Invited paper

Micro Manufacturing – Process and Part Quality Control
H.N. Hansen
Department of Mechanical Engineering, Technical University of Denmark, Denmark
Abstract – This paper presents the challenges related to process and part quality control in micro manufacturing. The
use of dimensional metrology is discussed with focus on precision and uncertainty estimation. Tolerance verificationand process qualification is depending upon measurements uncertainty, and in micro manufacturing the ratio U/T isoften large compared to conventional scale manufacturing. The challenges are presented using a number of cases, andvarious approaches to their solution are suggested.
Key Words: Micro manufacturing, metrology

1

INTRODUCTION

Micro manufacturing is a common term used for production of
products and components with critical dimensions in the sub-mm
range. Different definitions have been proposed over the years
trying to be more specific [1-4]. Currently research in the micro
manufacturing is focused on single manufacturing processes andtheir interaction with the materials being processed. Focus isgiven to size effects [5-7], process capabilities [8-10] and to someextent components quality [11-13]. The establishment of coherent
process sequences, to cover all necessary processing steps fromtooling over replication to assembly processes is necessary for theindustrial implementation. When integrating single processes into
production systems issues as material compatibility, relative
accuracy, alignment precision etc. become critical. The necessary
actions related to quality control comprise process validation andverification of tolerances as specified in the design. This paper
discusses challenges and possible solutions related to thisproblem complex and presents selected cases for illustration.
2

tolerance (T) in order to establish the so-called conformance zone
(Fig. 1). If the ratio U/T is close to 1, then a very little
conformance zone is present, and the consequence is a very little
allowable interval of variation for the manufacturing processes.
The so-called ‘golden rule of the gauge maker’ expresses that if
U/T < 0.1, then you are on the safe side. In micro manufacturing
U/T almost never is on that level. It is far more likely to see
0.5<U/T<1, and of course in this case verification becomes
critical.
Fig. 2 illustrates this connection in a different way. Variations
due to measuring instrument, chosen measurement procedure etc.
should be smaller than the expected process variation if themeasurements are used for process analysis.

PROCESS VALIDATION AND METROLOGICALCAPABILITIESIn micro manufacturing, like in ordinary scale manufacturing, it isimportant to control product characteristics such as dimensions,geometry, surface roughness, material properties etc. This implies
methods, procedures and instrumentation that can be applied insuch a way that results are meaningful and applicable in quality
and process control. Basic boundary conditions include:x

Capability of measuring critical dimensions on the μm oreven nm scale o requirements regarding instrument
resolution, traceability, measurement uncertaintyxCapability of measuring and/or combining dimensions
across several orders of magnitude on the same item orequirements regarding measurement range and resolution,measurement principle

x

Possibility to verify product tolerances on the μm scale omeasurement uncertainty must be determined and extremely
lowx

Process precision/accuracy/variation very high o similar or
lower measurement capability required

Due to these boundary conditions it is necessary to apply
metrological sound methods only. For the verification oftolerances the challenges are manifold [12-13]: tolerance zones
are reduced as absolute dimensions are scaled down;
measurement uncertainties are not reduced at the same rateleaving a smaller conformance zone for the verification (Fig. 1).
This link to the design and specification of components is a key
point in bringing micro manufacturing from the laboratory toindustrial applications. According to ISO 14253 [14] themeasurement uncertainty (U) must be subtracted from theFig. 1. Illustration of tolerance zone and measurement
uncertainty.

Process variations
Metrology procedure
Instrument
Fig. 2. Illustration of variations.
3

MICRO FLUIDICS

Micro fluidic systems are used to analyse small amounts ofliquids in the bio-chemical area. Such systems are characterisedby micro channels and other geometries of this order ofmagnitude. Dimensional accuracy of micro channels is of great
importance in micro fluidic systems. Fundamental measuring
tasks to be performed in quality control of micro fluidic systems
are the width (define as the distance between two opposing
surfaces) and the depth (defined as the distance between two
surfaces of same orientation but placed in a vertical direction) ofa micro channel [2]. A micro channel’s width and depth mustcomply with the dimensional specifications in order to be able tocontain the correct volume of reagents and biological fluids to be

ISUPEN 2013
International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.

