<PAGE>

                                                                   Exhibit 10.18


                              SEPARATION AGREEMENT
                              --------------------

     This Separation Agreement is entered into as of this 11th day of August,
1998 by and between Aseco Corporation, a Delaware corporation (the "Company"),
and Carl S. Archer, Jr. ("Archer").

                                   Recitals:
                                   --------

     WHEREAS, Archer has served as the President and Chief Executive Officer of
the Company;

     WHEREAS, Archer relinquished his position as President of the Company on
May 12, 1998 and as Chief Executive Officer of the Company on August 11, 1998;

     WHEREAS, Archer and the Company are parties to a Severance Agreement dated
as of December 30, 1996 (the "1996 Severance Agreement"); and

     WHEREAS, in connection with the termination of Archer's employment with the
Company, the Company and Archer wish to provide for the payment of certain
compensation and the provision of certain benefits to Archer by the Company in
lieu of Archer's rights under the 1996 Severance Agreement.

     NOW, THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, the parties agree as follows:

     1.  Interim Employment.  From the date of this Agreement until October 31,
1998 (the "Employment Period"), Archer shall remain a full-time employee of the
Company reporting directly to the Chief Executive Officer of the Company.

     2.  Consulting.  From November 1, 1998 until August 10, 1999 (the
"Consulting Period", and together with the Employment Period, the "Service
Period"), Archer shall render such executive-level consulting services as the
Chief Executive Officer of the Company may reasonably request; provided,
however, that Archer shall not be required to render more than 5 days of service
per month and shall not be required to travel outside of Massachusetts in
connection with performing such services. Archer's obligations under this
Section 2 shall terminate upon a Change in Control (as defined below).

     3.  Compensation.  As consideration for agreeing to provide the foregoing
employment and consulting services, the Company shall pay Archer during the
Service Period a fee of $200,000, payable in bi-weekly installments (net of all
applicable withholding taxes).  The Company's obligation under this Section 3
shall be absolute and unconditional, but shall terminate upon a Change in
Control.
<PAGE>

     4.  Severance Payments.  As consideration for past services rendered by
Archer, the Company shall pay him $15,000 per month (net of all applicable
withholding taxes) during the two-year period commencing on the first
anniversary of the date of this Agreement; provided, however, that upon a Change
in Control the Company shall pay Archer an amount equal to $560,000 minus the
total amount theretofore paid to him pursuant to Sections 3 and 4 of this
Agreement.  The Company, in its sole discretion, shall have the right to prepay
any or all amounts that it is required to pay pursuant to Sections 3 and 4 of
this Agreement.  As used in this Agreement, the term "Change in Control" shall
mean (i) the sale, lease, transfer or other disposition by the Company of all or
substantially all of its assets in a single transaction or a series of related
transactions; (ii) the merger or consolidation of the Company with another
entity in which the stockholders of the Company immediately prior to such merger
or consolidation hold less than 50% of the outstanding voting stock of the
surviving or resulting corporation immediately following such transaction; or
(iii) the sale or exchange (to or with any person or entity other than the
Company) by the stockholders of the Company of more than 50% of the outstanding
voting stock of the Company in a single transaction or series of related
transactions.

     5.  Benefits Continuation.  During the four-year period commencing on the
date of this Agreement, the Company shall provide to Archer the health, dental
and life insurance benefits made generally available to executive officers of
the Company; provided, however, that the Company shall have no obligation to
provide the foregoing benefits if the annual cost to the Company of doing so is
greater than $15,000.  In the event the annual cost to the Company of providing
the foregoing benefits is greater than $15,000 Archer may either pay the Company
an amount equal to the excess (and the Company shall thereafter be obligated to
provide such benefits) or Archer may designate certain of the foregoing benefits
the annual cost of which to the Company is less than $15,000 (and the Company
shall thereupon be obligated to provide the benefits so designated.)

     6.  Stock Options.  Exhibit A hereto lists all options held by Archer to
purchase shares of the Company's common stock. Notwithstanding the terms of the
stock option agreements pursuant to which such options were granted and the
termination of Archer's employment by the Company, each such option shall be
exercisable at any time prior to the earlier of (i) August 11, 2001 and (ii) the
expiration date of the option.  Each of the options listed on Exhibit A shall be
fully vested and therefore exercisable in full as of the date of this Agreement.

                                       2
<PAGE>

     7.  Release.  Archer does hereby release, acquit, remise and forever
discharge the Company, its agents, officers, directors, shareholders, employees,
affiliates, successors and assigns (collectively, "Released Parties") from all
claims (including, without limitation, any claim to employment or reemployment,
wages, or back wages, fees, expenses, benefits or compensation), damages,
demands, liabilities, equities and causes of action of every kind and character,
both known and unknown, in contract, tort or otherwise, including those past and
present, accruing to Archer against the Released Parties, or any of them, at any
time prior to the date of this Agreement.  Archer hereby acknowledges and
warrants that he is over eighteen years of age, is of sound mind, has fully read
and understands all terms and conditions set forth herein, has had an
opportunity to fully consult with his own counsel in connection herewith and is
entering into this Agreement voluntarily and without any promise or benefit
other than as set forth herein.

     8.  Miscellaneous.
         -------------

     (a) This Agreement constitutes the entire agreement between the parties and
supersedes all prior agreements and understandings, whether written or oral,
relating to the subject matter of this Agreement, including without limitation
the 1996 Severance Agreement.

     (b) This Agreement may be amended or modified only by a written instrument
executed by both the Company and Archer.

     (c) This Agreement shall be construed, interpreted and enforced in
accordance with the laws of The Commonwealth of Massachusetts.

     (d) This Agreement shall be binding upon and inure to the benefit of both
parties and their respective heirs, successors and assigns, including any entity
with which or into which the Company may be merged or which may succeed to its
assets or business.

     (e) The captions of the sections of this Agreement are for convenience of
reference only and in no way define, limit or affect the scope or substance of
any section of this Agreement.

     (f) In case any provision of this Agreement shall be invalid, illegal or
otherwise unenforceable, the validity, legality and enforceability of the
remaining provisions shall in no way be affected or impaired thereby.

                                       3
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have executed this Agreement as of
the day and year set forth above.


                              ASECO CORPORATION


                              By: /s/ Sebastian J. Sicari
                                  --------------------------------

                              Title: President
                                     -----------------------------

                              /s/ Carl S. Archer, Jr.
                              ------------------------------------
                              Carl S. Archer, Jr.

                                       4
<PAGE>

                                                                       Exhibit A
                                                                       ---------


<TABLE>
<CAPTION>
                   Number of                            Expiration
Grant Date          Shares          Exercise Price         Date
----------          ------          --------------         ----
<S>                <C>             <C>                  <C>

01/12/94            90,000             $ 5.375          01/12/2004
------------------------------------------------------------------
08/23/96           108,750             $10.375          08/23/2006
------------------------------------------------------------------
10/18/96            36,250             $ 9.875          10/18/2006
------------------------------------------------------------------
05/10/97            30,000             $ 9.875          05/10/2007
------------------------------------------------------------------
08/11/98            20,000             $  3.16          08/11/2008
------------------------------------------------------------------
</TABLE>


                                       5