Douglas R. Bettinger
Executive Vice President, Chief Financial Officer and Chief Accounting Officer at Lam Research
Great. Thank you, Tim. Good afternoon, everyone, and thank you for joining our call today during what I know is a busy earnings season.
We delivered strong financial results in calendar year 2023. Our revenue came in at $14.3 billion and diluted earnings per share at $27.33. We're pleased with the Company's execution during the year, where the memory WFE mix reached historic lows.
Let's look at the details of our December quarter results. Revenue for the December quarter was $3.76 billion, which was up 8% from the prior quarter and down 29% from a year ago. Our deferred revenue balance at the end of the quarter was $1.93 billion, which was an increase of $238 million from the September quarter, which was mainly tied to growth in customer advanced payments. We continue to have a higher deferred revenue balance versus historic levels given these customer advanced payments.
From a segment perspective, December quarter systems revenue in memory was 48%, which is an increase from the prior quarter level of 38%. The growth in the memory segment was led by DRAM, which was at record levels on a dollar basis, coming in at 31% of systems revenue compared with 23% in the September quarter. DRAM is benefiting from growth in high bandwidth memory capacity and the move to DDR5, which is needed to address AI related workloads and it's also benefiting from shipments to China.
As we've noted in the prior quarters, non-volatile memory WFE was at historic lows on a mix basis in 2023. For the December quarter, this segment represented 17% of our systems revenue, which was up a little bit from 15% in the prior quarter. The slight growth was predominantly related to investments in certain technology projects. NAND customers have aggressively reduced capacity throughout the year to bring inventory levels down.
The foundry segment represented 38% of our systems revenue, a little higher than the percentage concentration in the September quarter of 36%. Growth was driven by new fabs shipments in various regions across several process nodes. The logic and other segment was 14% of our systems revenue in the December quarter, which was down from the prior quarter level of 26%. The decline was driven by general mature nodes' softness as well as the timing of customer projects. Overall, in the foundry/logic segment, we performed well, delivering on the share gains that we've previously been discussing with you.
Now, I'll discuss the regional composition of our total revenue. The China region came in at 40%, which was down from 48% in the prior quarter. Most of our China revenue in the last two quarters was from domestic Chinese customers, and we expect spending from this region to be stable overall in 2024. China as a percent of our revenue is expected to stay relatively high in the March quarter, but it likely trends lower as the year progresses. Our next largest geographic concentration was Korea, that's 19% of revenue in the December quarter versus 16% in the September quarter. And finally, Japan and Taiwan rounded out the remaining of our top four regions.
The Customer Support Business Group generated revenue in the December quarter of nearly $1.5 billion, up 2% from the September quarter and 16% lower than the December quarter in calendar year 2022. Overall, the business was steady and we continue to see our memory customers operating the fabs at very low utilization rates. Given the strength of the installed-base units, we have a strong foundation for growth when technology conversions and utilization rates resume growing. Spares, followed by the Reliant product line, continue to be the two largest components of CSBG.
Turning to the gross margin performance. The December quarter came in at 47.6%, which is above the midpoint of guidance and generally in line with the September quarter level, which was 47.9%. We've improved elements of our cost structure during the year and delivered on our commitment to improve gross margin from the 2023 March quarter level by approximately 1 percentage point as we exited calendar year 2023 from those operational improvements.
December quarter operating expenses were $662 million, up from the prior quarter amount of $622 million. R&D as a percentage of spending was higher versus the September quarter, coming in at over 69% of total expenses. The increased spending reflects our ongoing focus on extending our product and technology differentiation across those critical inflections that Tim mentioned earlier. We will continue to grow investments across multiple market segments to support the long-term strategic objectives for ongoing Company outperformance.
Operating margin for the current quarter was 30%, in line with the September quarter level of 30.1% and above the midpoint of our guidance, primarily because of the stronger gross margin performance. Our non-GAAP tax-rate for the quarter was 12.3%, generally in line with expectations. Looking into calendar 2024, we believe the tax-rate will be in the low- to mid-teens, with the normal fluctuations quarter-by-quarter.
Other income expense for the December quarter came in at $5 million in income compared with $7 million in income in the September quarter. The slight fluctuation in OI&E was mainly due to variations in exchange rates. OI&E will continue to be subject to market related fluctuations that could cause some level of volatility each quarter.
On the capital return side, we allocated approximately $640 million to open market share repurchases. And we paid $264 million in dividends in the December quarter. In the 2023 calendar year, we returned 79% of our free cash flow, totaling $3.8 billion, which was largely consistent with our long-term capital return plans of 75% to 100%. December quarter diluted earnings per share was $7.52, over the midpoint of our guidance. Diluted share count rounded down to 132 million shares, on track with our expectations and down from the September quarter. During 2023, we repurchased nearly 5 million shares through our share buyback program. And I'd just mention, we have $2.1 billion remaining on our Board authorized share repurchase plan.
Let me pivot to the balance sheet. Our cash and short-term investments at the end of the December quarter totaled $5.6 billion, up from $5.2 billion in the September quarter. The increase was largely due to collections, offset by cash allocated to share repurchases, dividend payments and capital expenditures. Overall, 2023 was a record year for cash flows from operations, coming in at $5.3 billion. Days sales outstanding was 66 days in the December quarter, which was a decrease from 73 days in the September quarter. As a result of our operational focus and execution, I'm pleased to report that inventory turns improved to 1.8 times from the prior quarter level of 1.5. We will continue to work on bringing inventory down throughout calendar 2024.
Our non-cash expenses for the December quarter included approximately $70 million for equity compensation, $78 million for depreciation and $13 million for amortization. Capital expenditures for the December quarter were $115 million, up $38 million from the September quarter. Spending was primarily centered on product development activities and lab expansions in the United States and Asia, supporting our global lab investment strategy. We ended the -- excuse me, the December quarter with approximately 17,200 regular full-time employees, which was flat with the prior quarter.
Let's now turn to our non-GAAP guidance for the March 2024 quarter. We're expecting revenue of $3.7 billion, plus or minus $300 million. Gross margin of 48%, plus or minus 1 percentage point. This gross margin guidance is reflected -- reflective of continued favorable customer mix. I do expect this favorable mix to mitigate somewhat as the year progresses. Operating margins of 29.5%, plus or minus 1 percentage point. I would again highlight that the March 2024 quarter will have higher spending as it includes an extra week in the quarter, which occurs every several years. It's a 14 week quarter. And I will also remind you, we will be growing R&D spending this year. And finally, we're expecting earnings per share of $7.25, plus or minus $0.75, based on a share count of approximately 132 million shares.
We continue to be focused on improving our business operations to optimize efficiency and effectiveness as WFE growth occurs. Our profitability metrics reflect the progress we've made during the calendar year 2023, with the business realignment and transformational activities well underway. We'll see these activities continue in the first half of calendar year 2024. Including the costs incurred for these improvement activities and headcount reductions that we saw in calendar 2023, I now expect we'll spend in total $300 million for these actions, which will continue to be reported in our non-GAAP adjustments. I've previously told you we would spend $250 million over 12 months, it's now $50 million higher and six months longer.
So, let me conclude. Over many semiconductor cycles, Lam has established a proven track-record of successfully managing our business. With the actions we've taken over the course of the last several quarters, we expect to strengthen our operations and technology leadership and further enhance our profitability profile. When revenue scales into the next upturn, Lam will be stronger, better-positioned and more efficient.
Operator, that concludes our prepared remarks. Tim and I would now like to open up the call for questions.