5

Invited paper

analysed and to convey the requested flow rate designed for thefunction to be delivered. Surface topography is also of great
importance in polymer micro fluidics, hence the replication
capability of the process and the surface quality of the tool has tobe evaluated and possibly optimized. The surface finish on micro
fluidics has different roles, depending on the location on thesurface device. Firstly, low roughness is required to allow
assembly by for example laser welding or thermal bonding of alid on top of the micro fluidic unit for sealing purposes and,secondly, to diminish the interaction between the bio-fluid to beanalysed and the channel/reservoir surfaces [15].
The micro fluidic system presented in this paper was designed toseparate red blood cells from plasma [16]. The process chain for
manufacturing was rather complex and involved micro electricaldischarge machining of a Silicon wafer (to create the master
geometry), followed by PVD and electroforming of a Ni
counterpart. This Ni shim was finally used as an insert in an
injection moulding process to create polymer micro fluidic
systems. Fig. 3 illustrates the process sequence of creating the Ni
shim.
1

Micro EDM of silicon substrate(master)2Pre-treatments including cleaningand deposition of a thin layer of
Ti/Cu by PVD coating

3

Electroforming of nickel and copper
for the insert fabrication4

Selective etching of silicon in a
warm alkaline solution

5

Mechanical machining of the back of
the insert and of the external shape

6

Final cleaning and selective etchingof the Cu layer

Fig. 4. Examples of channel width and height measurement of
micro fluidic system: Si master, Ni shim and polymer part (fromleft to right) [16].

Fig. 5. Surface roughness analysis using WLI on micro fluidic
system [15].
Reservoir chamber surface
Micro tool surface (Nickel)
Ra = 448 nm (U = 27 nm)

Fig. 3. Process chain for making the Ni shim for the micro fluidic
device [17].
The precision moulding of a microfluidic system for blood
analysis was analysed and validated by using a metrologicalapproach applied to both micro channels dimensions (width anddepth) and to surface topography on a reservoir chamber and on
the microfluidic substrate surface. Dimensions were measured
using an optical coordinate measuring machine (CMM); opticalprofilometry such as white light interferometry (WLI) was used
for topographic characterization of polymer surfaces. Opticaltechniques provide fast non-contact measurements, which are in
general recommendable when in-line process controls are neededon soft materials and when sample contaminations are to be
avoided. Careful calibration of the instruments was performed
and the measurement uncertainties determined. In this respect, thereplication process involves a mirroring of the structure in such a
way that an indent on the shim is seen as a protrusion on thereplicated part adding the complication of establishingcomparable measurement strategies. In particular, the relocationof corresponding measuring points on different specimens is key
issue [16] that adds to the achievable lower measurement
uncertainty. Fig. 4 shows a reasonably good agreement between
the different steps of the process chain – and consequently aconclusion is that the replication of these features throughout theprocess chain is acceptable. However, the nominal width was notmet (Fig. 4) mainly due to problems with the micro electricaldischarge process and the control hereof.
Surface roughness results are shown in Fig. 5 and 6. Two areas
were analysed: reservoir chamber surface and micro fluidic
platform surface. The two levels of roughness are significantly
different.

6

Moulded part (PP)Ra = 301362 nm (U = 11
nm)

Microfluidic platform surface
Micro tool surface (Nickel)
Ra = 71 nm (U = 5 nm)

Moulded part (PP)Ra = 2848 nm (U = 2 nm)

Fig. 6. Comparison of surface roughness values on tool andmoulded part [15].
The results discussed above are interesting from two viewpoints.
First they can be seen as a verification of the part quality. In thiscase measurement results have to be compared with the tolerances
taking measurement uncertainty into account as alreadydiscussed. The other application is to use the results for process
analysis and process optimisation. In the current caseinvestigations were performed to analyse the injection moulding
process based on a statistical design of experiments. The purposeof these investigations was to find optimal settings of processparameters based on the influence of these process parameters on

ISUPEN 2013
International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.

Invited paper

achievable dimensions and surface roughness. Fig. 7 illustrates
how the process parameters melt temperature, mould temperatureand injection speed influences the depth of the channels whenvaried between a “low level” and a “high level” (two levelfactorial design). It can be seen that there are differences between
the observed channel depths when changing process parameters.
It can also be seen that the maximum differences are of the orderof some μm and the measurement uncertainty is determined to beof the order of ±5 μm. In this case, the observed changes are notsignificant compared to the measurement uncertainty. Fig. 7 also
contains the dimension of the tool (measured as the height of a
protrusion that results in a channel during moulding). A typicalobservation is that the tool dimensions usually can be determinedwith a lower uncertainty than the polymer parts. This is partly due
to material properties (the parts are measured using opticalmethods), partly due to the mirroring of the geometries during
replication.

