10. STOCK-BASED COMPENSATION - Stock-based compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended
Jun. 30, 2022	Jun. 30, 2021
Stock-based compensation expense by line item
Stock-based compensation expense	$ 638	$ 823
Cost of Sales [Member]
Stock-based compensation expense by line item
Stock-based compensation expense	60	70
Research and Development Expense [Member]
Stock-based compensation expense by line item
Stock-based compensation expense	351	470
Selling, General and Administrative Expenses [Member]
Stock-based compensation expense by line item
Stock-based compensation expense	$ 227	$ 283

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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5047-113901

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-URI http://asc.fasb.org/extlink&oid=122041274&loc=d3e301413-122809

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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