Patterning of Thick Parylene Films by Oxygen Plasma for 

Application as Exploding Foil Initiator Flyer Material 

by Eugene Zakar and Michael D. Grapes 

ARL-TR-4956 September 2009 

Approved for public release; distribution unlimited. 
NOTICES 

Disclaimers 

The findings in this report are not to be construed as an official Department of the Army position 

unless so designated by other authorized documents. 

Citation of manufacturer’s or trade names does not constitute an official endorsement or 

approval of the use thereof. 

Destroy this report when it is no longer needed. Do not return it to the originator. 
Army Research Laboratory 

Adelphi, MD 20783-1197 

ARL-TR-4956 September 2009 

Patterning of Thick Parylene Films by Oxygen Plasma for 

Application as Exploding Foil Initiator Flyer Material 

Eugene Zakar and Michael D. Grapes 

Sensors and Electron Devices Directorate, ARL 

Approved for public release; distribution unlimited. 
ii 

REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 0704-0188

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the 

data needed, and completing and reviewing the collection information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the 

burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. 

Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently 

valid OMB control number. 

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 

1. REPORT DATE (DD-MM-YYYY)

September 2009 

2. REPORT TYPE 

Progress 

3. DATES COVERED (From - To) 

June to August 2008 

4. TITLE AND SUBTITLE 

Patterning of Thick Parylene Films by Oxygen Plasma for Application as 

Exploding Foil Initiator Flyer Material 

5a. CONTRACT NUMBER 

5b. GRANT NUMBER 

5c. PROGRAM ELEMENT NUMBER 

6. AUTHOR(S) 

Eugene Zakar and Michael D. Grapes

5d. PROJECT NUMBER 

5e. TASK NUMBER 

5f. WORK UNIT NUMBER 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 

U.S. Army Research Laboratory 

ATTN: RDRL-SER-L 

2800 Powder Mill Road 

Adelphi, MD 20783-1197

8. PERFORMING ORGANIZATION 

 REPORT NUMBER 

ARL-TR-4956

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 

U.S. Army TACOM-ARDEC 

Fuze Division 

2800 Powder Mill Road 

Adelphi, MD 20783-1197 

10. SPONSOR/MONITOR'S ACRONYM(S) 

11. SPONSOR/MONITOR'S REPORT 

 NUMBER(S) 

12. DISTRIBUTION/AVAILABILITY STATEMENT 

Approved for public release; distribution unlimited. 

13. SUPPLEMENTARY NOTES 

14. ABSTRACT 

Parylene is an attractive flyer material for the exploding foil initiator due to its superior conformal step coverage and inert

properties. A process for patterning thick films of Parylene using a thin metal masking layer and oxygen plasma has been 

developed. Gold and titanium have been compared as masking materials on the basis of masking quality and compatibility 

with existing processes. Etch rate, directionality, and minimum feature size have been approximated for isotropic and 

anisotropic oxygen plasma etch recipes. Remaining obstacles to process integration are discussed. 

15. SUBJECT TERMS

Fuze, EFI, parylene, oxygen plasma 

16. SECURITY CLASSIFICATION OF:

17. LIMITATION 

 OF 

 ABSTRACT 

UU 

18. NUMBER

 OF 

 PAGES 

24 

19a. NAME OF RESPONSIBLE PERSON

Eugene Zakar 

a. REPORT 

Unclassified 

b. ABSTRACT 

Unclassified 

c. THIS PAGE 

Unclassified 

19b. TELEPHONE NUMBER (Include area code) 

(301) 394-1628 

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18
iii 

Contents 

List of Figures iv

List of Tables v

1. Introduction/Background 1

2. Experiment/Calculations 4

2.1 Parylene Etch Rate Calculation .......................................................................................4 

2.2 Metal Masking Feasibility ...............................................................................................5 

2.3 Etch Characterization ......................................................................................................6 

3. Results and Discussion 7

3.1 Parylene Etch Rate Calculation .......................................................................................7 

3.2 Metal Masking Material Feasibility ................................................................................7 

3.2.1 Ti Masking ..........................................................................................................7 

3.2.2 Au Masking .........................................................................................................8 

3.3 Etch Characterization ....................................................................................................11 

3.4 Outstanding Issues .........................................................................................................12 

4. Summary and Conclusions 13

5. References 14

List of Symbols, Abbreviations, and Acronyms 15 

Distribution List 16 
iv 

List of Figures 

Figure 1. Photograph of a typical EFI (top view). ..........................................................................1 