Fig. 7. Process optimization based on statistical design ofexperiments [15].
The diversity of features (e.g. channels, reservoirs) and theirdimensions (e.g. from 50 nm to 500 μm) makes the measurement
situation quite challenging. For each device, in principle you must
develop a specific procedure and this will ultimately affect theobtainable measurement uncertainty. So for the sake of processoptimization and process control we have suggested theintroduction of the so-called process fingerprint [18]. The
fingerprint contains a variety of features that cover a certain
dimensional range (see Fig. 8) and therefore can represent typicalfeatures of a variety of real micro fluidic systems. The fingerprint
should be put on the microfluidic system in a position that does
not influence functionality. This requires the fingerprint to have asmall footprint. By analysing certain parts of the fingerprint most
relevant to a specific micro fluidic system it is possible tooptimise the process. Fig. 8 illustrates a process fingerprint layout
based on crosses of various widths (10 m, 2 m and 500 nm
respectively) and same nominal depth (60 nm).

4

MICRO OPTICS

In a second case a process chain for production of polymer micro
optics was investigated. A mould geometry has been machinedusing ultra high precision machining techniques with a resulting
surface roughness of optical quality (Fig. 9 Top). These inserts
were mounted in an injection moulding machine (Fig. 9 Middle)
and the resulting polymer micro lenses are seen in Fig. 9 Bottom.
Two moulds were made and one was coated with TiN coating[19]. Polymer micro structured optical Fresnel lenses were
injection moulded using a commercially available optical grade
highflow
polycarbonate(Makrolon
2405
by Bayer
MaterialScience). Injection mouldings were executed on aconventional injection moulding machine with a reciprocatingscrew of diameter of 35 mm and a clamping force of 60 kN.
The dimensions of the tool inserts micro structures were
investigated at different production stages to evaluate the tool
wear. However, the characterization of highly reflective surfaces
with nanometer surface finish is a challenging task to be
performed. On one hand, tactile instruments could damage thesurface itself, and on the other hand, measurements carried out
with optical instruments could be hampered by the highreflectivity of the surface limiting their accuracy. Also, the inserts
have to be dismounted at each quality control step of the quality
control process to be positioned on the measuring instrument,
making the whole procedure lengthy and cumbersome. For thesereasons, an indirect surface metrology method based on surface
replication was developed and applied directly on the tool using asoft replica material (polydimethylsiloxane, PDMS) curing atroom temperature. A replication mould device that could be easily
mounted and removed from the mould was designed andmanufactured to solve the issue of disassembling the inserts fromthe injection moulding tool (Fig. 9 Middle) [19].
The tool wear was investigated by means of dimensional
measurements and scanning electron microscopy. A total numberof 24500 injection moulding cycles were run. Dimensional
measurements were carried out with the replica technique each1000Ǧ2000 cycles with the tool inserts mounted on the mould
during production. Fig. 10 illustrates the observed deviations of
selected rib heights over 24500 mouldings. Deviations are smaller
than 1 μm, and therefore measurement uncertainty becomes
critical. Furthermore measurements are performed on a replica of
the mould, and the variation of this replica process in
combination with the stability of the rubber material over a shortmedium period will influence measurement uncertainty.
Assuming that absolute traceability is of less importance because
we want to observe relative changes over time, still we need toassure stability of measurement procedure over time.
5

CONCLUSION AND OUTLOOKThe paper has described the challenges related to process andproduct quality control in micro manufacturing. When absolute
dimensions are scaled down, so are the tolerances, but usually at alower rate. This means that the ratio between measurement
uncertainty (U) and tolerance interval (T) becomes larger than inordinary scale manufacturing. This implies that care must be
taken to determine measurement uncertainties and to reduce theirsizes as much as possible. For process control metrologicalsolutions must be able to determine critical process fluctuations,and this again requires small measurement uncertainties andestablishment of traceability. These challenges have been
illustrated with two typical cases: a micro fluidic system andmicro optics.

Fig. 8. Process fingerprint for nano scale features [18].

ISUPEN 2013
International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.

