Note 4 - Stock-based Compensation - Stock-based Compensation Expense By Financial Statement Line Item (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Total stock-based compensation expense	$ 1,101	$ 849	$ 2,393	$ 1,758
Cost of Sales [Member]
Total stock-based compensation expense	21	30	112	59
Research and Development Expense [Member]
Total stock-based compensation expense	192	161	439	291
Selling, General and Administrative Expenses [Member]
Total stock-based compensation expense	$ 888	$ 658	$ 1,842	$ 1,408

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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