Figure 2. Schematic illustration side view of the flyer material operation in an EFI: a) a thin 

film copper (Cu) bridge and Parylene flyer material in line with the secondary, awaiting 

the firing signal; b) upon the fire command, the Cu bridge vaporizes and the Parylene 

flyer is sheared off and launched toward secondary explosive; and c) the flyer impacts the 

secondary explosive and the subsequent shockwave initiates detonation. ................................1 

Figure 3. Chemical structures for different types of Parylene. .......................................................3 

Figure 4. Process flow for patterning Parylene with a metal mask. ...............................................4 

Figure 5. Optical microscope images of metal mask patterns on silicon substrate: (a) 

crosses, (b) squares and dots, and (c) lines. ...............................................................................6 

Figure 6. Plots of average Parylene step height versus time for (a) isotropic and (b) 

anisotropic etch recipe. Note the generally linear trend. ..........................................................7 

Figure 7. Scanning electron micrograph showing the edge of a Ti mask and underlying 

Parylene film after anisotropic oxygen plasma etch. Note good pattern transfer as well as 

good Parylene step coverage over the Cu bridge structure. Stringy clumps in the lower 

right are as yet unidentified etch residues, possibly related to etch chamber 

contamination. ............................................................................................................................8 

Figure 8. Optical microscope images showing complications after the process of removing 

the Ti mask. In (a), the dot patterned Parylene film shows striations and discoloration 

along the edge of a metal Cu bridge that are indicative of peeling, while in (b), the 

Parylene dot film has been completely removed. ......................................................................8 

Figure 9. Dimensions of the mask cross patterns prior to etching. The upper cross (a) is 

slightly larger than the lower cross (b). .....................................................................................9 

Figure 10. Micrographs illustrating varying degrees of etch undercutting below the Au mask: 

(a) optical microscope image showing the Au mask patterns prior to Parylene etching, (b) 

SEM corner of large cross pattern transfer after anisotropic etch, (c) the same corner after 

isotropic etch (note greater undercutting), (d) SEM of two smaller crosses after 

anisotropic etch, and e) the same crosses after isotropic etch (the smaller cross has been 

completely undercut). ..............................................................................................................10 

Figure 11. Cracking of Parylene film from the isotropic etch recipe. Multiple cracking is 

evident in the partially etched Parylene film as well as through the Au masking layer. .......11 

Figure 12. Scanning electron micrograph overview of anisotropic etched features 200, 150, 

100, 60, 40, 20, 10, and 5 μm (from top to bottom). ...............................................................12 

Figure 13. Scanning electron micrographs depicting the approximate minimum feature size 

of line patterns after anisotropic etch: (a) 1000 times magnification of 10-μm-wide lines 

and (b) 600 times magnification of 5-μm-wide lines. .............................................................12 
v 

List of Tables 

Table 1. Oxygen plasma etch recipes tested. ..................................................................................5 
vi 

INTENTIONALLY LEFT BLANK. 
1 

1. Introduction/Background 

Microelectromechanical system (MEMS) exploding foil initiator (EFI) type fuzes are being 

explored to improve the performance and satisfy the requirements of MIL-STD-1316E, which 

mandates that any electrical fuze initiating an “uninterruptable explosive train” leading to 

ignition of the main charge in a warhead requires a voltage in excess of 500 V to fire. In the EFI 

an electrical current travels through a wide conductor path until it crosses a narrow neck, a.k.a. 

bridge, shown in figure 1, where it vaporizes and starts a chain of events with the flyer material. 

A reliable flyer material needs to be robust, able to withstand an impulsive launch, and remain 

intact at speeds approaching supersonic velocity. The operational sequence of events leading to 

detonation of an insensitive secondary explosive is illustrated in figure 2. EFIs eliminate the use 

of a sensitive primary explosive by requiring a voltage of 3000–4000 V for detonation, easily 

satisfying the 500 V requirement. 

Figure 1. Photograph of a typical EFI (top view). 

Figure 2. Schematic illustration side view of flyer material operation in an EFI: a) a thin film copper (Cu) 

bridge and Parylene flyer material in line with the secondary, awaiting the firing signal; b) upon the 

fire command, the Cu bridge vaporizes and the Parylene flyer is sheared off and launched toward 

secondary explosive; and c) the flyer impacts the secondary explosive and the subsequent shockwave 

initiates detonation. 

