Background

The currently available standards for length metrology at nanoscale suffer from two main obstacles which limit their possible application by stakeholders (semiconductor/nanotechnology industry, instrumentation manufacture, surface scientist, etc). Both, step height standards and lateral pitch standards, are only available down to specific limits, given by the production technology for these standards. These limits are 6 nm in case of step height and 70 nm for lateral resolution standards. Additionally the uncertainty limits for the current standards are too large (approximately a factor of 10) as required by the stakeholder.

The preparation of smaller step height prototypes suffers from their instability in air. For some research work 3 nm step height samples were produced at PTB, but the AFM images showed a very “noisy topography”. In research scientists try to use crystalline surfaces and single atomic steps produced on such surfaces. Typical materials used are graphite, mica, Au or glass. On graphite, however, the region near to a step is mostly loosely bonded to the underneath layer, and therefore the step height varies strongly.

In the case of lateral pitch standards a comparable situation has to be faced for application in atomic force microscopy (AFM) and interference microscopy. Here standards with lateral pitch values below p = 70 nm are missing and needed to fulfil market demands. By using averaging the standard measurement uncertainty can be very small. However, to check the linearity of AFM at that scale it is important to have accurate pattern with pitch values down to 10 nm over sufficiently large areas.

 

Need for the project

Nanotechnology is one of the driving forces in meeting the challenges of the 21th century, namely new functionalities, new or improved products, the climate change, sustainable energy production and life science applications. Regarding the semiconductor industry, its influence on society is obvious, since nanotechnology based products have already started to influence and change the lifestyle of the majority of the general public. Other parts of the nanoindustry, using functionalized surfaces, nano mechanical products or life science application are still emerging markets, with a high potential for future markets. The development of new dimensional standards, which exceed the current limits, are of fundamental relevance to enable the invention of new products tailored to the grand challenges of the 21th century.

European companies are worldwide leading vendors for analytical instrumentation at the nanoscale. This position in the market was established with state-of-art technology and superior functionality. This achievement can only be maintained through a continuous development of their products and the innovation of new measurement technology for next generation instrumentation for the nanoscale to lead the world market.

The currently used approach for the production of standards for dimensional measurement is a so called “top down” approach. This means that the existing standards are simply made by shrinking the dimension requested by stakeholders in that moment. For the production of these standards existing methods are used, pushing the technology to their limits (and beyond), to fulfil the required quality standards regarding to production variations and uncertainty limitations. After production, these traditional standards must be investigated very carefully.  Each sample and each structure is investigated by the use of very sophisticated metrological instrumentations at the NMIs and related standardisation institutes, to measure the dimensions and the related uncertainty. This is a very time consuming and expensive method. Concerning dimensional standards, this traditional approach has now come to its final limits, which is indicated by lack of reliable standards for h < 6 nm and p < 70 nm (1D as well as 2D), respectively.

This common approach ignores the availability of some invariants of nature and the existence of self-alignment and self-assembly in emerging systems. This can be utilized for setting up new type of standards in a “bottom up” approach. This specific bottom up approach is a very promising technique, if the required feature size undercuts certain limitations, as is currently the case.

 

Scientific and technical objectives

This project aims to develop a new type of dimensional standards for step height and lateral resolution measurements for scanning probe microscopy (SPM) and interference microscopy for instance. The design parameter will be tailored to the demands of stakeholders of semiconductor and nanotechnology industry as well as in biophysics and surface science. These standards will fill the gap at the nano- and subnanoscale, reflecting the ongoing shrinking process.

The project has the following scientific and technical objectives

  • Development of prototype sample for step height measurement with sub-nanometer step height and expanded uncertainty of u(h) = 10 pm for UHV-STM application.

  • Stabilisation of crystalline step height samples for in air SPM measurement.

  • Stability verification for in air application prototype samples of crystalline step height samples.

  • Development of a software tool for the simulation of tip-sample interaction regarding to crystalline step height standards and improved uncertainty calculations.

  • Provision of a guideline for measurement at crystalline step height samples.

  • Provision of an uncertainty budget for SPM measurement of step height prototypes.

  • Provision of laterally pitch sample prototypes for high resolution applications in UHV, based on the basis of the unit cell of crystalline Si.

  • Provision of self-aligned Pd:Si(111)-7x7 lateral pitch prototypes with p = 5 nm for SPM application in air.

  • Stability performance testing for crystalline prototypes under UHV, N2 and air conditions.

  • Performance of theoretical calculations based on DFT to investigate the influence of air to tip-sample interaction.

  • Provision of an uncertainty budget for lateral pitch measurement based on experimental results and quantum mechanical modelling.

  • Development of lateral and step height sample prototypes using the self-assembly power of diblock copolymers with tuneable feature size. Lateral pitch between 10 and 50 nm and step heights in the range between 3 and 20 nm.

