hence the monitoring of thin films and interface "chemical compositions" at the atomic scale becomes compulsory for the 65nm MOS at the process level.

Types of Metreology:

There are 2 types of metrology

In-line metrology: The parameters are measured and during the course of the process

Off-line metrology: The parameters are measured and they are processed after the process is over.They are used for the statistical analysis of the process and the post manufacturing defects.

Methods of monitoring the processes:

Many X-ray based methods can be used to monitor the manufacturing of microelectronic processes, like X-ray reflectivity (XRR), X-ray fluorescence(XRF), grazing incidence small angle X-ray scattering(GI-SAXS),X-ray photoelectron spectroscopy(XPS),X-ray diffraction(XRD) and its parent technique X-ray diffraction topography(XRT) [13].

The metrology tools are placed in the process chamber due to the requirement that the properties of the materials are to be measured asthe process happens and also to check the variation of the process parameters like the physical and the chemical properties.

REQUIREMENTS OF THE WAFER FAB FOR THE SEMICONDUCTOR MANUFACTURING:

The requirements of the wafer fabs match with the basic properties such that the x-ray metrology mechanisms are well suited for the semiconductor manufacturing processes. Also while due to the rapid scaling down the x-rays provide a non invasive and a non destructive testing And analysis of the processing. Also measurement spot size compatible with metrology test structure size(50-70 μm) and the measurement must be repeatable, reliable, quicker ,stable and automatic.

Almost all these requirements are fulfilled by the x-ray based metrology techniques that guarantees its potential application in this field.

1. X-ray fluorescence (XRF):

The absorption of the x-ray photon and the photoemission of the characteristic wavelength photon by the elements of the layers is used in this method. TheXRF efficiency is almost equal to 1 for the K layers and increases with the atomic number of

the fluorescent element.Rate = 10 s per point.

2. Grazing incidence small angle X-ray scattering (GI-SAXS):

The energy change produced by the x-ray scattering can be used for the characterisation of the porous layers,precipitates andquantum dots.The method has the rate of 400 to 600 s per point.

3. X-ray reflectivity (XRR):

X-ray reflectivity is a specific case of X-ray scattering at the layer of interface between two different refractive index surfaces. The refractive index of materials in the X-ray spectral range can be expressed as: n = 1− δ + iβ,

δ and β are characteristics of the dispersion and

absorption of X-rays.The refraction of the x-rays depending upon the characteristic of the refractive index of the material is used for the measurements.

4. X-ray diffraction (XRD):

The X-rays are reflected by the crystal planes of the materials and the parameters of the different materials like nature and the allotropic phase of the constitutive elements in the layers, as well as the film texture and the biaxial strain in the layer can be understood by this method

5. X-ray photoelectron spectroscopy (XPS):

XPS is based on the photoelectric effect, where the x-ray is absorbed by the material to emit the particular wavelength of the light. The whole process depends upon the frequency of the incident radiation .Since the x-ray has the matching frequency with the material properties,it is considered for this process.The main part of the XPS signal is emitted from the first 2 to 10 nm below the sample surface for the conventional

low X-ray energies used in XPS.

X-ray metrology of silicon substrate and ion implant:

X-ray diffraction techniques, such as high-resolution X-ray diffraction (HR-XRD) and X-ray diffraction topography (XRT) are perfectly suited for detecting and identifying the crystalline defects and quantifying the defect density. Ion-generates some crystal defects that can be identified using the XRD techniques performances.

X-ray metrology of nitrided silicon oxide gate dielectrics:

At 65 nm there is a need for the ultra thin dielectrics that required and the Silicon nitride thin films are used as dielectrics.Nevertheless, the nitrogen concentration needs to be meticulously controlled because the high amount of nitrogen will change the transconductance and thermal instability occurs. Because the escape depth of photoelectrons excited by the X-ray beam is limited to 10 nm, XPS is very well suited to monitor ultra-thin films like nitrided gate oxides(SiON).

X-ray metrology of NiSi thin films:In the 65 processes, NiSi films are used to reduce the overall contact resistance between the silicon part of MOS transistors (gate, source and drain) and the Cu interconnects. Hence if RTA of the Ni2Si film to form NiSi during its deposition in the wafer is not properly controlled may lead to high resistivity and junction leaks. Monitoring the NiSi formation from Ni deposition through the thermal anneals is delicate and extremely critical for the integration of nickel silicide in 65 nm.XRR is very well suited, since the contrast between the electron densities of the various nickel silicide phases is very large. The XRR is the quickest method that is available for this kind of conditions.

Process:

The XRR experiment was run after selective etch of the TiN cap and the unreacted Ni with an acquisition time of 30 s. The edge in the figure, while it is almost zero at the wafer centre and hence indicates that the Ni2Si => NiSi change starts at the wafer edge.

X-ray metrology of low-κ dielectrics:

In order to overcome the interconnect delays the low- k dielectrics are used to overcome the capacitances and resistances.In order to control the delicate process of depostition of the porous low-k dielectric the in-line monitoring is done survey the low-κ film thickness, its overall porosity, the average pore size and the pore size distribution. XRR and GI-SAXS are combined in the metrology of the manufacturing processes of 65 nm microelectronic devices.

X-ray metrology of Cu interconnects:The metrology is used for measuring individual thicknesses, the allotropic phase and texture of the Cu diffusion barrier layer, the texture of the thick ECD Cu layer, and monitoring of dishing in Cu lines. The combination of XRR and XRF is perfectly suited for the process control of film thickness in Cu Damascene.XRR is generally used during the monitor wafer process and the XRF,due to its small spot size, is used in the product wafer.The XRR is very sensitive to electron density while monitoring Ta/TaN barriers is very effective.

Conclusion:

Hence the effectiveness of the x-rays in the 65 nm domain compels one to acquire the technology for the monitoring of the processes involved in the ultra small ICs that are produced.Hence the X-rays are still having some good applications that are being dependent upon the high resolution sources and also the development of the detectors that may lead to the replacement of the costly synchrotron Detectors.Transmission X-ray microscopy and X-ray tomography are now used to investigate the reliability of Cu interconnects . μ-spot X-ray diffraction can be used to characterize stress in Cu/low-κ interconnects.

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