Cu bridge 

Flyer 

Cu bridge 

Flyer 

Insensitive 

Secondary 

Explosive 

Cu bridge 

Flyer

Insensitive 

Secondary 

Explosive 

a) b) c) 
2 

Parylene can be used as a flyer material and has several distinct advantages over other polymer 

materials, Kapton, and polyimide materials, due to its superior step coverage over circuit 

topography and inert quality owing to its low outgassing properties. These characteristics 

improve the performance, predictability, and stability of the EFI. The thickness requirement for 

a flyer material can vary depending on the energy requirements of the initiator, as it must be 

robust during launch, but generally speaking, it is near 1 mil (~25 μm). We use the term “thick” 

film Parylene for this application, because in comparison to other applications in integrated 

circuits “thin” films are generally near 1 μm. 

Currently, Parylene type “C” can only be chemical vapor deposited over the entire circuit 

surface, and selective removal is achieved only by taping the substrate surface prior to deposition 

and then peeling it off after the deposition. This method is inherently imprecise, and produces 

very uneven cut and dirty surfaces compared to clean room processing practices. An easily 

integrated process for patterning Parylene is needed to achieve high reliability and 

reproducibility both in production and use. 

The Parylenes (poly-p-xylylene and its derivatives) are polymeric films. They are chemical 

vapor deposited in a process originally developed by W. F. Gorham (1), in which Parylene dimer 

(di(poly-p-xylylene)) is vaporized (~165 °C), pyrolized to the monomer poly-p-xylylene 

(~670 °C), and then deposited at room temperature. The mechanism of deposition is given by 

Gazicki-Lipman as beginning with condensation of Parylene monomer on and diffusion to the 

surface, followed by chain initiation and propagation, in which the monomer molecules form 

long polymer chains (2). The activation energy for adding a monomer unit to an existing chain 

(propagation) is much lower than for the binding of the first two monomer units (initiation), so 

this deposition method tends to produce very high molecular weight polymers. Since 

polymerization is preceded by condensation, the Parylenes are highly transparent, conformal, 

pinhole-free coatings over large surface areas. 

The commercial name for unsubstituted poly-p-xylylene is Parylene N, known for its excellent 

dielectric properties. Monochloro-substituted Parylene C, used in this work, contains a single 

additional chlorine atom on the benzene ring. Parylene C is thermally stable and chemically 

inert, and is often used to passivate sensitive devices against hostile environments. Dichlorosubstituted Parylene D is also available. The chemical structures of the Parylenes are shown in 

figure 3. 
3 

Figure 3. Chemical structures for different types of Parylene. 

Unfortunately, the beneficial properties of Parylene C also complicate its patterning. Because it 

is chemically inert, a selective wet etch for Parylene does not exist. It cannot be patterned using 

conventional lift-off techniques, because it deposits very conformally over features. Some 

research has been done in the area of laser ablation of Parylene films for microelectrode tips (3), 

but this method is not suited for large (i.e., wafer-scale) patterning and would require investment 

in or construction of specialized equipment. Oxygen plasma is an attractive method for Parylene 

patterning. Oxygen plasma processes are frequently used for photoresist ashing and descum, so 

commercial tools to perform this process are already available. 

Plasma etching of silicon (using SF6 plasma) or silicon dioxide (using CF4 or CHF3 plasmas) can 

be accomplished using photoresist as a masking layer since these plasmas selectively etch only 

the target material. However, the selectivity between photoresist and Parylene in oxygen plasma 

is near unity, so for a thick film of Parylene an alternative masking layer is preferable to prevent 

the mask from eroding away before the completion of the etch. Metals are known to have a 

negligible etch rate in oxygen plasma and can be very good as a masking material for Parylene 

etching. Fine-line patterning of the metal masking can be achieved by plasma etching. We 

believe the same fine-line patterns can be transferred into the Parylene using oxygen plasma 

etching. 

We developed a scheme for patterning the Parylene as shown in figure 4. Parylene is chemical 

vapor deposited onto a substrate or existing structures (figure 4a). Next, the masking metal is 

evaporated or sputtered onto the Parylene surface (figure 4b). Photoresist is deposited and 

patterned (figure 4c) using conventional photolithographic techniques. The photoresist is used as 

an etch mask for chemical wet etching of the metal layer (figure 4d), and the Parylene is then 

etched in an oxygen plasma with the metal layer as a mask (figure 4e). During this step, the 

photoresist is also removed. Finally, the metal masking material is removed using the same wet 

etch process as previously, leaving the patterned Parylene structure (figure 4f). 