  • Provision of "find-me" structures for the identification of regions on interest on the sample protypes.

  • Propogation of the BC mask to silicon dioxide over the regions of interest (ROI) of 1 x 1 µm² to 10 x 100 µm² in area.

  • Fabrication and testing of nano objects produced by DNA nano-origami for their possible application in AFM based shape measurement.

 

Expected results and potential impact

The outcome of this project, which will be samples, procedures and good practice guidelines, will be fed into the pipeline of different standardisation bodies (DIN, ISO, etc.) to turn these prototype samples into standards for the nanotechnology market.

The expected impact is:

  • Instrumentation vendors (e.g. Omicron, Sensofar, Specs) can improve their market competitiveness by the demonstration and verification of their instrument’s capabilities via the measurement of the newly developed sample prototypes with reduced feature size and better uncertainty limitations.

  • Instrumentation manufactures in Europe are typically SMEs and worldwide leading companies in specific market segments. The outcome of this JRP, especially the length prototypes, will enable these companies to improve their equipment and will furthermore stimulate the invention of new instrumentation for higher resolution.

  • Standardisation institutes and calibration laboratories will benefit from the results of this JRP directly by setting up new standards for traceable measurement at the nanoscale. Furthermore the portfolio of calibration services can be extended for the first time into the sub-nanometre region for step height standards and nanometre range (5 nm – 50 nm) for lateral resolution standards. This improvement will also affect the uncertainty, which is expected to be in the range of some ten picometre.

  • The direct impacts to instrumentation manufactures and standardisation bodies will serve as a lever to enable and stimulate innovation in the bigger semiconductor market. Here the ongoing integration of semiconductor devices can only be maintained, if the appropriate reliable and traceable measurement technology is facilitated on-time. By the implementation of the new standards, the requests stated in ITRS will be fulfilled. The importance of the European semiconductor market can be seen in example of the two large microprocessor fabs in Dresden of Globalfoundries (formerly AMD operated), producing AMD CPU.

  • The reduction of the length standard prototypes down to the atomic scale will stimulate the application already in fundamental research projects. This is currently not the case, due to the lack of appropriate length standards in the sub-nanometre region of sub-nanometre heights or 5-50 nm for lateral resolution standards. This hinders, for instance, the traceable measurement of nanoparticles and carbon nanotubes. A reliable measurement of the particles is of paramount interest for environment and health care.

Thus far, the JRP has produced direct impact mainly for instrumentation vendors such as Omicron or Sensofar. Some of them (Omicron) are very actively involved in the upgrading of the UHV-STM to a traceable metrological STM. With regards to preparing for the introduction of new types of samples onto the market, the first users will be the instrumentation vendors, who need to demonstrate the potential of their instrumentation.

 

 

The JRP has made progess or started activities regarding the following scientific and technical objectives:

“Find-me” structures for the identification of regions of interest on the sample prototypes

The development of an appropriate “find-me” structure for each of the standards being produced in this JRP, including crystalline, BC, and DNA nano-origami standards, has already been completed. Led by NPL, all of the JRPs NMIs contributed to the development of these “find-me” structures. A technique was developed, which addressed the requirements for SPM and other related methods, such as simple identification of the region of interest for end users, without the structure interfering with the measurements. On the other hand, low production costs for the find-me structure could be achieved by including this step in a standard processing step during sample production.

Development of prototype samples for step height measurement with sub-nanometre step heights and an expanded uncertainty of u(h) = 10 pm for UHV-STM application

Prototype samples have been measured using AFM instrumentation at CMI, DFM, NPL and VTT.  These first handling tests are delivering a variety of information, required for writing the good practice guideline. The measurements are addressing the sample preparation and alignment of samples for measurement.  The measurements with different instruments furthermore ensures the general applicability of these prototype standards and this will help to provide a versatile tool for end user application on site.  Finally these strategies for data evaluation developed and tested by partners have started and used already existing ISO guidelines for step height measurement and this will offer the users continuation of their present work.  Additionally new algoriths are under development, to make use of the special features of the atomic step height standards. 

Stabilisation and verification of crystalline step height samples for in air SPM measurement and storage under UHV, N2, and air conditions

For the stability testing of the crystalline step height samples in the AFM instrumentation of CMI, DFM, NPL and VTT a special set of samples was produced by PTB. NPL has upgraded their storage options for N2. In air storage will be carried out by CMI, DFM and MIKES who will simulate different quality levels (well controlled lab conditions to “desktop”). Stakeholders will then be able to apply this in the practical application of the standards.

The first measurements of these samples revealed a twofold result. Immediately after opening the transport vessels and first measurements, the samples have a clean surface with atomically flat terraces and defined atomic steps. This was observed at all sites. After a period of time, varying from some hours to some days, the surface starts to become contaminated by particles. The first results are showing evidence for different contamination rates and there is a saturation of the contamination in the long term. Additionally XPS measurements by NPL, in conclusion with the other results, indicate that this problem originates within the fixing of the samples to steel plates, which is required for the measurement in the SPMs.

