Wafer applications

in surface metrology

Wafer applications in surface metrology

This article is not about sweets. Here we deal with the wafers from which microchips are made. Learn how wafers are manufactured and processed, what makes a perfect wafer and which measuring applications can be used for quality assurance. 

 

The manufacturing process

Microelectronic components and semiconductors are manufactured on round thin discs, so-called wafers. Wafers can be made of various conductive or non-conductive materials such as silicon, sapphire or glass with a typical diameter of 100, 150, 200 or 300 mm.

The blank wafers are subjected to various etching, grinding and polishing processes in the manufacturing process. The wafers are given an almost perfectly flat surface by these production processes. The components and structures are produced by a repeated sequence of additional structuring and deposition processes.

 

From the initial product ingot to high-quality wafers

Manufacturers in the fields of microelectronics, microsystems technology and photovoltaics have high demands on the production tolerances of the pre-product “wafer”. Even small deviations can have a negative impact on the quality in the downstream, cost-intensive process steps. This results in lower yields and reduced efficiency and reliability of the end products. High-quality, fully automatic multi-sensor measuring technology contributes to the control of process tolerances in wafer manufacturing and helps to maintain the required quality standards of the producers.

Microchip

Sawing, grinding and polishing - Tolerances and standards in wafer processing

Wafers are the substrate for the production of integrated circuits, light-emitting diodes, micromechanical (MEMS) components and solar cells. The particularly fine thin wafers are required primarily for the manufacture of 3D IC components. The micrometer-sized circuit packages are created by stacking and connecting in the vertical direction. Microchips mounted on thin wafers are assembled. This miniaturization allows even more efficient circuits to be realized, for example for more efficient solid-state disks, more compact CMOS image sensors and more powerful and energy-efficient logic components. The requirements for the specifications of the output product wafers are immense. Sawing, grinding and polishing require maximum precision.

Learn more about microchip production and why advanced packaging is so important for device performance.

Read now!

Sawing the starting product

The ingot, a block of semiconductor material (e.g. silicon) or a compound material (e.g. Gallium Arsenide (GaAs)), is the starting material for wafer production. As a rule, the ingot is drawn from the melt, usually using the Czochralski process, and doped during this process.

With a high-precision wafer saw, individual raw wafers are cut from it. This creates grooves whose width and depth must be checked. Optical measuring systems with multi-sensor configuration are ideal for this task. The saw contour cannot only be visualized three-dimensionally but also quantitatively and metrologically characterized - a task in which classical optical microscopes reach their limits. The quantitative measurement allows the sawing process to be better controlled, so that the deviations in this process step are significantly lower. In addition, the tool wear of the processing machines can be monitored and the behavior of different materials during the sawing process can be evaluated.

Wafer polished

Grinding evenly

After sawing the wafers are thinned using mechanical processes such as grinding and lapping. During the grinding of a raw wafer or structured wafer certain quality parameters must be mantained. For raw wafers, for example, the TTV value indicates uniform removal during grinding. The TTV value is statistical value based on a metrological wafer thickness measurement indicating the absolute thickness variation of the entire wafer. The point here is to determine the maximum difference between the thickest and thinnest point, the so-called Total Thickness Variation (TTV).

Total Thickness Variation set up TTV

Smooth polishing

In the thin wafer polishing process, a polishing pad applies pressure to make contact with the wafer surface and polishes it. A polishing paste (slurry) containing chemically effective substances and abrasive materials are used. The removal on the wafer surface occurs by friction. In a dry-polish process, the chemical and abrasive material is bound in a pad. In this process, a lower contact pressure is sufficient. Optical measuring systems are used for the monitoring of the polishing process, because the determination of the roughness allows conclusions to be made about the surface quality.