Parylene N Parylene C Parylene D 
4 

Figure 4. Process flow for patterning Parylene with a metal mask. 

Plasma processes have been shown to work previously with aluminum (4, 5) and silicon oxide 

(6, 7). It is the goal of the present work to extend the research using plasma processes with gold 

(Au) and titanium (Ti) masks to evaluate the resolution of pattern transfer in different etch 

recipes and to integrate the process with an EFI structure. 

2. Experiment/Calculations 

Process development was divided into three distinct phases: (1) determination of an etch rate for 

Parylene in oxygen plasma, (2) determination of the feasibility of Au and Ti masking materials, 

and (3) characterization of the etch quality in different etch recipes. Different samples were 

prepared and evaluated for each of the phases. 

2.1 Parylene Etch Rate Calculation 

In the first phase, etch rate tests were conducted to determine if useful Parylene etch rates can be 

achieved using oxygen plasma and to facilitate etch time prediction for subsequent phases. The 

lithography patterning method was skipped during this phase, and instead a crude and fast 

approach was used to reduce the process steps and time needed to perform the analysis. No 

chemicals or alignments were used in this phase of the investigation other than the oxygen 

plasma itself. The Parylene films were prepared on 100-mm diameter polished silicon 

substrates. A polyimide tape was applied across the substrate followed by the Parylene 

deposition. A step height was created in the Parylene after carefully scoring the taped surface 

edge with a razor blade and lifting the tape until the underlying silicon substrate was exposed. 

About 15% of the total Parylene surface area was removed using the tape method. By putting 

down several tapes on one substrate, we were able to determine the amount of etched Parylene at 

(a) (b) (c)

(d) (e) (f)
5 

each etch time interval before all the material was completely removed. Some edge peeling was 

unavoidable with the razor blade method, so scan distances were sufficiently long as to avoid the 

edge effects. 

The wafers were etched in multiple steps using two oxygen plasma recipes detailed in table 1. 

The intended plasma recipes were meant to produce both isotropic and anisotropic etch 

conditions. In the isotropic condition, the etched Parylene material eroded equally in all 

directions forming sloping sidewall cavities. In the anisotropic condition, the etching occurred 

faster in one direction and forming straight sidewalls. We used a commercial reactive ion etcher 

(RIE) for the anisotropic etching and a parallel plate oxygen plasma ashing system for the 

isotropic etching. Etch step times ranged from 5–10 min. After each etch step, the step height 

was re-measured at two points. In total, each wafer was etched in at least four steps, with step 

height measurements taken at two points after each step. To determine an etch rate, the two step 

heights were averaged to give an average step height, and linear least squares regression was 

conducted with cumulative etch time as the independent variable and average step height as the 

dependent variable. The etch rate was found as the negative of the slope of the linear least 

squares regression line. 

Table 1. Oxygen plasma etch recipes tested. 

Process Parameter Isotropic Etch Anisotropic Etch 

Pressure 1000 mtorr 500 mtorr 

O2 flow rate 500 sccm 50 sccm 

Bias voltage 0 V dc –540 V dc 

2.2 Metal Masking Feasibility 

In this second phase, tests were conducted to determine the survivability of Au or Ti metal 

masking materials to chemical processes and oxygen plasma. Mask patterning and alignment 

over the EFI structure was completed by the lithography method. The usefulness of the masking 

materials were evaluated on top of EFI structures consisting of Cu bridges with an overcoat of 

chromium (Cr) on a quartz substrate. These structures were coated with Parylene C by a 

contractor and later followed by either Ti (sputtered) or Au (e-beam evaporated) masking. 

Photoresist was spun onto the wafers, and exposed and developed in the positive mode using 

simple mask patterns (~2 mm diameter) dots. The Au metal mask was etched in an aqueous 

potassium iodide-iodine etching solution. Ti was etched in commercially available buffered 

oxide etch (BOE) solution. The Parylene on the test wafers was etched to completion using both 

isotropic and anisotropic oxygen plasma recipes as outlined in table 1. The voltage conditions 

were equivalent, but the oxygen pressure and flow varied intentionally to create the isotropic and 

anisotropic etch effects. The metal mask, after it is no longer needed, can be removed using the 

same aqueous etch chemicals as before. The processes were evaluated by optical and scanning 

electron microscopy (SEM). 
6 

2.3 Etch Characterization 

In the third phase, characterization tests were designed to rigorously qualify the isotropic and 

anisotropic oxygen plasma etching, the extent of mask undercutting, and the achievability of a 

minimum feature size of the Parylene material. Based on the previous feasibility tests and poor 

reliability of the Ti mask, only the Au masking was investigated in this phase of the project. 

