<PAGE>

                                                                    EXHIBIT 10.6
                                                                    ------------


                                PROMISSORY NOTE


$5,000,000.00                                              Boston, Massachusetts
                                                               November 27, 1998


     FOR VALUE RECEIVED, the undersigned Aseco Corporation, a Delaware
corporation (the "Borrower") hereby promises to pay to the order of FLEET
NATIONAL BANK (the "Bank") the principal amount of Five Million and 00/100
($5,000,000.00) Dollars or such portion thereof as may have been advanced by the
Bank or may hereafter be advanced by the Bank pursuant to (S)1.2 of that certain
letter agreement of even date herewith between the Bank and the Borrower (as
same may be from time to time amended, modified, supplemented and/or restated,
the "Letter Agreement") and remains outstanding from time to time hereunder
("Principal"), with interest, at the rate hereinafter set forth, on the daily
balance of all unpaid Principal, from the date hereof until payment in full of
all Principal and interest hereunder.  Capitalized terms used herein and not
defined herein shall have the respective meanings ascribed to them in the Letter
Agreement.

     Interest on all unpaid Principal shall be due and payable monthly in
arrears, on the first day of each month, commencing on the first such date after
the advance of any Principal and continuing on the first day of each month
thereafter and on the date of payment of this note in full, at a fluctuating
rate per annum (computed on the basis of a year of three hundred sixty (360)
days for the actual number of days elapsed) which shall at all times (except as
described in the next sentence) be equal to the sum of (i) one and one-half
(1.5%) percent plus (ii) the Prime Rate, as in effect from time to time (but in
no event in excess of the maximum rate permitted by then applicable law), with a
change in the aforesaid rate of interest to become effective on the same day on
which any change in the Prime Rate is effective.  Overdue Principal and, to the
extent permitted by law, overdue interest shall bear interest at a fluctuating
rate per annum which at all times shall be equal to the sum of (i) four (4%)
percent per annum plus (ii) the per annum rate otherwise payable under this note
(but in no event in excess of the maximum rate permitted by then applicable
law), compounded monthly and payable on demand.  As used herein, "Prime Rate"
means that variable rate of interest per annum designated by the Bank from time
to time as its prime rate, it being understood that such rate is merely a
reference rate and does not necessarily represent the lowest or best rate being
charged to any customer.  If the entire amount of any required Principal and/or
interest is not paid within ten (10) days after the same is due, the Borrower
shall pay to the Bank a late fee equal to five percent (5%) of the required
payment.

     All outstanding Principal and all interest accrued thereon shall be due and
payable in full on the first to occur of:  (i) an acceleration under (S)5.2 of
the Letter Agreement or (ii) the

                                      -1-
<PAGE>

                                                                    EXHIBIT 10.6
                                                                    ------------


Expiration Date. The Borrower may at any time and from time to time prepay all
or any portion of said Principal, without premium or penalty. Under certain
circumstances set forth in the Letter Agreement, prepayments of Principal may be
required.

     Payments of both Principal and interest shall be made, in lawful currency
of the United States in immediately available funds, at the office of the Bank
located at One Federal Street, Boston, Massachusetts 02110, or at such other
address as the Bank may from time to time designate.

     The undersigned Borrower irrevocably authorizes the Bank to make or cause
to be made, on the books of the Bank, at or following the time of making any
Revolving Loan (as defined in the Letter Agreement) and of receiving any payment
of Principal, an appropriate notation reflecting such transaction and the then
aggregate unpaid balance of Principal.  Failure of the Bank to make any such
notation shall not, however, affect any obligation of the Borrower hereunder or
under the Letter Agreement.  The unpaid Principal amount of this note, as
recorded by the Bank from time to time on such schedule or on such books, shall
constitute presumptive evidence of the aggregate unpaid principal amount of  the
Revolving Loans.

     The Borrower hereby (a) waives notice of and consents to any and all
advances, settlements, compromises, favors and indulgences (including, without
limitation, any extension or postponement of the time for payment), any and all
receipts, substitutions, additions, exchanges and releases of collateral, and
any and all additions, substitutions and releases of any person primarily or
secondarily liable, (b) waives presentment, demand, notice, protest and all
other demands and notices generally in connection with the delivery, acceptance,
performance, default or enforcement of or under this note, and (c) agrees to
pay, to the extent permitted by law, all reasonable costs and expenses,
including, without limitation, reasonable attorneys' fees, incurred or paid by
the Bank in enforcing this note and any collateral or security therefor, all
whether or not litigation is commenced.

     This note is the Revolving Note referred to in the Letter Agreement.  This
note is subject to prepayment as set forth in the Letter Agreement.  The
maturity of this note may be accelerated upon the occurrence of an Event of
Default, as provided in the Letter Agreement.

     THE BORROWER HEREBY KNOWINGLY, VOLUNTARILY AND INTENTIONALLY WAIVES THE
RIGHT TO A TRIAL BY JURY IN RESPECT OF ANY CLAIM BASED ON THIS NOTE OR ARISING
OUT OF, UNDER OR IN CONNECTION WITH THIS NOTE OR ANY RELATED DOCUMENTS OR OUT OF
ANY COURSE OF CONDUCT, COURSE OF DEALING, STATEMENTS (WHETHER ORAL OR WRITTEN)
OR ACTIONS OF ANY PERSON.  THIS WAIVER CONSTITUTES A MATERIAL INDUCEMENT  FOR
THE BANK TO ACCEPT THIS NOTE AND TO MAKE REVOLVING LOANS AS CONTEMPLATED IN THE
LETTER AGREEMENT.

                                      -2-
<PAGE>

                                                                    EXHIBIT 10.6
                                                                    ------------


     Executed, as an instrument under seal, as of the day and year first above
written.

CORPORATE SEAL                    ASECO CORPORATION

ATTEST:

/s/ Sebastian J. Sicari           By: /s/Mary R. Barletta
-------------------------             ----------------------
Witness                                       Name:  Mary R. Barletta
                                              Title:  Chief Financial Officer

                                      -3-