The Company grants stock options and restricted stock awards to employees and others. All outstanding stock options of the Company as of March 31, 2020 were granted with an exercise price equal to the fair market value on the date of grant. Outstanding stock options, if not exercised, expire 10 years from the date of grant.

The Company’s 2015 Amended and Restated Omnibus Incentive Plan (the “2015 Plan”) authorizes the grant of a variety of equity awards to the Company’s officers, directors, employees, consultants and advisers, including awards of unrestricted and restricted stock, restricted stock units, incentive and nonqualified stock options to purchase shares of common stock, performance share awards and stock appreciation rights. The 2015 Plan provides that options may only be granted at the current market value on the date of grant. Each share of stock issued pursuant to a stock option or stock appreciation right counts as one share against the maximum number of shares issuable under the 2015 Plan, while each share of stock issued pursuant to any other type of award counts as 1.8 shares against the maximum number of shares issuable under the 2015 Plan. The Company’s policy for issuing shares upon exercise of stock options or the vesting of its restricted stock awards and restricted stock units is to issue shares of common stock at the time of exercise or conversion. At March 31, 2020, a total of approximately 3,609,512 shares were available for future issuance under the 2015 Plan.

The following table summarizes stock-based compensation expense by financial statement line item in the Company’s consolidated statements of operations for the fiscal years ended March 31, 2020, 2019 and 2018:

	Fiscal Years Ended March 31,
		2020		2019		2018
	(in $000's)
Cost of revenue	$	2,641	$	2,643	$	1,721
Research and development		5,534		9,312		5,895
Selling, general and administrative		31,606		42,539		32,737
	$	39,781	$	54,494	$	40,353

The components of stock-based compensation for the fiscal years ended March 31, 2020, 2019 and 2018 were as follows:

	Fiscal Years Ended March 31,
		2020		2019		2018
	(in $000's)
Restricted stock units	$	28,895	$	45,998	$	34,559
Stock options		9,006		7,445		5,202
Employee stock purchase plan		1,880		1,051		592
	$	39,781	$	54,494	$	40,353

The following table summarizes stock option activity for the fiscal year ended March 31, 2020:

	Options (in thousands)			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (years)		Aggregate Intrinsic Value (in thousands)
Outstanding at beginning of period		853		$	87.14		5.57
Granted		120			257.88
Exercised		(85	)		44.02
Cancelled and expired		(18	)		224.77
Outstanding at end of period		869		$	112.03		5.30	$	60,113
Exercisable at end of period		652		$	64.38		4.23	$	59,445
Options vested and expected to vest at end of period		867		$	112.03		5.29	$	60,049

Stock options generally vest and become exercisable annually over three years. The remaining unrecognized stock-based compensation expense for unvested stock option awards at March 31, 2020 was approximately $12.9 million and the weighted-average period over which this cost will be recognized is 1.8 years.

The aggregate intrinsic value of options exercised for fiscal years 2020, 2019 and 2018 was $15.0 million, $174.0 million and $66.4 million, respectively. The total cash received as a result of employee stock option exercises during the fiscal years ended March 31, 2020, 2019 and 2018 was approximately $3.7 million, $12.9 million and $9.3 million, respectively.

The Company estimates the fair value of each stock option granted at the grant date using the Black-Scholes option valuation model. The weighted average grant-date fair values and weighted average assumptions used in the calculation of fair value of options granted during the fiscal years ended March 31, 2020, 2019 and 2018 were as follows:

	Fiscal Years Ended March 31,
		2020			2019			2018
Valuation assumptions:
Weighted average grant-date fair value	$	93.05		$	141.47		$	52.34
Risk-free interest rate		1.97	%		2.91	%		1.87	%
Expected option life (years)		4.14			4.04			4.07
Expected volatility		42.3	%		42.8	%		43.5	%

The risk-free interest rate is based on the U.S. Treasury yield curve in effect at the time of grant for a term consistent with the expected life of the stock options. Volatility assumptions are calculated based on the historical volatility of the Company’s stock. The Company estimates the expected term of options based on historical exercise experience and estimates of future exercises of unexercised options. An expected dividend yield of zero is used in the option valuation model because the Company does not pay cash dividends and does not expect to pay any cash dividends in the foreseeable future. Forfeitures are recorded as they occur instead of estimating forfeitures that are expected to occur.

The following table summarizes restricted stock unit activity for the fiscal year ended March 31, 2020:

	Number of Shares			Weighted Average Grant Date Fair Value
	(in thousands)			(per share)
Restricted stock units at beginning of period		620		$	193.53
Granted		161			258.59
Vested		(393	)		144.62
Forfeited		(68	)		304.90
Restricted stock units at end of period		320		$	263.92

Restricted stock units generally vest annually over three years. The remaining unrecognized compensation expense for outstanding restricted stock units, including performance-based awards, as of March 31, 2020 was $23.8 million and the weighted-average period over which this cost will be recognized is 1.5 years.

The weighted average grant-date fair value for restricted stock units granted during the fiscal years ended March 31, 2020, 2019 and 2018 was $258.59, $376.95 and $137.40 per share, respectively. The total fair value of restricted stock units vested in fiscal years 2020, 2019 and 2018 was $99.3 million, $171.3 million and $51.0 million, respectively.

Restricted stock units include certain awards that vest subject to certain performance. The remaining unrecognized compensation expense for outstanding performance restricted stock units as of March 31, 2020 was $4.5 million and the weighted-average period over which this cost will be recognized is 1.1 years.

In May 2019, performance-based awards of restricted stock units for the potential issuance of up to 196,580 shares of common stock were issued to certain executive officers and employees, which vest upon achievement of prescribed service milestones by the award recipients and the achievement of prescribed performance milestones by the Company. The Company did not meet the prescribed performance milestones in fiscal 2020 and therefore no shares will vest for these performance-based awards and the Company reversed all previously recorded stock stock-based compensation expense related to this award in the three months ended March 31, 2020.

In May 2018, performance-based awards of restricted stock units for the potential issuance of 114,000 shares of common stock were issued to certain executive officers and employees, which vest upon achievement of prescribed service milestones by the award recipients and performance milestones by the Company. The Company met the prescribed performance milestones in fiscal 2019 such that the remaining outstanding 47,000 shares of common stock as of March 31, 2020 will vest subject to service requirements for vesting for these employees and stock-based compensation expense is being recognized accordingly over the employee’s service term.

In May 2017, performance-based awards of restricted stock units for the potential issuance of 159,000 shares of common stock were issued to certain executive officers and employees, which vest upon achievement of prescribed service milestones by the award recipients and performance milestones by the Company. The Company met the prescribed performance milestones in fiscal 2018 such that the remaining outstanding 43,000 shares of common stock as of March 31, 2020 will vest subject to service requirements for vesting for these employees and stock-based compensation expense is being recognized accordingly over the employee’s service term.

The Company has an employee stock purchase plan, or ESPP. Under the ESPP, eligible employees, including officers and directors, who have completed at least three months of employment with the Company or its subsidiaries who elect to participate in the purchase plan instruct the Company to withhold a specified amount of the employee’s income each payroll period during a six-month payment period (the periods April 1—September 30 and October 1—March 31). On the last business day of each six-month payment period, the amount withheld is used to purchase shares of the Company’s common stock at an exercise price equal to 85% of the lower of its market price on the first business day or the last business day of the payment period.