During this phase, Parylene was deposited on only plain polished 75-mm diameter silicon 

wafers. The Au film mask layer was intentionally selected to be thin and yet sufficient to cover 

up the roughness features of the underlying Parylene surface and not have open holes. The 

strategy here was also intended to reduce the etch time needed to remove the metal mask after 

completion of the Parylene etch. If the metal mask was made too thick, a longer etch time was 

needed that could possibly affect the other surrounding structure integrity. Photoresist was spun 

onto a wafer in the positive mode with patterns containing several features of interest ranging 

from 5–200 μm in size according to figure 5. The Au metal mask was etched in an aqueous 

potassium iodide-iodine etching solution. The Parylene was etched to completion. The wafers 

were then diced by hand into smaller pieces and examined with a SEM. The Au masking was 

intentionally kept intact to characterize the etched surfaces and determine the amount of etch 

undercutting. 

Figure 5. Optical microscope images of metal mask patterns on silicon substrate: (a) crosses, (b) squares and 

dots, and (c) lines. 

(a) 

(b) 

60μ

50μm 

25μ

5μm 

10μ

(c) 
7 

3. Results and Discussion 

3.1 Parylene Etch Rate Calculation 

Plots of average step height versus time for the two etch recipes are shown in figure 6. A linear 

etch rate was predicted for Parylene etching in oxygen plasma. The data seemed to agree with 

this analysis, with linear regression producing values of R2

 = 0.9859 and R2 = 0.9982 for the 

isotropic and anisotropic etch recipes, respectively. The etch rates were comparable, with a rate 

of 232 nm/min for the isotropic etch recipe and 245 nm/min for the anisotropic etch recipe. 

These values are similar to those obtained elsewhere (8, 9) using similar recipes. It was noted 

during etch rate testing that the Parylene tended to etch faster on the edges than in the center. 

This was most obvious near the end of the etch, as the edges of the Parylene surface would begin 

to recede shortly before etch completion. Consequently, the etch rates obtained were slightly on 

the low side. In practice, this error was easily corrected by adding about 5 min to the calculated 

etch time. 

 Isotropic Etch Recipe

y = -0.2319x + 10.879

R 2

 = 0.9859

6

7

8

9

10

11

12

0 5 10 15 20 25

Time (min)

Step Height (μm)

 Anisotropic Etch Recipe

y = -0.2452x + 11.284

R 2

 = 0.9982

0

2

4

6

8

10

12

0 10 20 30 40 50

Time (min)

Step Height (μm)

Figure 6. Plots of average Parylene step height versus time for (a) isotropic and (b) anisotropic etch recipe. 

Note the generally linear trend. 

3.2 Metal Masking Material Feasibility 

3.2.1 Ti Masking 

Ti was rejected as a suitable masking material based on the initial feasibility tests conducted over 

sample EFI structures. A BOE solution was able to etch and pattern the Ti mask with no 

noticeable scarring or peeling of the Parylene, as shown in figure 7. However, during the second 

metal removal step (described in figure 4(e)), the BOE had a strong tendency to also peel up the 

unetched Parylene film along with the Ti. In the dot shaped mask test, some amount of peeling 

occurred along the edge of a Cu bridge, as in figure 8(a), and over 50% of the Parylene dots were 
8 

completely removed, as in figure 8(b). A second test, performed with an array of larger (~1 cm) 

Parylene square features, confirmed the problem. Although none of the square masks 

completely peeled off, they all exhibited some peeling on the edges of the Cu bridge structure. 

Such peeling undermines the effectiveness of the Parylene coating as a flyer material for EFI 

application.

Figure 7. Scanning electron micrograph showing the edge of a 

Ti mask and underlying Parylene film after 

anisotropic oxygen plasma etch. Note good pattern 

transfer as well as good Parylene step coverage over 

the Cu bridge structure. Stringy clumps in the lower 

right are as yet unidentified etch residues, possibly 

related to etch chamber contamination. 

Figure 8. Optical microscope images showing complications after the process of removing the Ti mask. In 

(a), the dot patterned Parylene film shows striations and discoloration along the edge of a metal Cu 

bridge that are indicative of peeling, while in (b), the Parylene dot film has been completely removed. 

