As the engineering node maintain on decreasing, the demand for advanced metrology tool is indispensable to run into the challenges grown by this device miniaturisation. The patterning of samples utilizing Focused Ion Beam method is a really popular technique in which assorted researches are ongoing to better the operation farther. The current best result of the research is utilizing Helium Ion Microscope ( HIM ) which has really high declaration, high stuff contrasts, good control over charge and high sensitiveness for the surface. The valuable information obtained from HIM exceeds and faster from that of SEM and other celebrated microscopes. HIM provides SEM-like ease-of-use with TEM-like declaration. The HIM achieved a surface declaration of 0.24 nanometers as the lowest. Such a capableness of HIM makes it possible failure analysis applications. This technique produces secondary negatrons, backscattered ions and familial ions supplying three different imaging options – secondary negatrons imaging, backscattered ion imagination and transmittal imagination. This combination of three imaging manners makes this microscope suited for different analysis of the stuff under trial.
The Helium Ion Microscope ( HIM ) is an advanced instrumentality tool in the field of charged atom microscopes. The signals provided by He ion beam are an indispensable thing in developing analytical value for the intent of imaging. There are three imaging manners in HIM which gives we look in-depth about its ability in making the image information. The Secondary Electron imagination has been discussed by looking into assorted qualities of the system. Then the Rutherford backscattering imagination ( RBI ) method has been discussed along with its restrictions and characteristics. Then transmittal imagination has been discussed although this is least explored. Fig.1 shows conventional diagram of HIM with incident ion beam and imaging sensors. The basic apparatus is similar to that of FIB system.
Annular MCP Detector
Fig.1. Conventional diagram of incident ion beam and assorted sensors
Atomic Level Ion Source ( ALIS ) acts as ion gun which provides a stable He ion beam with really high brightness. This gives sub-nm investigation size for the sample.
The scattered secondary negatrons ( SE ) from the sample in the He ion microscope are detected usually utilizing an Everhart-Thornley ( ET ) sensor. ET sensor collects the ejected SE from the sample surface which provides high declaration images along with high signal to resound ratio. The backscattered He sensor collects the backscattered He ions to bring forth an image. The incident ion beam through the Si substrate produces a big figure of electron-hole braces after accomplishing batch of energy loss mechanisms. The figure of electron-hole braces is relative to incident ions energy. Thus the sensor detects this accrued charge by utilizing backscattered He ion counts and this is the information for imagination.
1. Why go for Helium?
Helium ions have really high beginning brightness and short De Broglie wavelength ( reciprocally relative to their impulse ) . This makes it possible to acquire qualitative and quantitative information of the stuff of all time achieved.
He ion beam produces both SE and backscattered ions.
SE output is big.
Interaction volume of He ion beam is smaller near the surface.
Wavelength of He ions is smaller than negatrons for same energy.
Faster operation than SEM.
Interaction volume: Electron beam with 1 keV will be holding larger interaction volume. But 30 keV He beam good collimated below the SE flight deepness and therefore holding a smaller interaction volume with the sample than that of low keV SEM. Besides the declaration of HIM is better than that of SEM with high keV. This rule is shown in Fig.2.
Fig.2. He Ion Beam-Sample Interaction
The capableness of HIM makes it to use in Failure Analysis ( FA ) . Some of the FA applications are:
Imagination of uncoated dielectric stuffs at upper limit He ion energy and declaration.
Imagination of interface movies and surface taints with really high declaration.
Imagination of thick insulators to place implicit in clears or trunkss.
High declaration imaging to place defects in high-aspect-ratio contacts, vias and trenches.
2. Different Imaging Modes
2.1. Secondary Electron Imaging
An incident He ion on a sample has both possible energy ( PE ) and kinetic energy ( KE ) in which KE usually varied from 5 to 45 keV whereas ionisation potency energy is changeless which is 24.6 electron volt. An incident ion beam will chuck out negatrons and photons from the sample because of its PE and KE. The emanation through PE is said to be possible emanation, and emanation through KE is said to be kinetic emanation. Therefore the maximal value of energy from SE will be two thousand times lower than that of the energy of the ion beam. The energy of the SE which is emitted from the sample is to be 2eV, independent of the composing of the sample. This energy when compared with that of the SEMs electron KE which ranges few electron volts to several keV. In Si, negatron with 2eV energy has a scope less than 1 nanometers. So information of HIM SE volume is in sub-nm. The investigation size of HIM is besides sub-nm scope and hence the declaration of HIM SE is evidently below 1nm. The 2eV energy of SE indicates that HIM is more of surface sensitive. As indicated before SEs from the sample in HIM are detected usually utilizing an Everhart-Thornley ( ET ) sensor.
2.1.1. SE imaging – An illustration
An image of aluminium fiduciary grade taken by HIM SE imagination is shown in Fig.3. Here it is seen that taint on the surface and the surface inside informations of underlying barrier bed are clearly imaged. This information is non easy seen by SEM.
Al Oxide or Contamination
Fig.3. HIM-SE image of aluminium fiduciary grade
2.2. Back Scattered Ion Imaging
Back sprinkling, here usually called as Rutherford backscattering phenomenon contains two relationships. They are,
The energy of backscattered He ion is related to the mass of the sample. So, if the energy of backscattered He ion is higher, it reflects the higher mass of the mark.
