New material request: polystyrene microspheres in aqueous suspension

Aaron Hryciw ahryciw at
Wed Oct 10 17:10:43 PDT 2007

Dear SpecMat Committee –

Please regard this email as a request for a new material for use at SNF; a
general MSDS for this class of materials (from the chemical supplier from
whom the material has been purchased) is attached.
Contact information*
Aaron Hryciw
Coral login:  ahryciw
Phone:  3-5840 (office); (650) 353-0347 (cell)
ahryciw at
PI:  Mark Brongersma (Mat. Sci. Eng.)

* Chemical or material name*
Polystyrene microspheres (diameters between 100 nm and 1 um) suspended in DI
water, 10% solids by weight, 15 mL vials.
MSDS from supplier attached.
Will be diluted with methanol or ethanol before use.
Common/trade names:  polymer microsphere suspension, polystyrene
microspheres, polystyrene nanospheres, latex microspheres suspensions.
Storage group identifier:  *G*. Non-Reactive Materials and Non-Hazardous
Main hazard class:  11. Non Hazardous.

*Vendor/manufacturer info
*Duke Scientific Corporation
2463 Faber Place
P.O. Box 50005
Palo Alto, CA

Phone: 1-800-334-3883 or 1-650-424-1177
Fax:  1-650-424-1158
info at (in particular:

Reason for request
*My interest in polystyrene microspheres is due to their use as a
self-assembled monolayer mask, as a means of fabricating large (~cm^2) arrays
of silicon nanowires (NWs) with controlled size, length, and areal density
(please refer to Huang et. al, attached).  Nanosphere lithography (NSL)
comprises a rapid, parallel approach to fabricating well-controlled NWs
without requiring conventional photolithographic techniques.  The material
used for the mask (viz., the polystyrene microspheres) is also very benign
and poses no health or safety hazards when used properly for this technique.

* Process Flow*
I intend to follow the general process flow detailed in the attached
paper (Huang et.
al).  if necessary, the entire process flow needn't occur in SNF (only use
of the RIE tool is strictly required), although, for the sake of
cleanliness, I would prefer it.

A full process flow in SNF would be as follows:

   1. Clean (100) n-type Si wafers or pieces (ca. 1x1 cm^2) in an
   ultrasonic cleaner using acetone (10 min), then methanol (5 min).
   2. Clean in Piranha (4:1 v/v H2SO4:H2O2) and RCA solution (1:1:5 v/v/v
   NH3:H2O2:H2O @ 80 °C) for 1 hr.
   3. Rinse in DI water.
   4. Place substrate in a Petri dish (I will have my own dedicated
   glassware) and cover with DI water.
   5. Pipette polystyrene microsphere solution (5% solids in 15 mL DI
   water + 15 mL methanol) onto water surface and gently tilt Petri dish to
   encourage self-assembly of large single-crystal hcp arrays.
   6. Allow water to evaporate slowly, depositing microspheres onto
   substrate. [The foregoing six steps could be performed at wbgeneral,
   wbmetal, or wbnonmetal, as necessary.]
   7. RIE microsphere array to desired sphere diameter using O2 plasma
   etch:  40 sccm, 30 W RF power, 5 Pa pressure. [Performed on one of the
   drytek tools, consistent with previous material groups.]
   8. Deposit 40 nm of Ag. [innotec or metallica]
   9. Etch Si NWs in an etchant consisting of HF (4.6 M) , H2O2 (0.44 M),
   and H2O. [wbgeneral]
   10. Remove microspheres by soaking in toluene for 2 hr. [wbsolvent]
   11. Remove Ag film by soaking in nitric acid for 15 min. [wbgeneral]

If necessary, steps 1–6 and 7–11 could be performed outside SNF (in our lab
in McCullough Bldg., for instance).

*Amount and form
*Two or three 30-mL vials containing microspheres (of two or three different
sizes) suspended in 15 mL DI water + 15 mL methanol.  Once the 15-mL
DI-water-suspended material (as purchased) has been diluted with methanol,
no further mixing will be required.

If the entire process flow is approved for use in SNF, I would store the
vials of microsphere aqueous/methanoic suspensions at SNF.  If only step 7
is approved for SNF, I would store them in our group's lab.

As per the MSDS, waste materials containing microspheres could be disposed
of in SNF's hazardous materials bins in a sealed, labeled plastic bag, due
to their non-hazardous nature.

Thank-you for considering this new material for use at SNF.  I would enjoy
the opportunity to discuss possible amendments to the abovementioned process
flow if necessary to allow it in the cleanroom.

Best regards,

 – Aaron

Aaron Hryciw
Postdoctoral Scholar
Geballe Laboratory for Advanced Materials
Stanford University
476 Lomita Mall (04-490)
McCullough Building, Rm. 325
Stanford, CA  94305-4045

Tel.:  (650) 723-5840
Fax.:  (650) 736-1984
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