3.2.2 Au Masking 

Au etchant showed good compatibility with the mask. No peeling or scarring of the Parylene 

surface occurred when the Au was removed after the oxygen plasma etch. Based on these 

results, Au was chosen as the masking material for further investigation leading into the 

characterization phase. 

a) b)

400 μm 

Cu bridge Cu bridge 

Parylene 

dot 

Parylene 

missing

Ti mask 

Substrate 

Ti mask

Cu 

bridge 

Substrate
9 

Mask patterns with cross features were used to generalize the etch characteristics of both the 

isotropic and the anisotropic etch recipes. We paid attention to a particular set of mask 

dimensions noted in figure 9 for determining the minimum etch limits for the isotropic etch 

recipe. 

Figure 9. Dimensions of the mask cross patterns prior to etching. The upper cross (a) is slightly larger than the 

lower cross (b). 

As can be seen from an overall mask perspective in figure 10(a) and the close-ups of the 

respective regions, all the patterns transferred successfully for the isotropic and anisotropic etch 

recipes. The isotropic etch recipe in figure 10(c) clearly produced more undercutting than the 

anisotropic recipe in figure 10(b). The Au mask introduced ripples around the edges where the 

undercutting occurred. The limitations of isotropic etching became more apparent with the 

smaller features. As a consequence, some smaller features failed to transfer due to total 

undercutting failure (figure 10e). Based on comparative observations between figure 9 and 

figure 10(e), we roughly estimated that the minimum feature achieved was 50 μm using the 

isotropic etch recipe for this thick film Parylene. The feature with the minimum size of 40 μm 

was not standing after completion of the isotropic etch. 

90 μm

50 μm 40 μm

80 μm

(a) (b)
10 

Figure 10. Micrographs illustrating varying degrees of etch undercutting below the Au mask: (a) optical microscope 

image showing the Au mask patterns prior to Parylene etching, (b) SEM corner of large cross pattern 

transfer after anisotropic etch, (c) the same corner after isotropic etch (note greater undercutting), (d) SEM 

of two smaller crosses after anisotropic etch, and e) the same crosses after isotropic etch (the smaller cross 

has been completely undercut). 

Another interesting observation we made regarding the isotropic etching was occasional cracking 

of the Parylene film. In the dot and square mask pattern, about 40 min into the isotropic etch, 

cracking of the Parylene layer was observed, as in figure 11. The cracks broke through the thin 

Au mask layer in many areas, and by the time most of the Parylene was removed, much of the 

Anisotropic Etch 

Recipe Isotropic Etch 

R i

a) 

b

)

c) 

d

)

e) 
11 

Au mask was flaking off and the sample was deemed unsuitable for SEM. While this 

phenomenon may require more investigation, initial assessment is that it was a result of a 

combination of heating effects and undercutting caused by over-etching. Parylene has been 

observed to crack under thermal stress (10), and significant undercutting was observed in the 

previous characterization test. Parylene etch rates in oxygen plasmas have been noted to be 

much less inside trenches and between dense features. This was confirmed here, as the edges 

etched relatively quickly, while the Parylene in the center of the wafer took much longer to etch. 

Most of the Au mask damage occurred during this extended period of over-etching needed to 

compensate for the differences, so the isotropic etch seems to be limited to patterning relatively 

large, low dense features. 

Figure 11. Cracking of Parylene film from the isotropic 

etch recipe. Multiple cracking is evident in 

the partially etched Parylene film as well as 

through the Au masking layer. 

3.3 Etch Characterization 

The initial assessment of the anisotropic etch recipe was that a wide variety of mask shapes can 

be transferred accurately on the order of 1 μm scale without damaging the Parylene, as shown in 

figure 12. A wide range of mask dimension for the square and dot patterns were still standing 

except for the ones approaching 1 μm. The final etched features were smaller in width compared 

to the mask dimensions. An example of this is shown in figure 13(a), where a set of 10-μm-wide 

line patterns appear to be standing but the width of the Parylene material is less than the mask 

standing above it. However, the next smaller set of 5-μm-wide line patterns in figure 13(b) 

appear collapsed. The effects of undercutting became pronounced with decreasing mask 

dimensions. The minimum achievable etch feature size for the anisotropic etch recipe was 

estimated to be approximately 10 μm, near the same order of magnitude as the thick film 

Parylene. 