The back sprinkling derived function cross subdivision rises with square of the atomic figure of sample ( Z2 ) and increases reciprocally with square of the ion energy.
Back dispersing differential cross subdivision is really a step of He ion which is scattered back from the sample by any sensor. But for Rutherford backscattered imagination ( RBI ) , HIM usually use annulate MicroChannel home base ( MCP ) sensor with negative prejudice as shown in Fig.1. MCP sensor absorbs about 80 % of the backscattered ions and rejects the SEs and signifiers an image. And as for 2nd Rutherford relationship, the image from HIM RBI contains image contrast and picture grey degree which are dependent on atomic figure of the sample.
2.2.1. Coincident Comparison of SE and RBI Images
HIM has a capableness in capturing both SE and RBI images at the same time with the aid of two parallel sensing channels. This capableness can be used in failure analysis ( FA ) of stuffs. The Fig.4 shows sample of a tin-coated Cu electrical connection in which it has been seen that Sn hair’s-breadth ( defect indicant ) has been grown.
( a ) ( B )
Fig.4. Image of Tin-coated Cu electrical connection
( a ) HIM SE image shows alterations in contrast along the hair’s-breadth
( B ) HIM RBI image, demoing small to no contrast alterations
Passage is seen in the hair’s-breadth at the point of Orange pointers ( indistinguishable sites in each image )
These types of beards finally turn over clip and eventually pass through from one electrical connection to another. This is turn cause electronics to neglect in operation. Lot of infinite orbiters have been suffered by entire or partial loss of its map since due to this phenomenon. The HIM images shown in Fig. 4 a, B shows complementary information from this type of hair’s-breadth. The HIM SE image gives the topology of the sample. The different subdivisions along the length of the connection are with different contrast degrees which show some alterations happened while the beards are turning. Wherever the beards have a crick or crook, these types of alterations have been seen. The orange arrows as in Fig.4 point such type of countries.
The RBI in contrast shows that the stuff is unvarying all over the length. This shows that the construction of the Sn is altering physically or electronically in some other manner and non by material fluctuation. RBI image shows the indicant of different elements at the passage points ( orange pointer in Fig. 4 ) on the hair’s-breadth. Here the two topographic points indicated by pointers has been seen brighter contrast. This information supply a manner for growing analysis. It should be noted down that most of the atoms as seen in hair’s-breadth of SE image are non seen in RBI image. This indicates that parts are most likely same as Sn, similar same as of hair’s-breadth.
( a ) ( B )
Fig.5. Simultaneous comparing of both SE and RBI images ( a ) SE image ( B ) RBI image
In the current tendency of HIM instrument ‘s constellation, there are two independent video signal ironss which allow coincident imagination of both SE and RBI image. Fig. 5 shows a tin/lead solder sample with coincident comparing of both SE and an RBI image, where it has been shown that SE image shows surface contaminations ( i.e. flow residue ) and topography nowadays in the Sn and lead solder. But alternatively, RBI image ( backscattered image ) differentiates lead from that of Sn. The information from both images will be utile in stuff analysis and mistake localisation.
1.3. Transmission Imaging
Transmission imaging mode although least explored, this can be used in with different imaging manners at the same time to acquire high declaration images. There is an option in obtaining image information from five different types of transmittal signals in HIM. They are: 1. bright field transmitted ions, 2. dark field i.e. , Rutherford scattered familial ions, 3. top-side SE signal, 4. bottom-side SE signal and 5. diffracted ion signal.
Let us take an illustration of cut sample of gate part of a typical IC. Fig.6 ( a ) shows SE manner image as discussed antecedently. Here, the music directors are seen to be brighter and dielectrics, like gate spacers and interlayer insulator are seen as darker. It should be noted that thin beds, like line drive around the Cu, imaged strongly due sensitiveness of the surface and the material contrast of HIM SE imagination. Fig.6 ( B ) shows a bright field image of the same sample country. The bright field ion images usually give lower declaration. Fig.6 ( degree Celsius ) shows bottom side transmitted SE image. This reveals spreads in the nitride, silicide contacts quality and identifies low denseness country of the wolfram he-man in which via fill procedure has been choked off. Fig.6 ( vitamin D ) shows carbon black ‘s dark field and the declaration here is 0.45nm.
Fig.6. Transmission Imaging ( a ) Top-side SE ( B ) Bright field ( hundred ) Bottom-side SE ( vitamin D ) Dark field
Therefore the working rule, characteristics and different manners of imagination of Helium Ion Microscope ( HIM ) have been studied. The applications particularly associating to blame localisation and failure analysis has been discussed along with imaging manners. The consequences from the high declaration images have been seen really utile in analysing the stuff. It has been besides seen that high declaration images allows atom degree elemental analysis. Although SE imaging gives much of information of the sample, Rutherford and transmittal imagination are besides utile in some specific applications as seen before. The mistake localisation complexness keeps on increasing due device miniaturisation which in bend requires advanced metrology tool. With the benefits of advanced HIM, there will be addition in usage of HIM in semiconducting material industries in the hereafter.