Wafer polished

SAW MARKS

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Automated detection and reporting of saw marks

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Area measurement of saw marks on a Si wafer

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Topography of a Si wafer along the blue line

ROUGHNESS

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DIN/ISO-compliant roughness and waviness measurement

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Area measurement of a polished Si wafer, sRa = 0,766 nm

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Profile measurement of a Si wafer, Ra = 13,8 nm

FLATNESS

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Flatness measurement

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Topography of a CMP macro area

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Profile measurement of a CMP macro area, TIR = 53 nm

DEFECT INSPECTION / VOIDS

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Defect inspection

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3D topography of a Si wafer with voids

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3D topography of a dimple of a Si wafer

wafer flatness

It has to be flat: The perfect wafer

The surface finish of the 'round disc' is characterized by its flatness. The flatter the surface, the more perfect the wafer is. Differences in height may lead to contacting problems during subsequent stacking to a 3D IC wafer package.

In addition, the wafers must be ground to an exact thickness and the doping introduced into the substrate should not be removed too far. The wafer edge can also be checked at random with a 2D profile measurement. From this, the wafer manufacturer can determine the wear of the grinding tools and optimize the process parameters. Waste at the edge of the wafer also plays an important role. The more efficiently this is done, the more microchips can be produced on the wafer later and the so-called fill factor increases. Processed wafers that are equipped with bump structures or solder balls, for example, can be measured in terms of height, width and coplanarity.

FRT offers the right measurement technology for your application. We will be glad to help you solve your measurement needs by creating the best system configuration for you.

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Special Interest: Advanced microchip packaging

Advanced packaging technologies are rapidly evolving in the semiconductor industry to achieve the functionality, speed and form factor required for the mobile market.

As wafer level packaging (WLP) and heterogeneous integration (HI) approaches become more relevant, metrology processes begin to creep into back-end process control, where measurement becomes trickier and more diversified. The dawn of fan-out (FO) processes both at the wafer and panel level has added more diversity to metrology needs. The addition of 2.5D and 3D heterogeneous integration, and now chiplet technologies further expands the diversity of applications.

The fabrication of an integrated circuit (IC) requires a variety of physical and chemical processes performed on a semiconductor (e.g., silicon) substrate. By creating structures of various components, millions of transistors can be built and wired together to form the complex circuitry of a modern microelectronic device.

Three-dimensional integrated circuit (3D IC) and 2.5D IC with Si interposer are regarded as promising candidates to overcome the limitations of Moore’s law because of their advantages of lower power consumption, smaller form factor, higher performance and higher function density. To achieve 3D and 2.5D IC integrations, several key technologies are required. The cost and complexity of these new packaging technologies require cost-effective inspection and metrology solutions across the entire process to ensure product quality and yield.

Learn more about Advanced Packaging now!

Download process steps Read blog series Download MicroProf® AP info

Meet the measurement requirements

Whether for measurements with several layers, for example with bonded or taped wafers, edge evaluation of the wafers or the evaluation of the coplanarity of bump structures, with modern optical measuring systems you can easily measure your wafers with high accuracy and keep pace with the increasing demands on accuracy and reproducibility.

TTV setup: Option for two-sided sample inspection 

The system can be equipped with an opposed sensor configuration (TTV setup) consisting of two non-contact chromatic white light sensors.

It is possible to measure both the roughness and the Total Thickness Variation (TTV) with very high resolution using the FRT MicroProf® surface measuring tool.

This multi-sensor tool measures the complete wafer surface and determines thickness, TTV, bow, warp and flatness as well as high resolution 3D topography, 2D profile and roughness measurements. The optical sensors are fast and very accurate. Furthermore, the system can be equipped with Atomic Force Microscope (AFM).

WAFER THICKNESS / TTV

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SEMI-compliant thickness and TTV measurement

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Full wafer thickness map in 3D view, polished Si wafer

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4 profiles of a polished Si wafer showing thickness variation

BOW / WARP

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SEMI-compliant bow and warp measurement

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Full wafer map of a Si wafer in 3D view showing bow

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Profile of the top topography of a Si wafer

GLOBAL / LOCAL WAFER PARAMETERS

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Measurement of local and global wafer parameters

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Full thickness wafer map in 3D view

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Wafer map with local parameters (LTIR, LTV, LT, LFPD, etc.)

Metrological measurement applications

The following metrological measurement applications can be performed automatically:

TTV, Bow and Warp

Determination of TTV, Bow, Warp and a variety of other parameters on the basis of punctual measurement data, profile measurements and 3D measurement data. Evaluation of sample thickness and bow measurement according to SEMI standard. Determination of the parallelism of upper and lower topography measurement and the roll-off amount over the wafer edge.