Au 

mask

Parylene
12 

Figure 12. Scanning electron micrograph overview of anisotropic 

etched features 200, 150, 100, 60, 40, 20, 10, and 5 μm 

(from top to bottom). 

Figure 13. Scanning electron micrographs depicting the approximate minimum feature size of line patterns 

after anisotropic etch: (a) 1000 times magnification of 10-μm-wide lines and (b) 600 times 

magnification of 5-μm-wide lines. 

3.4 Outstanding Issues 

While oxygen plasma is generally thought to etch polymers primarily, during the feasibility tests 

it noticeably attacked a Cr protective top layer of the EFI structure in both etch recipes, in some 

cases exposing the bulk Cu layer. This is a concern for compatibility with the existing process, 

but was still under investigation at the time of this report. Metal etching in oxygen plasmas has 

been documented elsewhere (11), but not with Cr and not at the rates that would be required to 

remove the Cr layer. It is possible that the outdated nature of the wafers received from a 

contracted facility during the feasibility tests introduced unknown variables into the process. 

(b) (a) 
13 

Another as yet unexplained issue is the presence of small, stringy clumps of material on the 

etched surface of all samples in figure 7 etched using the anisotropic etch recipe. The occurrence 

of surface roughness and “grass” has been documented previously after oxygen plasma RIE of 

Parylene (12), but these observations were made on incompletely etched Parylene, and the 

resolution of the SEM images provided in that publication does not permit detailed comparison. 

It was postulated by Majid that the surface roughness was due to preferential etching in areas 

where the polymer is weak, but if that were the case one would not expect to see this 

phenomenon when doing complete Parylene removal. Additional oxygen plasma treatment of 

surfaces covered with the clumps had no noticeable effect. Cleaning with organic solvents and 

BOE was also attempted, again with no effect. It was also speculated that the clumps were due 

to etch chamber contamination. 

4. Summary and Conclusions 

A method for patterning thick film Parylene has been investigated. The etch rates and degree of 

anisotropy have been assessed for two different oxygen plasma etch recipes, and approximate 

minimum feature sizes have been obtained for each recipe. Preliminary compatibility studies 

have been conducted to determine the compatibility of an Au masking layer with EFI structures. 

Remaining obstacles to the integration of oxygen plasma Parylene removal into the existing 

process flow for the EFI structure have been identified, and potential next steps have been 

suggested. Based on these results Au masking and oxygen plasma shows great promise as an 

option for selective Parylene patterning for flyer material application. 

c) 

e) 
14 

5. References 

1. Specialty Coating Systems. Discovery and Early History of Parylene. 

http://www.scscoatings.com/parylene_knowledge/history.aspx. (accessed 21 July 2008). 

2. Gazicki-Lipman, M. Vapor Deposition Polymerization of para-Xylylene Derivatives—

Mechanism and Applications. Journal of the Vacuum Society of Japan June 2007, 50, 601–

608. 

3. Schmidt, E. M.; Bak, M. J.; Christensen, P. Laser Exposure of Parylene-C Insulated 

Microelectrodes. Journal of Neuroscience Methods April 1995, 62, 89–92. 

4. Mastrangelo, C. H.; Saloka, G. S. A Dry-Release Method Based on Polymer Columns for 

Microstructure Fabrication. Proc. of 1993 IEEE Conf. on MEMS, 77–81, February 1993. 

5. Zahn, J. D.; Gabriel, K. J.; Fedder, G. K. A Direct Plasma Etch Approach to High Aspect 

Ratio Polymer Micromachining with Applications in BioMEMS and CMOS-MEMS. Proc. 

of 15th Intn’l IEEE Conf. on MEMS, 137–140, January 2002. 

6. Tacito, R. D.; Steinbrüchel, C. Fine-Line Patterning of Parylene-n by Reactive Ion Etching 

for Application as an Interlayer Dielectric. Journal of the Electrochemical Society June 

1996, 143, 1974–1977. 

7. Yeh, J.T.C.; Grebe, K. R. Patterning of Poly-pary-xylylenes by Reactive Ion Etching. 

Journal of Vacuum Science and Technology A June 1983, 1, 604–608. 

8. Meng, E.; Tai, Y. Parylene Etching Techniques for Microfluidics and BioMEMS. Proc. of 

18th Intn’l. IEEE Conf. on MEMS, 568–571, February 2005. 