7

Invited paper

[2]
[3]
[4]
[5]
[6]
[7]
[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

Fig. 9. Tool and polymer parts for micro optical elements [20].
[16]

[17]

[18]

Fig. 10. Investigation of wear phenomena in micro injection
moulding [19].

[19]

6 REFERENCES[1]
Alting L., Kimura F., Hansen H.N., Bissacco G., Microengineering, Keynotepaper, Annals of the CIRP, Vol.
52/2 (2003), p. 635-658.

[20]

8

Hansen H.N. et al, Dimensional micro and nanometrology, Keynote paper, Annals of CIRP Vol. 55/2
(2006), p.721-744.
Masuzawa, T., 2000, State of the art of
micromachining, Annals of the CIRP, 49/2:473-488.
Brinksmeier E. et. al, 2001, Machining of precisionparts and microstructures, Proc. of the 10th ICPE, p.19.
Geiger M. et al, 2001, Microforming, Keynote paper,
Annals of the CIRP, 50/2:445-462.
Vollertsen F. et al, Size effects in manufacturing of
metallic components, Keynote paper, Annals of CIRP
Vol. 58/2 (2009), p. 566-587.
Bissacco G. et al, Modelling the cutting edge radius
size effect for force prediction in micro milling, Annalsof CIRP Vol. 57/1 (2008), p. 113-116.
Hansen H.N. et al, Complex process chains for
manufacturing of invisible displays integrated in bulk
metal panels, Annals of CIRP Vol. 56/1 (2007), p.237240.
Islam A. et al, Process chains for the manufacture ofmolded interconnect devices, International Journal ofAdvanced Manufacturing Technology, Volume 42 (910), (2009), p. 831-841.
Hansen H.N. et al, Precision tooling technologies for
micro forming, Invited paper, 2nd International
Conference on New Forming Technology, Bremen,
September 20-21, 2007, p. 157-165.
Hansen H.N., Dimensional metrology in micro
manufacturing, Keynote paper, 3rd International
Conference on Multi-Material Micro Manufacture(4M2007), Borovets (Bulgaria), 3-5 October 2007, p.
29-35.
Hansen H.N. et al, Tolerancing issues in micro
manufacturing, 10th Anniversary International
Conference of the European Society for Precision
Engineering and Nanotechnology (Euspen), Zurich(Switzerland), 18-22 May 2008, Vol. II, p.317-320.
Tosello G. et al, Applications of dimensional micro
metrology to the product and process quality control inmanufacturing of precision polymer micro components,
Annals of CIRP Vol. 58/1 (2009), p. 467-472.
ISO 14253-1:1998 Geometrical Product Specifications(GPS) – Inspection by Measurement of Workpieces
and Measuring Equipment – Part 1: Decision Rules for
Proving Conformance or Non-Conformance with
Specifications,International
Organization
forStandardization, pp. i–v, 1–15.
ToselloG.,
Marinello
F.,
HansenH.N.,
Characterization and analysis of micro channels andsub-micron surface roughness of injection moulded
microfluidic systems using optical metrology, Polymer
Process Engineering ‘09 Conference (PPE’09),
Bradford (United Kingdom), 27-28 October 2009,
pp.199-232, ISBN 13 978 1 85143 262 2
Tosello G., 2008, Precision Moulding of Polymer
Micro Components – Optimization, Simulation,
Tooling, Quality Control and Multi-Material
Application, Ph.D. Thesis, Technical University ofDenmark, Department of Mechanical Engineering,pp.1-242, ISBN: 978-87-89502-77-9.
Tosello G. et al, High aspect ratio micro tool
manufacturing for polymer replication using μEDM ofsilicon, selective etching and electroforming,
Microsystems Technology (2008) 14:1757-1764.
Calaon M. et al, 2012, Production quality control ofmicrofluidic chip designs, Proceedings of the 3rd
European Conference on Microfluidics - Microfluidics
2012 - Heidelberg, December 3-5, 2012.
Tosello G. et al., Surface wear of TiN coated nickel
tool during the injection moulding of polymer micro
Fresnel lenses Annals of CIRP, Vol. 61/1 (2012), 535538
Gasparin S., 2012, Verification of Tolerance Chains in
Micro Manufacturing, Ph.D. Thesis, Technical
University of Denmark, Department of MechanicalEngineering, pp.1-232.

ISUPEN 2013
International Symposium on Ultraprecision Engineering and Nanotechnology, Tokyo, Japan, March 13, 2013.