Wafer Thickness, Center Thickness, Wafer TTV, Warp, Wafer Warp Front, Bow BF, Bow BF Front, Wafer Sori, Wafer Taper, TIR, TIR95, NTD, NTV, Sag X, Sag Y, Sag (avr), Profile Thickness, Profile Warp, Profile Sori, Profile Taper, TV5, TV9, GBID, GF3D, GF3R, SBDI, SF3D, SF3R, SFLD, SFQR, Wafer FPD, LTIR, LTV, LFPD, Sample Thickness (Thickness (Default), Min., Max., Std., TTV), Bow, Parallelism, Roll-Off Amount (ROA Front, Avr. ROA Front Side, Min. ROA Front, Max. ROA Front, ROA Back, Avr. ROA Back Side, Min. ROA Back, Max. ROA Back, ROA TTV, Avr. ROA TTV, Min. ROA TTV, Max. ROA TTV, ROA Measurement Point)

Bumps, Vias, Trenches

Evaluation of bumps in 3D-measurements, automatic detection of steps in profiles and evaluation of the height or depth of features in area measurements with statistical
evaluation.

Bumps (Height, Diameter, Number, Coplanarity), Step Height (Height/Depth, Inner Width, Outer Width, Inner Distance, Outer Distance, Number, statistics for each result parameter)

Critical Dimension and Overlay

Determination of critical lateral structure sizes based on 3D measurement data or camera images. Determination of overlay parameters such as offset (x,y) and rotation. For this calculation, the results are obtained from two separate “Critical Dimension” results (Hybrid Evaluation).

Length, Width and Diameter of the detected feature, center coordinates related to the measured element (die), the measured sample (wafer), xy table coordinates, Offset (x,y), Rotation, Overlay Vector Map, statistics and single values, Shape Quality, Inside Shape (Average, Min., Max., Range, Std.), Spill Out (Average, Min., Max., Range, Std.), Shape Area Difference

Roughness, Waviness and Flatness

Calculation of roughness and waviness values according to DIN EN ISO 4287. Determination of flatness using the TIR (Total Indicator Reading) value.

Ra, Rq, Rz, Rt, Rp, Rv, Rmax, Rz25, Rmax25, Wt, Lc, Ls, sRa, sRq, sRz, sRt, sRp, sRv, sRmax, sRz25, sRmax25, sWt, TIR

Step Height

Automatic recognition of (multiple) steps in profiles, evaluation of height or depth of features in area measurements with statistical evaluation.

Height/Depth, Inner Width, Outer Width, Inner Distance, Outer Distance, Number, statistics for each result parameter

Saw Marks

Evaluation of the depth of saw marks based on profile measurements.

Groove Depth, wafer position of the groove, profile position of the groove

Film Thickness

Evaluation of the film thickness of a single layer and layer stacks (several layers simultaneously).

Film thickness, Min., Max., Range, Std.

Wafer Stress

Determination of the stress of a wafer by measuring profiles (star-shaped through the center) before and after a coating process. Mapping of local stress is also possible
with more than 8 profiles.

Stress (avr), Min., Max., Range, Std., value in center of wafer

Lens Shape

Star-shaped profile measurements over the lens apex to determine the shape of the lens and automatic determination of the shape of the lens via a fit of an ideal shape.

Radius of Curvature, Chi-Square (Average, Min., Max., Range, Std.), Standard Deviation (Average, Min., Max., Range, Std.)

Angle Evaluation

Determination and output of the angle between selected profile regions. Evaluation of profile measurement data and extracted profiles.

Angle

multi-sensor configuration wafer

Multi-sensor concept

Based on our multi-sensor concept, the 3D surface measuring devices can be equipped with point, line and field of view sensors for topography analysis as well as with film thickness sensors. In addition, atomic force microscopy can also be applied. As a result, complex measurement tasks can be solved with the use of different sensors by collecting the data from each sensor and subsequently combining the different results.

The decisive step is now that the instrument, respectively the recipe used, knows the complete measuring task and fully implements it. This means not only the data collection with all necessary sensors is automated, but the software also records the different results and calculates the desired parameters.