Tests at PTB are demonstrating the stability of the samples over a period of more than six weeks.  A sample has been measured directly after production with a Bruker AFM (Type ICON). During the six week summer break the sample was stored in the AFM without changing its position under ambient lab conditions.  This measurement has revealed a slightly increased roughness of the sample, but without negative impact on the step height surface measurement.  Besides this, no negative impact on the samples has been observed and the surface is practically unchanged.

Development of a software tool for the simulation of tip-sample interactions with crystalline step height standards and improved uncertainty calculations and DFT based simulations to investigate the influence of the air on tip sample interaction

FZU has developed software for ab initio calculation of the tip-sample interaction. First calculations have been carried out with simplified models of the tips. The results of these simulations have been reported to the other partners in order to discuss the boundary conditions and the output parameter of the simulation. This will help to improve our understanding of the measurement process and will support the setup of the uncertainty budget. A copy of the simulation software has been installed at CMI.

Simulation of different tip and sample configurations are still being carried out.

Development of lateral and step height sample prototypes using the self-assembly power of diblock copolymers with tuneable feature size. Lateral pitch between 10 and 50 nm and step heights in the range between 3 and 20 nm

Block copolymers (BC) form regular patterns which relate to their chemical composition and to the processing parameter (such as temperature, ramping and time). For the project a regular alignment of BC pillars has been produced, serving as a lithography mask for transferring the pattern into silicon. REG(PMO) and INRIM have determined the optimal set of parameters in relation to the time and temperature during annealing for the production of regular BC pillars. The process of self-assembly is furthermore supported by pre registering of the Si substrate with trenches of defined length and width. This pre-registering has been carried out by PMO and samples have been delivered to REG(PMO) for carrying out the self-assembly of the BC.

By optimizing the processing parameters INRIM was able to produce within the pre-registered trenches mono domain gratings with self-assembled BSs.  Enlarging the undisturbed area with a periodical grating is a key requirement for the reduction of the uncertainty for the lateral pitch measurement.  The development has now come to a point, where it is possible to create prototypes of lateral solution standards in a controlled and reproducible way.  Consequently INRIM will now produce sets of lateral resolution samples to be distributed to the consortium.  AFM inspection, comparable to the work already conducted for step height samples, will prepare the way to outline criteria for a good practice guideline.  This will pave the way towards round robin comparisons of such prototype standards for interested stakeholders.

Propagation of the BC mask to silicon dioxide over the regions of interest (ROI) which are 1 x 1 µm² to 10 x 100 µm² in area

After the controlled production of BC based etching masks for laterial resolution standards, INRIM is now working on the transfer of the regular lateral BC pattern into the SI substrate via reactive ion etching (RIE).  This step will increase the usability of the prototypes, since the transfer to Si will increase the stability of the prototypes.

As the work on the BC based samples has progressed very well, INRIM has already processed samples with a domain structure into the final Si structure. These samples were distributed to DFM and VTT for first feasibility tests on their SPM instrumentation. The results of these first measurements are proving the lateral regularity of the BC structures and are promising for the successful production of BC based lateral pitch samples for application.

Fabrication and testing of nano objects produced by DNA nano-origami for their possible application in AFM based shape measurement

Aalto has designed DNA origami structures by caDNAno software and accordingly purchased the DNA strands needed for the structure formation (scaffold strands and synthetic staple strands). The optimal conditions for folding the above-mentioned nanostructures have been studied and the folding quality has been verified using agarose gel electrophoresis, UV-Vis spectroscopy and TEM imaging. Furthermore, the excess staple strands have been efficiently separated from the folded structures by spin-filtering. This has resulted in purified high-quality samples containing DNA origami nanostructures.

Finally Aalto was able to produce DNA-nanoorigami in liquid phase with a high level of purification.  These samples were handed over to VTT for checking their application according to certification.  The focus on this feasibility study is:  preparation of samples for measurement, stability of the molecules during AFM measurement, identification of wrong molecules, statistical variation of the DNA-nanoorigami and stability during storage.

In the last year of the project the activities which have already started will continue and finish.  Additionally activities for the following objectives will be carried out:

  • Provision of a guide line for measurements using crystalline step height samples.
  • Provision of an uncertainty budget for SPM measurement of step height prototypes.
  • Provision of lateral pitch sample prototypes for high resolution applications in UHV, based on the unit cell of crystalline Si.
  • Provision of self-aligned Pd:Si(111)-7x7 lateral pitch prototypes with p = 5 nm for SPM application in air.
  • Provision of an uncertainty budget for lateral pitch measurement based on experimental results and quantum mechanical modelling.