9. Williams, K. R.; Gupta, K.; Wasilik, M. Etch Rates for Micromachining Processing—Part 

II. Journal of Microelectromechanical Systems December 2003, 12, 761–778. 

10. Hsu, J.; Kammer, S.; Jung, E.; Rieth, L.; Normann, A. R.; Solzbacher, F. Characterization of 

Parylene-C Film as an Encapsulation Material for Neural Interface Devices. Proc. of the 3rd

Intn’l Conf. on Multi-Material Micro Manufacture, 2007. 

11. Chou, C. H.; Philips, J. Platinum Metal Etching in a Microwave Oxygen Plasma. Journal of 

Applied Physics September 1990, 68, 2415–2423. 

12. Majid, N.; Dabral, S.; McDonald, J. F. The Parylene-Aluminum Multilayer Interconnection 

System of Wafer Scale Integration and Wafer Scale Hybrid Packaging. Journal of Electronic 

Materials March 1989, 18, 301–311. 
15 

List of Symbols, Abbreviations, and Acronyms 

Au gold 

Cr chromium 

Cu copper 

EFI exploding foil initiator 

BOE buffered oxide etch 

MEMS microelectromechanical systems 

RIE reactive ion etcher 

SEM scanning electron microscopy 

Ti titanium 
16 

NO. OF 

COPIES ORGANIZATION 

 1 ADMNSTR 

 ELEC DEFNS TECHL INFO CTR 

 ATTN DTIC OCP 

 8725 JOHN J KINGMAN RD STE 0944 

 FT BELVOIR VA 22060-6218 

 1 DARPA 

 ATTN IXO S WELBY 

 3701 N FAIRFAX DR 

 ARLINGTON VA 22203-1714 

 1 CD OFC OF THE SECY OF DEFNS 

 ATTN ODDRE (R&AT) 

 THE PENTAGON 

 WASHINGTON DC 20301-3080 

 1 US ARMY RSRCH DEV AND 

 ENGRG CMND 

 ARMAMENT RSRCH DEV AND 

 ENGRG CTR 

 ARMAMENT ENGRG AND 

 TECHNLGY CTR 

 ATTN AMSRD AAR AEF T J MATTS 

 BLDG 305 

 ABERDEEN PROVING GROUND MD 

 21005-5001 

 1 PM TIMS, PROFILER (MMS-P) 

 AN/TMQ-52 

 ATTN B GRIFFIES 

 BUILDING 563 

 FT MONMOUTH NJ 07703 

 1 US ARMY INFO SYS ENGRG CMND 

 ATTN AMSEL IE TD A RIVERA 

 FT HUACHUCA AZ 85613-5300 

 1 COMMANDER 

 US ARMY RDECOM 

 ATTN AMSRD AMR 

 W C MCCORKLE 

 5400 FOWLER RD 

 REDSTONE ARSENAL AL 

 35898-5000 

 1 US GOVERNMENT PRINT OFF 

 DEPOSITORY RECEIVING SECTION 

 ATTN MAIL STOP IDAD J TATE 

 732 NORTH CAPITOL ST NW 

 WASHINGTON DC 20402 

NO. OF 

COPIES ORGANIZATION 

 1 US ARMY RSRCH LAB 

 ATTN RDRL CIM G T LANDFRIED 

 BLDG 4600 

 ABERDEEN PROVING GROUND MD 

 21005-5066 

 12 US ARMY RSRCH LAB 

 ATTN IMNE ALC HRR 

 MAIL & RECORDS MGMT 

 ATTN RDRL CIM L TECHL LIB 

 ATTN RDRL CIM P TECHL PUB 

 ATTN RDRL SER E P AMIRTHARAJ 

 ATTN RDRL SER L B PIEKARSKI 

 ATTN RDRL SER L B ROD 

 ATTN RDRL SER L E ZAKAR 

 ATTN RDRL SER L L CURRANO 

 ATTN RDRL SER L M DUBEY 

 ATTN RDRL SER L R POLCAWICH 

 ATTN RDRL SER L W CHURAMAN 

 ATTN RDRL SES S B MARY 

 ADELPHI MD 20783-1197 

 5 US ARMY TACOM-ARDEC 

 ATTN RDAR MEF FA D ERRERA 

 ATTN RDAR MEF FA J D CAMPBELL 

 ATTN RDAR MEF FA C ROBINSON 

 ATTN RDAR MEF FA 

 J F HARTRANFT 

 ATTN RDAR MEF FA W KONICK 

 ADELPHI MD 20783-1197 

TOTAL: 26 (1 ELEC, 1 CDS, 24 HCS)