Hybrid metrology: Determine parameters that cannot be measured directly!

A hybrid measuring concept increases the precision of measurements on samples for which a single sensor or a single measuring principle is simply not enough. Depending on the application, it can include measurements with different topography and (layer) thickness sensors, which are fully automated by a single recipe. Controlled by FRT software, these sensors automatically acquire different data sets to create new information by combining data which is not directly accessible with a single sensor.

HYBRID METROLOGY

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Combination of different sensor, automated calculation of the desired sample parameters

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Example of hybrid mertrology

The software SEMI-compliant Acquire Automation XT

Select from a variety of software packages the measurement and evaluation routines for your measurement applications. For recurring structures, a layout wizard with a graphical user interface (GUI) can help you to teach the desired measuring positions. In addition, a fine alignment of the samples is available via pattern recognition.

Interested in more info?

Download data sheet

Software Recipe

The software offers comprehensive possibilities, from manual measurements to fully automatic, recipe driven measurements as well as integration into production control systems, e.g. via a SECS/GEM interface. The recipes are then triggered by the host and the measuring results are automatically transferred to the Fab control.

 Software Result Wafer

You can configure different measurement applications with different sensors so that they run in series within a recipe. This includes performing the measurements, processing and analysis with intelligent algorithms, outputting and visualizing the results in reports and exporting the results in various data formats.

The tools are driven by the SEMI-compliant Acquire Automation XT software. This software enables recipe-based measurement and data analysis of structured and unstructured wafers, as well as panels.

Learn even more in data sheet.

The MicroProf® series: standard and special solutions to improve your production efficiency

Whether you need a standalone, non-contact wafer measuring tool for your lab or a fully automated, integrated tool in your front-end or back-end areas, FRT offers the appropriate standard and special solutions to improve your production efficiency. The degree of automation extends from manually operated or semi-automated tools such as the MicroProf® 300, to fully automated tools with wafer handling. It includes automatic pre- and fine alignment in the MicroProf® MHU for very high-volume processing of microelectronics and wafers. Further options are the handling of thin wafers and an ionizer bar.

MicroProf® 300
MicroProf® MHU

In addition, tools can be equipped with powerful image acquisition hardware for intelligent pattern recognition. FRT systems can be configured to handle SEMI-compliant and non-standard wafers often used in the MEMS industry. We also offer a variety of solutions for the growing market segment of 3D IC manufacturing from thin film measurement to trench measurement to inspection of critical dimensions in the whole manufacturing process.

For semiconductor applications, FRT offers tools for clean room manufacturing: the MicroProf® FE, MicroProf® FS and MicroProf® AP. As a standard the MicroProf® tools are equipped with filter fan units (FFU) that ensure ISO Class 3 clean room conditions within the unit.

MicroProf® FE
MicroProf® AP

CRITICAL DIMENSION

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Measurement of critical dimension of features

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Topography measurement of pre-filling vias

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Profile measurement of a pre-filling via, CD and depth

OVERLAY

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Overlay measurement

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Automatic recognition of marks and determination of overlay shift

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Overlay map: example of display of overlay errors

TRENCHES

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Measurement of smallest trenches

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Area measurement of a RDL test structure

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Profile measurement of RDLs (width, height and spacing)

FILM THICKNESS / LAYER STACK

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Thin film and layer stack measurement

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Thickness map of a photoresist coating on a Si wafer

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Coating thickness profiles of a photoresist on a Si wafer

COPPER NAIL PROFILE

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Copper nail dimensions an coplanarity

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Area measurement of a copper pillars

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Surface profile of copper pillars

The MicroProf® can process 150/200 mm and 200/300 mm wafers in one system (FOUP, SMIF and open wafer cassettes possible). The SEMI-compliant software GUI enables interactive or automated use, easy creation of measurement and evaluation recipes and integration into existing production control systems via the SEMI-compliant SECS/GEM interface. This interface transfers measurement information to the next step in the production line.

FRT offers the right measurement technology for your application. Do not hesitate to contact our experts if you have any questions. We will be glad to help you solve your measurement needs by creating the best system configuration for you.

Get in contact

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